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The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 1. Overview: The Nervous System
Anatomical planes
The brain can be dissected for study in several ways:
A medial cut or section divides the brain into right and left halves of equal size, separating the right
and left hemispheres from one another.
A sagittal cut runs parallel to the medial cut, but divides the brain into right and left portions of
unequal size. A medial section may be considered to be a type of sagittal cut. However, a sagittal
section is not a type of medial cut.
A coronal cut runs from ear to ear, separating the brain into front and back portions.
Horizontal (equal halves) or transverse cuts are perpendicular to coronal, medial, and sagittal cuts.
They divide the brain into upper and lower sections.
When describing the nervous system, anatomists use the terms anterior and posterior to indicate
front and back.
Superior and inferior are used to refer to the upper and lower parts of the nervous system.
Cranial and cephalic may be used as synonyms for superior.
Rostral, which literally means "toward the beak," is also sometimes substituted for superior.
The antonym for rostral is caudal, a term that means "toward the tail," and may be used to replace
inferior in descriptions of the brain and spinal cord.
Ventral means "toward the belly" and dorsal means "toward the back." Structures in the lower part of
the brain may be described as ventral.
Medial means toward the center while the term lateral signifies toward the sides.
The neuraxis or central nervous system consists of the brain and spinal cord
The Brain is made up of the cerebral cortex , sub cortical structures, brain stem
and cerebellum
The spinal cord consists of grey and white matter surrounded by meninges in which cerebro-spinal
fluid circulates. It runs from just below the medulla to small of back. Below that the cauda equina
consisting of projections from the spinal cord goes down to the coccygeal area.
Peripheral Nervous System
Cranial Nerves
There are twelve pairs of cranial nerves,
Ten of them have their cell bodies in the brain stem.
Some are motor; some are sensory, and some are both motor and sensory.
Six of them are involved in speech and swallowing.
Spinal Nerves
They connect the central nervous system to the body.
There are thirty-one pairs each of which is both sensory and motor.
Autonomic Nervous System
Distribution
Involved in control of all automatic and glandular functions, it is controlled by the
hypothalamus
It works with the the endochrine system for control of hormonal secretion.
Sympathetic and Parasympathetic Divisions
Sympathetic prepares the body for flight or fight.
Parasympathetic helps, among other things, to bring the body back to normal.
CSU Chico | Glossary | References | Neuroscience on the Web | SPPA362 Home | Next
Other courses in the Neuroscience on the Web series:
SPPA 336, Neuropathologies of Language and Cognition | SPPA 342 (Neuropathologies of Swallowing and Speech)
Copyright, 1998/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.
The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 2. General Description of the Central Nervous System
The Cortex
The two hemispheres of the brain are covered by a layer of cells called the cortex. (Cortex means "bark" in
Latin.) The surface of the cortex is ridged, as it is made up of gyri and sulci.
A gyrus is a raised fold of tissue.
A sulcus is a groove lying between two gyri. A particularly deep sulcus may be called a
fissure.
A convolution includes both gyri and sulci.
Because it is convoluted, a large amount of cortical tissue fits into a relatively small area.
The Subcortical Structures
The Basal Ganglia
The basal ganglia are made up of two structures, the caudate nucleus and the lenticular nucleus.
The caudate nucleus is bounded on one side by the lateral ventricle and is divided into a
head, body and tail.
The lenticular nucleus are lens-shaped structures that have two components, the globus
pallidus and the putamen. The putamen is the more lateral of the two.
The Internal Capsule
The internal capsule lies between the lenticular and caudate nuclei. It is a group of
myelinated ascending and descending fiber tracts including the pyramidal tract that connect
the cortex to other parts of the central nervous system. It begins as a corona radiata (radiating
crown) from motor cells in the premotor, primary motor, and primary sensory areas of the
cortex and converges into the internal capsule. The capsule itself ends within the cerebrum,
but the axons that pass through it continue down to the brain stem, and spinal cord.
Because so many axons join together to pass through this area, the internal capsule is
sometimes referred to as a bottleneck of fibers.
Despite its close proximity to the caudate nucleus and lenticular nucleus, the internal capsule
is not part of the basal ganglia.
The internal capsule and the basal ganglia are collectively referred to as the corpus striatum.
The Limbic System
This is the most ancient and primitive part of the brain. It is also called the rhinencephalon
as the term "rhino" means nose in Latin and much of this area is dedicated to the processing
of olfactory stimuli.
The limbic system is involved, among other things, with emotion and memory.
The Thalamus
This subcortical structure sits within the brain at the level of the temporal lobe. It is well
protected in this location.
The thalamus is made up of three parts, including two thalamic bodies and the tissue that
connects them which is called the massa intermedia, or the interthalamic adhesion. The
thalamic bodies are separated by the third ventricle, one of the spaces in the brain that is filled
with cerebral spinal fluid. The massa intermedia lies within the ventricle.
The thalamus receives and organizes sensory information from the periphery. Messages from
all sensory modalities with the exception of smell pass through the thalamus on their way to
cortical centers and other structures for further processing. (Information about smell travels
directly to the temporal lobe.)
Sensory information, including touch and kinesthesia passes from the thalamus to the parietal
lobe.
Auditory information comes into the thalamus from the inferior colliculi of the midbrain. It is
processed by the medial geniculate bodies of the thalamus before being sent to the
temporal lobe. In the temporal lobe, it first arrives at Heschl's gyrus, which is the primary
auditory area. From there it is sent to association areas for further processing.
Visual information comes into the thalamus from the superior colliculi of the midbrain. It is
processed by the lateral geniculate bodies of the thalamus and sent to the primary visual
area in the occipital lobe. The visual association areas, also in the occipital lobe, further
analyze the information.
Lesions in the thalamus can cause a type of aphasia.
The Hypothalamus
The hypothalamus is located immediately below the thalamus. Part of it is also slightly
anterior to the thalamus. The hypothalamus regulates the functioning of the pituitary gland, so
it controls basic biological functions like appetite, body temperature, sex drive, etc. The
hypothalamus is the part of the brain that makes you shiver when you are cold and sweat
when you are hot. A part of the hypothalamus monitors the level of glucose in the blood and,
when it notices a significant decrease, it sends messages to the stomach producing
sensations of hunger.
Diabetes Insipidus, the most serious type of diabetes, is caused by lesions in the
hypothalamus, or between the hypothalamus and the pituitary gland.
While the thalamus is an input structure, sending messages to higher brain areas, the
hypothalamus is an output structure, sending messages to glands and other parts of the body.
The Brain Stem
The Midbrain (Mesencephalon)
This is the most superior part of the brain stem. The corpora quadrigemina, the red
nucleus, the substantia nigra, the cerebral peduncles, and the cell bodies of two cranial
nerves are located in the midbrain.
The corpora quadrigemina consists of the tectum which is the roof of the brain stem, and
of four protrusions located on the tectum which are called colliculi.
The two superior colliculi are involved in vision. They relay information to the
lateral geniculate bodies of the thalamus.
The two inferior colliculi are involved in hearing. They relay information to the
medial geniculate bodies of the thalamus.
The red nucleus is part of the extrapyramidal tract and connects the cerebellum to the
thalamus and spinal cord.
The substantia nigra is a group of dark colored cell bodies which produce dopamine. It is
also part of the extrapyramidal tract.
The cerebral peduncles connect the pons to the cerebrum.
The nuclei of cranial nerve III, the oculomotor cranial nerve, and of cranial nerve IV, the
trochlear cranial nerve which both provide innervation for eye movement are also located in the
midbrain.
The Pons
The word "pons" is Latin for "bridge." Fibers found there connect the brain stem to the
cerebellum.
The cell bodies for cranial nerves V and VI, the trigeminal and abducens, as well as nuclei of
cranial nerve VII, the facial nerve, are located there.
The Medulla Oblongata
This structure, which is the most inferior part of the brain stem, sits on top of the superior end
of the spinal cord. Because it has a rounded shape, it was once called "the bulb." (The term
"bulbar" refers to the brain stem.) It is involved in circulation and respiration and has several
important landmarks.
The pyramids, which mark the decussation of the pyramidal tract, lie on either side of the
median fissure.
The olivary nuclei are posterior to the pyramids. They are involved in the processing and
relay of auditory information.
The cell bodies of cranial nerves VIII-XII are located here. Some of the nuclei of CN VII are also
found in the medulla.
The Cerebellum
The word "cerebellum" means "little brain" in Latin. This structure has two hemispheres, each
of which is divided into lobes and is covered by the cortex. It is one of the newer parts of the
brain and is very important for the production of speech. It organizes muscle activity and plays
a role in the coordination of fine motor movements and also in balance.
The cerebellum receives both motor and sensory input, and so is the center of a feedback
loop. All motor messages that leave the brain also go to the cerebellum, including information
about the strength of the impulses. The cerebellum integrates motor output so that
movements are smooth and coordinated. Muscle spindles, joints and tendons send
information about movement back to this area. The cerebellum then relays these messages to
the cortex, completing the feedback loop.
Lesions here will cause cerebellar or ataxic dysarthria which involves jerky, uncoordinated
movements of the speech musculature.
The cerebellum is connected to the brain stem by three pairs of tracts called the cerebellar
peduncles.
The Spinal Cord
The spinal cord contains the cell bodies of the spinal nerves as well as their afferent and efferent fibers. It
begins below a large opening in the base of the skull called the magnum foramen and extends downward,
surrounded and protected by the vertebral column. It does not continue through the whole length of the
column, terminating instead slightly above the level of the waist. The part of the vertebral column that lies
below the spinal chord is called the cauda equina, which is Latin for horse's tail.
CSU Chico | Glossary | References | Neuroscience on the Web | SPPA362 Home | Next
Other courses in the Neuroscience on the Web series:
SPPA 336, Neuropathologies of Language and Cognition | SPPA 342 (Neuropathologies of Swallowing and Speech)
Copyright, 1998/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.
The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 3. The Meninges and Cerebrospinal Fluid
The Meninges
The meninges are three layers of protective tissue called the dura mater, arachnoid mater, and the pia mater
that surround the neuraxis. The meninges of the brain and spinal cord are continuous, being linked through the
magnum foramen.
Dura Mater
The dura mater is the most superior of the meningeal layers. Its name means "hard mother" in
Latin and it is tough and inflexible. This tissue forms several structures that separate the cranial
cavity into compartments and protect the brain from displacement.
The falx cerebri separates the hemispheres of the cerebrum from one another.
The falx cerebelli separates the lobes of the cerebellum from one another.
The tentorium cerebelli separates the cerebrum from the cerebellum.
The dura mater also forms several vein-like sinuses that carry blood which has already given its
supply of oxygen and nutrients to the brain back toward the heart.
The superior sagittal sinus runs across the top of the brain in an anterior-posterior direction.
Other sinuses include the straight sinus, the inferior sinus, and the transverse sinus.
The epidural space is a potential space that may exist between the dura mater and the skull. If
there is hemorrhaging in the brain, blood may collect here. Adults tend to bleed here as a result of
closed head injury.
The subdural space is another potential space that may exist between the dura mater and the
medial layer of the meninges, the arachnoid mater. When bleeding occurs in the cranium, blood may
collect here and push down on the lower layers of the meninges. If bleeding continues, brain damage
will result from this pressure. Children are especially likely to have bleeding in the subdural space in
cases of head injury.
Arachnoid Mater
The arachnoid or arachnoid mater is the middle layer of the meninges. In some areas, it projects
into the sinuses formed by the dura mater. These projections are the arachnoid granulations or
villi . They transfer cerebrospinal fluid, the fluid found in the ventricles, back into the bloodstream.
The subarchanoid space lies between the arachnoid and pia mater. It is filled with cerebrospinal
fluid. All blood vessels entering the brain, as well as cranial nerves pass through this space. The term
arachnoid refers to the spider web like appearance of the blood vessels within the space.
Pia Mater
The pia mater is the innermost layer of the meninges. Unlike the other layers, this tissue adheres
closely to the brain, running down into the sulci and fissures of the cortex. It fuses with the
ependyma, the membranous lining of the ventricles to form structures called the choroid plexes
which produce cerebrospinal fluid.
Cerebrospinal Fluid
Purpose
Cerebrospinal fluid is a clear liquid produced within spaces in the brain called ventricles. It is also
found inside the subarachnoid space of the meninges which surrounds both the brain and the spinal
chord. In addition, a space inside the spinal chord called the central canal also contains
cerebrospinal fluid.
It acts as a cushion for the neuraxis, also bringing nutrients to the brain and spinal cord and removing
waste from the system.
Choroid Plexus
All of the ventricles contain choroid plexuses which produce cerebrospinal fluid by allowing certain
components of blood to enter the ventricles. The choroid plexuses are formed by the fusion of the pia
mater, the most internal layer of the meninges and the ependyma, the lining of the ventricles.
The Ventricles
These four spaces are filled with cerebrospinal fluid and protect the brain by cushioning it and
supporting its weight.
The two lateral ventricles extend across a large area of the brain. The anterior horns
of these structures are located in the frontal lobes. They extend posteriorly into the
parietal lobes and their inferior horns are found in the temporal lobes.
The third ventricle lies between the two thalamic bodies. The massa intermedia
passes through it and the hypothalamus forms its floor and part of its lateral walls.
The fourth ventricle is located between the cerebellum and the pons.
The four ventricles are connected to one another.
The two foramina of Munro, which are also know as the interventricular foramina,
link the lateral ventricles to the third ventricle.
The Aqueduct of Sylvius which is also called the cerebral aqueduct connects the third
and fourth ventricles.
The fourth ventricle is connected to the subarachnoid space via two lateral foramina
of Luschka and by one medial foramen of Magendie.
Subarachnoid Space
Although cerebrospinal fluid is manufactured in all of the ventricles, it circulates through the system
in a specific pattern, moving from the lateral ventricle to the third, and then from the third to the fourth.
From the fourth ventricle, the cerebrospinal fluid passes into the subarachnoid space where it
circulates around the outside of the brain and spinal cord and eventually makes its way to the
superior sagittal sinus via the arachnoid granulations or arachnoid villi. In the superior sagittal sinus,
the cerebrospinal fluid is reabsorbed into the blood stream.
The cerebrospinal fluid of the neuraxis is regenerated several times every twenty-four hours.
Endolymph and perilymph, the fluids of the inner ear, are derived from cerebrospinal fluid.
Currently, there is no consensus regarding the manner in which cerebrospinal fluid enters the inner
ear. Osmosis may be involved.
A condition called hydrocephalus occurs when, for some reason, too much cerebrospinal fluid is
produced and the ventricles swell, putting pressure on the tissue of the brain. Tumors are one
potential cause of an over-production of cerebrospinal fluid.
Hydrocephalus should not be confused with hydroencephali. The term hydroencephali literally
means "water brain" and refers to a rare birth defect in which the cerebrum is absent and the space
where it should be is entirely filled with cerebrospinal fluid.
In the past, before CT and MRI technology existed, a technique involving cerebrospinal fluid called
pneumoencephalography was used to view the brain. A small amount of cerebrospinal fluid was
removed from the ventricular system and replaced with air or some other inert gas. This allowed the
examiner to view the ventricles in a scan and make inferences about brain pathology. Tumors and
hemorrhages could sometimes be located by examining the shapes and sizes of the ventricles.
Because space within the cranium is limited, growths or coagulated blood (hematoma) will displace
white and gray matter, pushing them into the ventricular system.
Cerebrospinal fluid can be analyzed to make judgements about a person's general health. A sample
is taken from the spinal cord via a lumbar puncture which is also known as a spinal tap.
CSU Chico | Glossary | References | Neuroscience on the Web | SPPA362 Home | Next
Other courses in the Neuroscience on the Web series:
SPPA 336, Neuropathologies of Language and Cognition | SPPA 342 (Neuropathologies of Swallowing and Speech)
Copyright, 1998/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.
The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 4. Cerebral Lobes, Cerebral Cortex, and Brodmann's Areas
The Cerebral Lobes
Each cerebral hemisphere is divided into four lobes; the frontal, parietal, temporal, and the occipital.
The Frontal Lobe is the most anterior lobe of the brain. Its posterior boundary is the fissure of Rolando,
or central sulcus, which separates it from the parietal lobe. Inferiorly, it is divided from the temporal lobe
by the fissure of Sylvius which is also called the lateral fissure.
This lobe is associated with higher level cognitive functions like reasoning and
judgement. Most importantly for speech pathologists, the frontal lobe contains several
cortical areas involved in the control of voluntary muscle movement, including those
necessary for the production of speech and swallowing.
Broca's Area is found on the inferior third frontal gyrus in the hemisphere that is
dominant for language. This area is involved in the coordination or programming of
motor movements for the production of speech sounds. While it is essential for the
execution of the motor movements involved in speech it does not directly cause
movement to occur. The firing of neurons here does not generate impulses for motor
movement; that is the function of neurons in the motor strip. The neurons in Broca's area
generate motor programming patterns when they fire.
This area is also involved in syntax which involves the ordering of words in speech.
Injuries to Broca's area may cause apraxia or Broca's aphasia.
The precentral gyrus, which may also be called the primary motor area or, most
commonly, the motor strip is immediately anterior to the central sulcus. It controls the
voluntary movements of skeletal muscles; cell bodies of the pyramidal tract are found on
this gyrus.
The amount of tissue on the precentral gyrus that is dedicated to the innervation of a
particular part of the body is proportional to the amount of motor control needed by that
area, not just its size. For example, much more of the motor strip is dedicated to the
control of the articulators than to the legs.
The premotor area or supplemental motor area is immediately anterior to the motor strip.
It is responsible for the programming for motor movements. It does not, however
program the motor commands for speech as these are generated in Broca's area which is
also located in the frontal lobe.
The most anterior part of the frontal lobe is involved in complex cognitive processes like
reasoning and judgment. Collectively, these processes may be called biological
intelligence. A component of biological intelligence is executive function. According to
Denckla, 1996, executive function regulates and directs cognitive processes. Decision
making, problem solving, learning, reasoning and strategic thinking are all part of
executive functioning. Some characteristics of right hemisphere syndrome are considered
problems of the executive function. They include left side neglect where there is a lack of
awareness of the left side of the body.
The Parietal Lobe is immediately posterior to the central sulcus. It is anterior to the occipital lobe, from
which it is not separated by any natural boundary. Its inferior boundary is the posterior portion of the
lateral fissure which divides it from the temporal lobe.
The parietal lobe is associated with sensation, including the sense of touch, kinesthesia,
perception of warmth and cold, and of vibration. It is also involved in writing and in some
aspects of reading.
The postcentral gyrus which is also called the primary sensory area or the sensory
strip is immediately posterior to the central sulcus. This area receives sensory feedback
from joints and tendons in the body and is organized in the same manner as the motor
strip.
Like the motor strip, the sensory strip continues down into the longitudinal cerebral
fissure and so has both a lateral and a medial aspect.
The presensory, secondary sensory, or sensory association areas are located behind the
postcentral gyrus. These areas are capable of more detailed discrimination and analysis
than is the primary sensory area. They might, for example, be involved in sensing how
hot or cold something is rather than simply identifying it as hot or cold. Information is
first processed in the primary sensory area and is then sent to the secondary sensory
areas.
The angular gyrus lies near the superior edge of the temporal lobe, immediately posterior
to the supramarginal gyrus. It is involved in the recognition of visual symbols.
Geschwind described this area as "the most important cortical areas of speech and
language" and the "association cortex for association cortices". He also claims that the
angular gyrus is not found in non-human species.
Fibers of many different types travel through the angular gyrus, including axons
associated with hearing, vision, and meaning. The arcuate fasciculus, the groups of
fibers connecting Broca's area to Wernicke's area in the temporal lobe connects to this
area.
The following disorders may result from damage to the angular gyrus in the hemisphere
that is dominant for speech and language: anomia, alexia with agraphia, left-right
disorientation, finger agnosia, and acalcula.
Anomia is a difficulty with word-finding or naming. Someone suffering from anomia can list the functions of an object
and explain it meaning, but cannot recall its name.
Alexia with Agraphia refers to difficulties with reading and writing.
Left-right disorientation is an inability to distinguish right from left.
Finger agnosia or tactile agnosia is the lack of sensory perceptual ability to identify by touch.
Acalcula refers to difficulties with arithmetic.
The Temporal Lobe is inferior to the lateral fissure and anterior to the occipital lobe. It is separated from
the occipital lobe by an imaginary line rather than by any natural boundary.
The temporal lobe is associated with auditory processing and olfaction. It is also
involved in semantics, or word meaning, as Wernicke's area is located there.
Wernicke's Area is located on the posterior portion of the superior temporal gyrus. In
the hemisphere that is dominant for language, this area plays a critical role in the ability
to understand and produce meaningful speech. A lesion here will case Wernicke's
aphasia.
Heschl's Gyrus, which is also known as the anterior transverse temporal gyrus, is the
primary auditory area.
There are two secondary auditory or auditory association areas which make important
contributions to the comprehension of speech. They are not completely responsible for
this ability, however, as many areas, including Wernicke's area, are involved in this
process.
The Occipital Lobe, which is the most posterior lobe, has no natural boundaries. It is involved in vision.
The primary visual area receives input from the optic tract via the thalamus.
The secondary visual areas integrate visual information, giving meaning to what is seen
by relating the current stimulus to past experiences and knowledge. A lot of memory is
stored here. These areas are superior to the primary visual cortex.
Damage to the primary visual area causes blind spots in the visual field, or total
blindness, depending on the extent of the injury. Damage to the secondary visual areas
could cause visual agnosia. People with this condition can see visual stimuli, but cannot
associate them with any meaning or identify their function. This represents a problem
with meaning, as compared to anomia, which involves a problem with naming, or wordrecall.
The Island of Reil or Insula is a cortical area which lies below the fissure of Sylvius and
is considered by some anatomists to be the fifth lobe of the cerebrum. It can only be seen
by splitting the lateral fissure. Little is known about the connections of this area, but it
may be linked to the viscera. Drunkers, 1996 feels that it may be involved in programming
for speech for speech sounds.
It is important to remember that, while some functions can be localized to very specific parts of the brain, others cannot be classified in
this way because many areas are involved in their performance. Word-finding, for example, is associated with several different areas.
Also, we cannot say that all higher level cognitive functioning is associated with the frontal lobe; the processing of word meaning
carried out by Wernicke's certainly involves a sophisticated type of cognition. Also, right hemisphere lesions often result in
cognitive/perceptual problems.
The Cerebral Cortex
The cortex is about four millimeters thick and is composed of six layers. Listed from most superior to most inferior, these
layers are; the molecular layer, the external granular layer, the internal pyramidal layer, the internal granular layer,
the ganglionic layer, and the fusiform or multiform layer.
The molecular layer is the most superior layer of the cortex. It contains the cell bodies of neuroglial
cells.
The external granular layer is very dense and contains small granular cells and small pyramidal cells.
The external or medial pyramidal layer contains pyramidal cells arranged in row formation. The cell
bodies of some association fibers are found here.
The internal granular layer is thin, but its cell structure is the same as that of the external granular
layer.
The ganglionic layer contains small granular cells, large pyramidal cells as well as the cell bodies of
some association fibers. The association fibers that originatehere form two large tracts: The Bands of
Baillarger and Kaes Bechterew.
The fusiform layer is also known as the multiform layer; its axons enter white matter, it function is
unknown.
All layers are present in all parts of the cortex. However, they do not have the same relative density in all areas.
Depending upon the function of a particular area, some of these layers will be thicker than others in that location.
The cortex wraps around the brain, covering its inferior surface and lining the gap between the right and left cerebral
hemispheres, which is called the longitudinal or interhemispheric fissure.
The part of the cortex covering the sides of the hemispheres is called lateral cortex while the part covering the sides of
the hemispheres that lie within the longitudinal cerebral fissure are called medial cortex.
Brodmann's Classification System
Studies done by Brodmann in the early part of the twentieth century generated a map of the cortex covering the lobes
of each hemisphere. These studies involved electrical probing of the cortices of epileptic patients during surgery.
Brodmann numbered the areas that he studies in each lobe and recorded the psychological and behavioral events that
accompanied their stimulation.
The Frontal Lobe contains areas that Brodmann identified as involved in cognitive functioning and in
speech and language.
Area 4 corresponds to the precentral gyrus or primary motor area.
Area 6 is the premotor or supplemental motor area.
Area 8 is anterior of the premotor cortex. It facilitates eye movements and is involved in
visual reflexes as well as pupil dilation and constriction.
Areas 9, 10, and 11 are anterior to area 8. They are involved in cognitive processes like
reasoning and judgement which may be collectively called biological intelligence.
Area 44 is Broca's area.
Areas in the Parietal Lobe play a role in somatosensory processes.
Areas 3, 2, and 1 are located on the primary sensory strip, with area 3 being superior to
the other two. These are somastosthetic areas, meaning that they are the primary sensory
areas for touch and kinesthesia.
Areas 5, 7, and 40 are found posterior to the primary sensory strip and correspond to the
presensory to sensory association areas.
Area 39 is the angular gyrus.
Areas involved in the processing of auditory information and semantics as well as the appreciation of
smell are found in the Temporal Lobe.
Area 41 is Heschl's gyrus, or the primary auditory area.
Area 42 immediately inferior to area 41 and is also involved in the detection and
recognition of speech. The processing done in this area of the cortex provides a more
detailed analysis than that done in area 41.
Areas 21 and 22 are the auditory association areas. Both areas are divided into two
parts; one half of each area lies on either side of area 42.
Area 37 is found on the posterior-inferior part of the temporal lobe. Lesions here will
cause anomia.
The Occipital Lobe contains areas that process visual stimuli.
Area 17 is the primary visual area.
Areas 18 and 19 are the secondary visual areas.
The Homunculus
A pedagogical device called the homunculus, which literally means "little man," is often used to explain the
organization of the motor strip and to demonstrate that specific areas of this gyrus are responsible for sending
commands to specific parts of the body. The body is represented on the motor strip in an upside-down fashion. The
lower parts of the body, like the feet and the legs, receive motor movement commands from the superior part of the
precentral gyrus. Parts of the face, on the other hand are innervated by the inferior part of the motor strip.
The motor strip extends down some distance into the longitudinal cerebral fissure. The portion inside
this fissure is its medial aspect. The part on the lateral surface of the hemisphere is called its lateral
aspect. The medial cortex controls the movements of the body from the hips on down while the lateral
aspect sends commands to the upper body including the larynx, face, hands, shoulders, and trunk.
The medial and lateral aspects of the motor strip have different blood supplies. Blood comes to the
medial area from the anterior cerebral artery while the lateral portion is supported by the middle
cerebral artery.
CSU Chico | Glossary | References | Neuroscience on the Web | SPPA362 Home | Next
Other courses in the Neuroscience on the Web series:
SPPA 336, Neuropathologies of Language and Cognition | SPPA 342 (Neuropathologies of Swallowing and Speech)
Copyright, 1998/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.
The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 5. The Corpus Striatum, Rhinencephalon, Connecting Fibers, and
Diencephalon
The Corpus Striatum
The corpus striatum, or "striped body" consists of the basal ganglia and the internal capsule. The
basal ganglia is made up of nuclei, so it is gray matter. The internal capsule is a group of tracts
containing myelinated axons, so it is white. Because the internal capsule runs between the caudate
and lenticular nucleus of the basal ganglia, the group of structures forms a stripe.
The Basal Ganglia
The basal ganglia, which is the largest subcortical structure in the brain, is located at
the level of the thalamus. Its components are the caudate nucleus and the lenticular
nucleus, which consists of the putamen and the globus pallidus.
The caudate nucleus is bounded on one side by the lateral ventricle and is divided into
a head, body and tail. It contains endorphins, chemicals that, among other things,
produce a positive emotional state.
The lenticular nucleus is also know as the lentiform nucleus. (Lentiform means lensshaped in Latin). It is located between the caudate nucleus and the Island of Reil with
its anterior aspect being attached to the head of the caudate nucleus. The putamen is
the most lateral part of the structure. When the globus pallidus, the more medial part of
the lenticular nucleus, is probed, sensations of thirst are produced.
Some anatomists consider the claustrum to be part of the basal ganglia.
The amygdala, which is involved in emotion, was once classified as part of the basal
ganglia, but is no longer categorized in this way. It is still considered to be a part of the
limbic system. It is attached to the tail of the caudate nucleus.
The subthalamic nuclei and the substantia nigra are both functionally related to the
basal ganglia, but are not considered to be part of that structure.
The Internal Capsule
The internal capsule lies between the lenticular and caudate nuclei. It is a group of
myelinated ascending and descending fiber tracts including the pyramidal tract that
connect the cortex to other parts of the central nervous system. The capsule itself ends
within the cerebrum, but the axons that pass through it continue down to the brain stem
and spinal cord. They descend through the midbrain within two large bundles called the
cerebral peduncles or cruz cerebri.
Because so many axons join together to pass through this area, the internal capsule is
sometimes referred to as a bottleneck of fibers. This makes it a very bad place to get a
lesion.
The striata, a branch of the middle cerebral artery, brings blood to the internal capsule.
The striata is called the "artery of stroke" because it is prone to hemorrhage and
supports so many important nerve fibers. If there is a problem with the blood supply in
this area, many efferent and afferent tracts can be damaged.
Despite its close proximity to the caudate nucleus and lenticular nucleus, the internal
capsule is not part of the basal ganglia. As stated previously it forms part of the corpus
striatum along with the caudate and lenticular nuclei.
The Rhinencephalon
The term "rhinencephalon," which literally means "smell-brain," is used by many
anatomists to refer to the limbic system. Others make a distinction between these two
terms considering the olfactory tract and olfactory bulbs to be part of the rhinencephalon
but not of the limbic system.
The limbic system is sometimes called the limbic lobe. (This term is only descriptive;
it does not imply that this group of structures is comparable to the lobes of the cerebral
hemispheres which are neo-cortex. The limbic structures are much older.)
The limbic system consists of both cortical and subcortical structures which are located
on the medial, inferior surfaces of the cerebral hemispheres. Its components are
phylogenetically related, being some of the most ancient parts of the brain.
The cortical areas classified as part of the limbic system include the hippocampus,
the cingulate gyrus, and the subcallosal gyrus.
The hippocampus, is a gyrus found on the medial edge of the temporal lobe. It is
named for its shape, as hippocampus literally means "sea horse." The hippocampi are
very close to the basal ganglia and to the lateral ventricles.
The cingulate gyrus is immediately superior to the corpus callosum.
The subcallosal gyrus is immediately inferior to the corpus callosum.
The subcortical components of the rhinencephalon include the olfactory pathways, the
amygdaloid bodies, the mamillary bodies, the dorsal-medial and ventral-anterior
nuclear groups of the thalamus, parts of the reticular formation, and the septal
region.
The olfactory pathway originates in the nasal area. As it passes posteriorly to enter
the temporal lobe at the hippocampal gyrus the olfactory tract is immediately superior
to the optic tract.
The mamillary bodies are also known as the mamillary nucleus. They are connected
to the hippocampus, the thalamus and the fornix.
The septal region includes both the septum pellucidum, which is a double walled
structure located between the corpus callosum and the fornix, and the septal nuclei.
The fornix is a group of fibers that arises from each hippocampus, and project to the
contralateral hippocampus. It links the rhinencephalon to both the thalamus and the
hypothalamus. It is connected to the septal nuclei, the mamillary bodies, and anterior
nucleus of the thalamus.
The limbic system is involved in recent memory, emotion and in motivation and
reinforcement. According to Love and Webb (1992) responses mediated by the limbic
system include pleasure, satiety, guilt, punishment, inhibition, wakefulness, alertness,
excitement, and autonomic activity.
Based on the behavioral correlates of lesions to the limbic system, it may also be
involved in cortical speech and language behavior. The nature of this involvement is not
known at the present.
Lesions to the limbic system can also cause a variety of behaviors, including
aggression, extreme fearfulness, altered sexual behavior, and changes in motivation.
Damage specifically to the hippocampus can affect recent memory and emotion.
Lesions on the olfactory pathways can cause anosmia, which is a loss of the sense of
smell.
An uncinate fit is an epileptic seizure that is receded by an olfactory hallucination.
Connecting Fibers
There are three major types of axons, or nerve fibers, in the brain.
Efferent fibers take messages from the brain to the peripheral nervous system. They are almost
always motor fibers.
Afferent fibers take messages from the periphery back to the brain. They are almost always sensory
fibers.
Interconnecting fibers connect structures within the brain. There are two types of interconnecting
fibers: commissural fibers, and association fibers.
Commissural fibers connect the hemispheres of the brain. The corpus callosum, the anterior
commissure, and the posterior commissure are all composed of commissural fibers. Some
sources consider them to be association fibers.
The corpus callosum, which is Latin for "large body" is the major group of commissural fibers. It is
located some distance down inside the longitudinal cerebral fissure, the split that separates the
hemispheres. It contains at least 200 million axons. Most of these fibers connect mirror image sites
on the two hemispheres; axons might connect Brodmann's area 3 in the parietal lobe of the right
hemisphere to area 3 in the parietal lobe of the left hemisphere. Not all of the connections follow this
pattern, however. For example, area 17 in the occipital lobe is connected to areas 18 and 19 of the
other hemisphere rather than to area 17.
One treatment for severe epileptic seizures is commissurectomy, an operation that severs the
corpus callosum. Both Sperry and Gazzaniga conducted experiments on split-brain patients and
noted differences between the functions of the left and right cerebral hemispheres. The right
hemisphere appears to be involved in the intuitive, holistic processing of information and in spatial
reasoning. The left hemisphere, on the other hand, seems to be more adapted for logic and analytical
reasoning. Most importantly for speech pathologists, it was found that the left hemisphere plays a
dominant role in the speech and language abilities of most people.
Of course, both hemispheres are apparently involved to some extent in any cognitive process.
The other two groups of commissural fibers are called the anterior commissure and the posterior
commissure. Both are connected to the corpus callosum.
Many of the commissural fibers that connect the two temporal lobes pass through the anterior
commissure. The anterior commissure also connects the temporal lobe to the amygdala and to the
occipital lobe in the other hemisphere. The anterior commissure is used by neurosurgeons to locate
the circle of Willis.
Almost all parts of the cortex receive commissural fibers. The "hand area" of the primary sensory strip
is one of the few areas that does not.
Association fibers connect areas within the same hemisphere. The cell bodies of association fibers
are the most prevalent type of neuron found in the cortex.
Long association fibers connect areas that are located in different lobes of the brain. For example,
the arcuate fasciculus, which connects Broca's area in the frontal lobe with Wernicke's area in the
temporal lobe, is composed of long association fibers. The term arcuate fasciculus means "arching
bundle" in Latin. Lesions to this particular bundle of long association fibers will cause conduction
aphasia.
Short association fibers connect areas that are located in the same lobe. For example, the fibers
which connect Heschl's gyrus with the auditory association areas are short association fibers.
The Diencephalon
Location and Description
The diencephalon consists of the thalamus,epithalamus (includes pineal gland), subthalamus
and hypothalamus. Some sources classify the diencephalon as part of the brain stem. This is not
the view of the majority and, for the purposes of this class, the diencephalon should be considered
part of the cerebrum.
Both the thalamus and hypothalamus are located in the center of the brain at the level of the temporal
lobe. They are very well protected in this area.
The thalamus is located below the caudate nucleus and the fornix and is medial to the lenticular
nucleus. It is composed of two bodies which are separated from one another by the third ventricle,
with one lying in each hemisphere. The two thalamic bodies are connected to one another by another
part of the thalamus, the massa intermedia or thalamic adhesion, which makes up part of the
ventricle.
The epithalamus includes the pineal gland and is involved autonomic functions.
The subthalamus is located ventral to the thalamus and is important for motor movement. It has
connections to the basal ganglia, thalamus and brainstem.
The hypothalamus is a solid structure that is located immediately inferior to the thalamus. Part of it is
also anterior to the thalamus. It forms the floor and part of the lateral walls of the third ventricle.
The Thalamus
The thalamus has been described as the switchboard for the cortex. It receives
information from the cerebellum, the basal ganglia and from all sensory pathways with
the exception of the olfactory tract; it integrates the messages and sends them on to
the cortex for further processing.
The thalamic bodies are composed of several different nuclei which are divided from one
another by lamina or thin walls of tissue. The thalamic nuclei can be divided into four
groups based on their functions; the specific relay nuclei, the association nuclei,
the non-specific nuclei, and a subcortical nucleus.
Five areas of the thalamus are classified as the ventral nucleus complex. These include
the lateral geniculate body, the medial geniculate body, the ventral
posterolateral nucleus, the ventral posteromedial nucleus, and the ventral
lateral/ventral anterior nuclei. All of the above receive sensory information and are
considered specific relay nuclei. The ventral lateral and ventral anterior are motor relay
nuclei
The lateral geniculate body is part of the visual information pathway. It receives
information from the superior colliculus of the midbrain and then relays it to the visual
areas of the cortex in the occipital lobe.
The medial geniculate body processes auditory information. It receives messages
from the inferior colliculus of the midbrain and transfers them to the auditory areas of the
cortex in the temporal lobe.
The ventral posterolateral nucleus or VPL is involved in the processing of
somatosensory information. Messages come in from the spinothalamic tract and the
medial lemniscus and are passed on to the somato-sensory cortex found in the parietal
lobe. This nucleus mediates sensations of pain and temperature as well as
proprioception.
The ventral posteromedial nucleus or VPM also handles sensory information. It
receives input from the trigeminothalamic tract which it passes on to the somatosensory cortex of the parietal lobe. Sensory information mediated by the trigeminal
nerve is processed in this area. For example, information about toothaches is carried by
this tract.
The ventral lateral and ventral anterior or VL/VA are motor relay nuclei of the
thalamus. They process motor information. They gets input from the cerebellum and the
basal ganglia and sends output to the motor and premotor cortex in the frontal lobe. As
lesions here will affect motor abilities, knowledge of these nuclei is extremely important
for speech-language pathologists.
Association nuclei form connections between different areas of the thalamus. They are
involved in the integrating and correlating processes that it performs. Three parts of the
thalamus are classified as association nuclei. These include the pulvinar, the lateral
posterior nucleus, and the dorsal medial nucleus.
The pulvinar receives information from other thalamic nuclei and from the superior
colliculus. It sends output to association areas in the parietal, occipital and temporal
lobes. The medial and lateral geniculate bodies are located on top of the pulvinar but are
not a part of it.
The lateral posterior nucleus, or the LP, integrates input that it receives from oth
The dorsal medial nucleus receives input from the amygdaloid bodies, the
hypothalamus, and from other thalamic nuclei. It sends information to the pre-frontal
cortex. As this nucleus connects parts of rhinencephalon with one of the motor areas of
the frontal lobe, it may be involved in the hypothesized connection between the limbic
system and communication.
Two parts of the thalamus are categorized as non-specific nuclei. They are the
intralaminar nucleus and part of the ventral anterior nucleus (the motor relay).
Information comes to the intralaminar nucleus from the basal ganglia, the reticular
formation, and other thalamic nuclei. It sends output to many different cortical areas.
The motor relay of the ventral anterior nucleus receives input from other structures
as well as from within the thalamus and sends it to the pre-motor and pre-frontal cortex
of the frontal lobe. The ventral lateral nucleus is part of the motor feedback circuit
between the cortex and the cerebellum.
One portion of the thalamus, the reticular nucleus, is classified as subcortical. This
name does not refer to the location of the structure, as all parts of the thalamus are
subcortical according to this definition. Instead, the reticular nucleus is labeled this way
because it does not project to the cortex. Its input comes from other thalamic nuclei
and its output is passed on to thalamic nuclei.
The Hypothalamus
The hypothalamus is considered a nodal point in the pathways mediating autonomic,
emotional, endocrine, and somatic functions. It is involved in the following specific
functions:
Release of some hormones from the pituitary gland.
Temperature regulation of the body.
Intake of food and water.
Autonomin nervous system pathways:
The hypothalamus is connected to reticular nuclei in the brain stem that relay axons
that control autonomic motor functions. They are particulary involved with coughing and
vomiting reflexes as well as the reflexes involved in expelling inspirated substances. The
larynx spasms violently in response to food (liquid or solid) getting into the laryngeal
glottis.
Emotion:
The hypothalamus is connected to the septum and the amygdaloid bodies, which are
part of the limbic system, via a group of fibers called the stria terminalis. Reciprocal
connections exist between the hypothalamus and the thalamus, pituitary gland, brain
stem, and the temporal lobe.
The hypothalamus is composed of several groups of nuclei and regions, including the
preoptic area, the supraoptic area, the paraventricular nucleus, the dorsalmedial nucleus, the ventral-medial nucleus, the lateral region, and the posterior
region.
The preoptic area is located in the anterior portion of the hypothalamus. It is involved
in temperature regulation of the body, including the dilation of peripheral blood vessels
and sweating.
The supraoptic area is inferior to the preoptic area and just above the optic chiasm. It
is connected to the pituitary gland and regulates water intake and output via control of
the kidneys.
The paraventricular nucleus is also involved in water regulation. Both the supraoptic
area and the paraventricular nucleus produce ADH or antidiuretic hormone.
The dorsal-medial and the ventro-medial nuclei are involved in the control and
expression of emotions like rage, fear, and extreme anxiety.
The lateral region contains the apostat, which monitors the level of glucose in the
blood and sends messages to the stomach provoking hunger when the blood sugar level
drops to a certain level.
The posterior region is involved in controlling the body's temperature in cold
environments. This area produces shivering by causing the muscles of the body to
vibrate at a rate of between seven and thirteen Hertz. This area also controls sexual
behavior.
Lesions in the hypothalamus may cause obesity, loss of the ability to control body
temperature, or loss of interest in sex. Damage to the hypothalamus or to connections
between the hypothalamus and the pituitary gland may cause diabetes insipidus which
is far more serious than sugar diabetes, the more common form of the disease.
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Unit 6. The Midbrain, Pons, Medulla, and Reticular Formation
The Midbrain (Mesencephalon)
The mesencephalon is the most superior part of the brainstem. It is divided into an anterior and a
posterior section by the Aqueduct of Sylvius which connects the third and fourth ventricles. Motor
tracts, including the fibers of the pyramidal system, pass downward on the midbrain's anterior surface.
Sensory axons, including those of the spinothalamic tract also ascend, along the front of the midbrain
behind the motor tracts.
The corpora quadrigemina, which is located on the posterior surface of the midbrain, is composed
of two superior colliculi and two inferior colliculi. The superior colliculi are part of the visual
system, relaying input from the optic tract to the lateral geniculate bodies of the thalamus. The inferior
colliculi are part of the auditory pathway and send information to the medial geniculate bodies of the
thalamus.
Several important nuclei are located in the midbrain, including the red nuclei, the substantia nigra,
and the nuclei of cranial nerves III and IV.
The red nuclei connect the midbrain to the cerebellum.
The substantia nigra is a group of dark-colored, dopaminergic cells.
CN III is the oculomotor nerve.
CN IV is the trochlear nerve.
Both of these cranial nerves provide innervation for motor movements of the eyes.
The cerebral peduncles (cruz cerebri) are two very large bundles of axons which are a
continuation of the efferent projections within the internal capsule.
The Pons
The pons, which is also part of the brain stem, is inferior to the midbrain and superior to the medulla.
Its posterior border is separated from the cerebellum by the Aqueduct of Sylvius, and more inferiorly,
by the fourth ventricle. Motor and sensory tracts traverse the anterior surface of the pons. The sensory
fibers are located behind the motor fibers.
The nuclei of cranial nerves V and VI are located in the pons. CN V, or the trigeminal, sends motor
messages to the jaw and receives sensory messages from the teeth, tongue, and parts of the face.
CN VI, or the abducens, provides motor innervation to the eye.
The motor nucleus of cranial nerve VII, the facial nerve, is also located in the pons. This part of the
nerve innervates the muscles of facial expression including the eye lids, forehead and the lips.
The Medulla Oblongata
The medulla is the most inferior part of the brain stem. The cell bodies of the following cranial
nerves are located there:
Lower Motor Neuron Cell Bodies
CN VIII, the auditory nerve
CN IX, the glossopharyngeal nerve
CN X, the vagus nerve
CN XI, the spinal accessory nerve
CN XII, the hypoglossal nerve
Because the nuclei of the vagus nerve are found in the medulla, it is considered to be a center for
circulation and respiration. It is also quite important to swallowing. It controls muscles of the pharynx,
larynx and velum for swallowing.
(Note: Most of the cranial nerves important for speech and swallowing are located in the medulla.)
The Reticular Formation
The reticular formation is a set of interconnected nuclei that are located throughout the brain stem. Its
dorsal tegmental nuclei are in the midbrain while its central tegmental nuclei are in the pons and
its central and inferior nuclei are found in the medulla.
The reticular formation has two components:
The ascending reticular formation is also called the reticular activating system. It
is responsible for the sleep-wake cycle, thus mediating various levels of alertness. This
part of the reticular system projects to the mid-line group of the thalamus, which also
plays a role in wakefulness. From there, information is sent to the cortex.
The descending reticular formation is involved in autonomic nervous system activity
as it receives information from the hypothalamus. The descending reticular formation
also plays a role in motor movement.
Interneurons of the reticular formation receive some of the cortico-bulbar fibers from the
motor cortex. It is those fibers that innervate the three cranial nerves involved in eye
movement. Other cortico-bulbar fibers innervate cranial nerves directly. The descending
reticular nuclei in the brain are involved in reflexive behavior such as coughing and
vomiting.
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Unit 7. The Cerebellum
The Cerebellum
Location and Description
The cerebellum is located in the posterior fossa of the skull, dorsal to the pons and medulla from
which it is separated by the Aqueduct of Sylvius and the fourth ventricle.
Like the cerebrum, the cerebellum is covered by cortex and consists of two hemispheres, each of
which is divided into lobes. The hemispheres are separated from one another by a thin structure
called the vermis, or "worm."
The anterior lobe, or paleocerebellum, is the second oldest part of the cerebellum. It
receives proprioceptive input from the spinal cord and controls the anti-gravity muscles
of the body, thus regulating posture.
The posterior lobe, or neocerebellum, is the newest part of the cerebellum. It is
involved in the coordination of muscle movement via the inhibition of involuntary
movement. Inhibitory neurotransmitters, especially GABA, are found here. This lobe
plays an important role in fine motor coordination.
The flocculonodular lobe consists of the flocculi, the most ancient part of the
cerebellum, and the nodulus, the narrowest and most inferior part of the vermis. This
lobe is involved in the maintenance of equilibrium.
Four different nuclei are located deep within each cerebellar hemisphere; the dentate
nucleus, the emboliform nucleus, the globose nucleus, and the fastigal nucleus.
These deep nuclei have axons that project to the brain stem, sending messages out
to be conveyed to other parts of the central nervous system.
The deep nuclei are regulated by radish-shaped cells located in the cerebellar cortex
called Purkinje cells. The Purkinje cells control the output of the cerebellum by
inhibiting the firing of the deep nuclei. The Purkinje cells located in the lateral cortex of
the cerebellum project to the dentate nuclei, while those in the intermediate cortex
synapse with the emboliform and globose nuclei. The fastigial nuclei receive input from
Purkinje cells found in the cerebellar cortical covering of the vermis.
Importance of the Cerebellum
The cerebellum is involved in a feedback loop for muscle movement. When the cortex sends a
message for motor movement to the lower motor neurons in the brain stem and spinal cord, it also
sends a copy of this message to the cerebellum. This is conveyed from pyramidal fibers in the cortex
on the corticopontinecerebeller tract to the cerebellum. In addition, information gets to the cerebellum
from muscle spindles, joints and tendons. This information (proprioception and kinesthesia) lets the
cerebellum know about the movements that have been executed, so that it can determine how well
motor commands coming from the cortex are being carried out. This has ben called its comparator
function.
The cerebellum plays a major role in the coordination of muscle activity for the production of smooth
movement through its connections with the pyramidal and extrapyramidal systems and the
descending reticular formation. Due to its role in the coordination of fine motor movements, the
cerebellum makes important contributions to the control of rapid, alternating muscle movements
necessary for speech and swallowing.
The Cerebellar Peduncles
Three fiber bundles called peduncles connect the cerebellum to the brain stem. On these tracts,
information runs in both directions, with all messages sent and received by the cerebellum traveling
on these fibers.
The superior cerebellar peduncle or the superior brachium conjuctivum connects
the cerebellum to the midbrain and contains efferent fibers from the dentate, emboliform,
and globose nuclei. These are the axons that send feedback to the motor cortex in the
frontal lobe via the red nucleus in the midbrain and thalamus. Afferent fibers traveling in
this peduncle bring proprioceptive information to the cerebellum from the lower body.
This information ascends along the spinal cord in the ventrospinocerebellar tract,
before entering the cerebellum.
The middle cerebellar peduncle or the middle brachium pontis is the largest of the
peduncles and links the cerebellum with the pons. Via this connection, the cerebellum
receives a copy of the information for muscle movement that the pyramidal tract is
carrying down to lower motor neurons.
The inferior cerebellar peduncle or the restiform bodies connects the cerebellum
with the vestibular nuclei located in the lower pons and medulla and also with the
reticular formation.
Proprioceptive information from the upper body, travelling along the dorsospinocerebellar tract
enters the cerebellum on the inferior cerebellar peduncle.
Feedback Pathways
Sensory and motor input to the cerebellum:
The cerebellum receives both proprioceptive and kinesthetic information from the periphery. It also
gets information about the strength and type of muscle movements occurring.
Proprioception, according to Love and Webb, 1992, refers to sensory information about pressure,
movement, vibration, position, muscle pain, and equilibrium received from muscles, joints and
tendons.
Kinesthesia is a more specific term than proprioception. It refers to feedback that comes only from
muscle spindles. Kinesthesia is the "ability to detect the range and direction of movements of the
limbs" (Bhatnager & Andy, 1995, pg. 341).
Sensory information ascends to the cerebellum along the spinal cord. The two main tracts that bring
information from the periphery to the cerebellum are the ventrospinocerebellar tract and the
dorsospinocerebellar tract.
The ventrospinocerebellar tract contains proprioceptive fibers from the lower body. (The axons of
this tract decussate and travel upward on the contralateral side of the spinal cord for some distance
before crossing again and continuing upward ipsilaterally.) The axons of this tract enter the
cerebellum on the superior cerebellar peduncle.
The dorsospinocerebellar tract takes proprioceptive information from the upper body that reaches
the cerebullum on the inferior cerebellar peduncle (restiform body).
The reticulocerebellar tract carries messages received by the reticular nuclei in various parts of the
brain stem from the cortex, spinal cord, vestibular system and red nucleus.
The vestibulocerebellar tract brings information from the semi-circular canals of the inner ear to the
cerebellum via the vestibular nucleus located in the lower pons and medulla. These fibers travel to the
flocculi on the inferior cerebellar peduncle.
Cortical Input
The corticopontinecerebellar tract brings motor information to the cerebellum from the frontal lobe.
It leaves the precentral gyrus and descends in the internal capsule along with pyramidal tract fibers.
Its axons synapse with cells in the pons. These pontine nuclei then send second order neurons to the
cerebellum on the middle cerebellar peduncle. The axons of the corticopontine tract bring the
cerebellum a copy of the information that the corticobulbar and corticospinal aspects of the pyramidal
tract are conveying to the cranial and spinal nerves. Thus, the cerebellum "knows" the details of the
messages being sent to lower motor neurons by the upper motor neurons of the pyramidal tract. The
messages sent by the corticopontinecerebellar tract includes information about the nature of the motor
impulse being sent by the precentral gyrus, its destination, its strength and its speed. This
connection is contralateral, meaning that information from the frontal lobe of one hemisphere is sent to
the posterior lobe on the opposite side of the brain.
The cerebellum acts on the motor messages carried by the corticopontinecerebellar tract. In an
unknown fashion, it integrates the information and exerts control over the message through the firing
of Purkinje cells.
The corticopontinecerebellar tract is considered by many to be part of the extrapyramidal system,
although it originates in the precentral and postcentral gyri with pyramidal tract fibers it synapses with
cells in the pons rather than with spinal or cranial nerves. The portion of the tract from the cortex to
the pons consist of first order neurons; and from the pons to the cerebellum, second order. The first
order neurons may be considered by some to be part of the pyramidal system.
Output from the cerebellum
The tracts discussed below may be considered part of the extrapyramidal system since they carry
involuntary, and automatic information and are indirect and multisynaptic. As mentioned previously
some sources consider the extrapyramidal tract to consist of the basal ganglia only.
Output to the red nucleus
The dentate nuclei, which receive inhibitory messages from the Purkinje cells in the lateral cerebellar
cortex, and the emboliform and globose nuclei, which are controlled by the Purkinje cells in the
intermediate cortical areas all project to the contralateral red nucleus in the midbrain via the superior
cerebellar peduncle.
Some of the information sent on the superior cerebellar peduncle travels from the red nucleus to the
ventrolateral nucleus of the thalamus. From there it passes on to the precentral gyrus of the
frontal lobe. Other ascending projections travel to the thalamus directly. These connections allow the
cerebellum to perform its comparatot function by providing feedback to the motor cortex regarding the
messages that it has received from corticopontinecerebellar fibers as well as feedback from the
muscles innervated by lower motor neurons that receive impulses from the pyramidal and extra
pyramidal tracts.
The Structure of the Feedback Loop for Motor Movement
Precentral gyrus --> Pontine nuclei -- > Cerebellum -- > (after input from muscles
innervated by lower motor neurons, and comparisons made by the cerebellum between
output from the cortex and input from muscles) --> Red nuclei --> Thalamus -->
Precentral gyrus where adjustments are made
Fibers from the red nucleus also descend to synapse with the spinal nerves, forming the rubrospinal
tract.
The vestibulospinal tract
The fastigal nucleus, which receives inhibitory control from the Purkinje cells of the
vermis, sends messages to the vestibular nuclei in the lower pons and medulla. From
the vestibular nuclei, information is sent to lower motor neurons in the brain stem and
spinal cord. This tract brings information about the body's position in space to the
antigravity muscles.
The reticulospinal tract
This tract carries information related to the functioning of the autonomic nervous system
including circulation of the blood, dilation of blood vessels, respiration and visceral
activity. (Note that while this tract influences autonomic activity, it is not part of the
autonomic nervous system.)
Ataxia, which is an incoordination of motor movement, results from cerebellar lesions. The term ataxia is also used
to describe the unsteady walk and usual postures seen in patients who have suffered injury to the cerebellum.
People sometimes compensate for these problems by walking with their feet far apart in a broad-based gait (Love
& Webb,1992).
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Copyright, 1999/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.
The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 8. The Spinal Cord, Spinal Nerves, and the Autonomic Nervous System
The Spinal Cord
Description
The spinal cord begins below the medulla and ends just above the small of the back at the
conus medularis. Recall that the area within the vertebral column beyond the end of the
spinal cord is called the cauda equina.
Meninges
Recall also that the spinal cord is protected by the vertebrae and the meninges. The dura
mater, arachnoid mater and pia mater of the spinal cord are continuous with those of the brain.
Cerebrospinal fluid is in the subarachnoid space that lies between the arachnoid and pia
mater and in the central canal, a space in the middle of the gray matter of the cord. It
provides a hydraulic cushion for the spinal cord.
Internal Composition
Dorsal (sensory) and ventral (motor) horn cells
When the cord is viewed in a cross-section, its gray matter is "H" shaped or, as
described by Bhatnagar and Andy,1995, butterfly shaped. It has two ventral and
two dorsal horns. The white matter surrounding the cell bodies of the cord is
made up of ascending and descending fibers. Motor tracts are found on the
ventral and lateral aspects of the cord while sensory tracts run along its dorsal
area.
Neuronal types
Motor neurons
These lower motor neurons are located on the ventral aspect of the
cord. They are either alpha or gamma cells.
Alpha cells are the principle lower motor neurons of the spinal
cord and form the main portion of the final common pathway. They
conduct rapid motor impulses, with each alpha cell innervating
approximately 200 muscle fibers.
Gamma neurons are also part of the final common pathway
according to some sources but they are only half as numerous as
alpha cells. Gamma cells conduct slow motor impulses. Their
major function is to stretch muscle spindles.
Association neurons
Interneurons connect the anterior and posterior horns of the gray
matter and are involved in the reflex arc. They work within the
same segment of the spinal cord, with a segment being defined as
the horizontal section of the cord that gives rise to one pair of
spinal nerves.
Internuncial Neurons travel between segments, sending
projections up to the brain stem and cerebellum. They project in
an ascending, not descending manner.
These association neurons are found throughout the central
nervous system. They are much more numerous than motor
neurons; the ratio between the two types of cells is 30:1.
The main function of the association neurons in the spinal cord is
that of inhibitory control. They also interconnect other cells with
one another.
Some sources, including Bhatnager and Andy, (1995), do not
distinguish between interneurons and internuncial neurons. Even if
these two types of association neurons are grouped together, they
should definitely be distinguished from the spinal nerves which are
lower motor neurons, forming a final common pathway for
information descending from the brain.
The Spinal Nerves
Description
There are thirty-one pairs of spinal nerves. These nerves are mixed, having
both a sensory and a motor aspect. Their motor fibers begin on the ventral part
of the spinal cord at the anterior horns of the gray matter. The roots of their
sensory fibers are located on the dorsal side of the spinal cord in the posterior
root ganglia. When the motor and sensory fibers exit the spinal column
through the intervertebral foramina and pass through the meninges, they join
together to form the spinal nerves.
Spinal nerves receive only contralateral innervation from first order neurons.
Eight pairs of spinal nerves are located in the uppermost,
cervical region of the cord:
Twelve pairs are found in the thoracic region.
Five pairs are in the lumbar area.
Five pairs are in the sacral area.
One pair is found in the most inferior, coccygeal region.
Function
These second order lower motor neurons, the spinal nerves, form part of the final
common pathway for information traveling from the central nervous system to
the periphery. The spinal nerves provide innervation to body areas below the neck
while cranial nerves (also second order neurons) carry impulses only to the head
and neck, except for the vagus. (You will understand shortly that cranial nerves
can be sensory, motor or both).
Reflex arc
Also, the sensory and motor fibers of the spinal nerves form a
reflex arc. This type of reflexive behavior occurs when a message
from sensory fibers causes a motor reaction directly, without
traveling to the brain. For example, if you touch a hot burner on
the stove, sensory information about the temperature of the burner
travels along spinal nerves to your spinal cord and are carried
directly to their motor nuclei by interneurons; the motor command
goes out along the axons of the lower motor neuron causing you
to move your hand away from the stove. As messages do not have
to travel up to the brain to be processed, reactions mediated by
this reflex arc can occur very rapidly. Of course you WILL feel pain
shortly thereafter (milliseconds) as the information gets to the
parietal lobe via the thalamus
The Autonomic Nervous System
The autonomic nervous system is involved in the control of the heart, glands and smooth muscles of the
body and plays a major role in regulating unconscious, vegetative functions. It works together with the
endocrine system to control the secretion of hormones and is itself controlled by the hypothalamus.
Because motor fibers make up the bulk of the autonomic system, some anatomists consider it to be purely
motoric although it does include some afferent axons that carry information from the viscera.
Although the autonomic nervous system is considered to be one of the three main divisions of the human
nervous system in its own right, parts of both the central nervous systems and the peripheral nervous
systems play a role in its functions.
The autonomic nervous system has two components, the sympathetic system and the parasympathetic
system. These two aspects have antagonistic functions.
Sympathetic System
The sympathetic system prepares the body for fight or flight reactions. Action
of this system results in accelerated heart rate, increased blood pressure and
blood flow away from the periphery and digestive system toward the brain, heart
and skeletal muscles. It also causes adrenaline to be released, temporarily
increasing physical strength.
Parasympathetic System
The parasympathetic system brings the body back to a state of equilibrium. It
slows heart rate and decreases the release of hormones into the blood stream.
The activity of the parasympathetic system causes more localized reactions
than does the sympathetic system as much of its output is to specific organs.
The autonomic nervous system consists of four chains of nuclei or ganglia, two of which are
located on either side of the spinal cord. The outer chains of nuclei form the
parasympathetic division of the system while those closest to the spinal cord make up its
sympathetic element.
Rami communicantes
The rami of the autonomic nervous system are the axons of pre-ganglionic and
ganglionic cells.
Pre-ganglionic cells of the autonomic nervous system are neurons located in
some of the cranial nerves of the brain stem and in some of the spinal nerves
that project to the ganglionic chains of the autonomic nervous system. The
autonomic nervous system is closely connected with the central and peripheral
nervous systems due to this arrangement.
Ganglionic cells originate within the ganglia. They project to post-ganglionic
neurons.
Post-ganglionic cells are neurons that are located in the target organs and
muscles of the autonomic nervous system.
It can be said that the motor pathways of the autonomic nervous system are
made up of its pre-ganglionic and ganglionic cells.
The fibers and ganglionic chain of the parasympathetic chain are not as welldefined as those of the sympathetic chain. All preganglionic neurons of the
sympathetic system synapse with the sympathetic chain. This is not true of the
parasympathetic preganglionic cells, however. Some of them synapse with the
chain, but others go directly to end organs or muscles.
CSU Chico | Glossary | References | Neuroscience on the Web | SPPA362 Home | Next
Other courses in the Neuroscience on the Web series:
SPPA 336, Neuropathologies of Language and Cognition | SPPA 342 (Neuropathologies of Swallowing and Speech)
Copyright, 1999. Patrick McCaffrey, Ph.D. This page is freely distributable.
The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 9. The Cranial Nerves (including lower motor neurons involved in
swallowing and speech)
The cranial nerves innervate the muscles of the jaw, face, tongue, neck, pharynx, and larynx. Some of them are
motor, some are sensory and some are mixed nerves, containing both sensory and motor fibers. Six of them are
involved in speech and swallowing, and are therefore very important to the speech, language pathologist.
The motor nuclei and processes of the cranial nerves are lower motor neurons as they form a final common pathway
for information descending from the cerebrum to the periphery. Because the motor roots of the cranial nerves are
located in the brain stem, messages from the precentral and postcentral gyri reach the cranial nerves on the
corticobulbar portion of the pyramidal tract. With the exception of part of CN VII (the facial nerve) and parts of CN
XII the hypoglossal nerve, this innervation is bilateral.
Along with the spinal nerves, the cranial nerves make up the peripheral nervous system.
There are twelve pairs of cranial nerves.
Cranial Nerves Involved in Smell and Vision
CN I, the olfactory nerve, is a purely sensory nerve. It has receptors within the mucous membrane
of the nose. Information runs posteriorly along the olfactory tract and through the olfactory bulb to the
temporal lobe where it is processed. Remember that this is the only sense not mediated by the
thalamus.
CN II, the optic nerve, is also a sensory nerve. Visual information from the retina is carried back to
the superior colliculus of the midbrain on the optic tract which is immediately below the olfactory tract
in the more anterior part of the brain. From the superior colliculus, messages are first passed on the
lateral geniculate body in the thalamus and then to the cortex of the occipital lobe.
CN III is the oculomotor nerve. The nucleus of this motor nerve is located in the midbrain. It
mediates movements of the eyeball and constriction and dilation of the pupil.
CN IV or the trochlear nerve, is a motor nerve. Its nucleus also lies in the mesencephalon. This
nerve also innervates eye movements and damage to it will cause double vision.
CN VI is the abducens nerve. The nucleus of this motor nerve is found in the pons. This nerve
provides innervation for movements of the eyeball.
The Cranial Nerves Involved in Speech, Hearing and Swallowing
CN V is the trigeminal nerve.
The motor nucleus of this large mixed nerve originates in the pons. It provides motor
innervation to the muscles that control the mandible (jaw), the tensor veli palatini
muscle of the velum, and the tensor tympani muscle of the middle ear.
It mediates sensation from the head, jaw, face, some of the sinuses and tactile
sensation from the anterior two thirds of the tongue.
CN VII or the facial nerve, is most often classified as a motor nerve, but can also be considered a
mixed nerve.
Its motor nucleus which is located in the pons innervates all of the muscles of facial
expression including those in the forehead, cheeks, and lips, as well as the stapedius
muscle of the middle ear. It also sends motor impulses to the rest of the ear; if you can
wiggle your ears, this action is mediated by CN VII.
The part of the nucleus that sends commands to the upper part of the face receives
bilateral (ipsalateral and contralateral) innervation from upper motor neuronal tracts.
However, the portion that controls the lower part of the face receives only contralateral
(unilateral) innervation. This means that unilateral lesions of the pyramidal tract may
have noticeable effects on voluntary movements of the cheeks and lips. Note that
involuntary facial expressions of emotion will not be impaired in the case of a pyramidal
tract lesion as they are controlled by the extrapyramidal tract. They will be affected by
lower motor neuron lesions.
The sensory aspect of the facial nerve mediates taste in the anterior two thirds of the
tongue.
CN VIII is the auditory/vestibular nerve. The two branches of this sensory nerve carry information
from the cochlea and from the vestibular end organs in the inner ear. The auditory nerve originates in
the medulla.
CN IX, or the glossopharyngeal nerve, is a mixed nerve.
Its motor aspect contributes to the action of the middle pharyngeal constrictor muscle
and innervates the stylopharyngeus muscle.
Its sensory aspect carries input from the posterior one third of the tongue, the velum,
and the pharynx including the tonsils.
CN X is the vagus nerve. This mixed nerve originates in the medulla.
One of the motor nuclei of the vagus innervates the majority of the viscera, including the
heart, respiratory system, and digestive system.
Another motor nucleus sends motor commands to the pharyngeal constrictor muscles
and completely controls the intrinsic musculature of the larynx. The superior branch of
the vagus innervates the cricothyroid muscle and so is involved in pitch changes. Its
recurrent branch innervates all of the other intrinsic laryngeal musculature.
The vagus also innervates the glossopalatine and levator veli palatine muscles, making
it primarily responsible for palatal functioning.
CN XI is the spinal accessory nerve, a motor nerve that originates in the medulla. It innervates the
trapezius and sternocleidomastoid muscles of the neck. It also sends some motor messages to the
uvula and the levator veli palatine (raises the velum).
CN XII, is the hypoglossal nerve, another motor nerve that originates in the medulla. It controls
tongue movement, innervating both the intrinsic and extrinsic tongue muscles. The part of the nucleus
that innervates the genioglossus, the muscle involved in tongue protrusion, is connected only to
contralateral fibers from the pyramidal tract. Thus, unilateral upper motor lesions can affect this type of
movement. All other tongue muscles receive bilateral innervation for voluntary movements.
If there is damage to the hypoglossal nerve itself, the tongue may reveal the presence of
the lower motor neuron lesion by fasiculating (twitching).
The Six Cranial Nerves Involved in Speech and Swallowing
CN V - - the trigeminal nerve
CN VII - - the facial nerve
CN IX - - the glossopharyngeal nerve
CN X - - the vagus nerve
CN XI - - the spinal accessory nerve
CN XII - - the hypoglossal nerve
Stages of Deglutition (Logemann,1989, 1994. Morrell,1984. In Groher,1984.)
In the Oral-preparatory stage, food is moved around the mouth, chewed and tasted. Time needed
for this stage is variable.
During the oral stage, food is moved to the back of the mouth by the tongue. This
stage lasts for about 1 second, ending when the bolus touches the back of the
oropharynx.
In the first part of the pharyngeal stage, or laryngeal substage, a number of things happen
simultaneously. The larynx moves up and forward, the vocal folds approximate and the epiglottis falls
over the top of the larynx.
During the second part of this stage, the bolus of food is moved down the pharynx by
the stripping action of the pharyngeal constrictor muscles. This phase of the
pharyngeal stage ends when the cricopharyngus muscle, also known as the p.e.
segment, opens, allowing the food to enter the esophagus. It is not known what leads
the p.e. segment to open at the right time during the swallowing sequence. Some think
that it opens due to stretching caused by the elevation of the larynx. The total time
required for the entire pharyngeal phase is 1 second.
During the esophageal stage, food travels down the esophagus via a wave-like motion called
peristalsis. This phase lasts between 8 and 20 seconds.
Note that peristalsis occurs only in the esophagus and colon, not in the pharynx. In
some older literature on swallowing, there is confusion about this, and the stripping
action of the pharyngeal constrictor muscles is mislabeled as peristalsis.
Swallowing is not just a motor reflex; it requires a combination of sensory and motor
control. The swallowing center, which is located in the nucleus ambiguous of the
medulla, recognizes a pattern that consists of both sensory and motor elements and
then triggers the swallowing response. This is why the swallow is considered to be a
type of patterned response or pattern recognition system.
Sensory input that initiates the swallow response comes from the trigeminal, facial and
glossopharyngeal nerves.
The trigeminal nerve is involved in the oral preparatory and oral stages. It provides the
innervation that controls jaw movement for chewing and it also mediates tactile
sensation in the anterior two thirds of the tongue.
During the oral preparatory and oral stages, taste is carried from the anterior two thirds
of the tongue by the facial nerve.
The glossopharyngeal nerve is responsible for taste in the posterior one third of the tongue and for
tactile sensation to the posterior part of the oral cavity, including the velum, tonsils, and walls of the
oropharynx. It provides the feedback that is most important in the elicitation of the swallow.
Feedback from motor movements, especially tongue movements which are mediated by the
hypoglossal nerve, also help to trigger the swallow.
Input from both the cerebral cortex and the cerebellum is responsible for the coordination and timing
of the motor movements involved in swallowing.
Mnemonic for the Cranial Nerves
On
(olfactory)
Some
(sensory)
Old
(optic)
Say
(sensory)
Olympus's
(oculomotor)
Marry
(motor)
Towering
(trochlear)
Money
(motor)
Top
(trigeminal)
But
(both)
A
(abducens)
My
(motor)
Finn
(facial)
Mother*
(motor)
And
(auditory)
Says
(sensory)
German
(glossopharyngeal)
Bad
(both)
Vended
(vagus)
Business
(both)
At
(accessory)
Marry
(motor)
Hopps
(hypoglossal)
Money
(motor)
The facial nerve could also be classified as both sensory (taste for anterior two thirds of tongue) and
motor, in which case the word in this part of the rhyme would change to "brother." It is usually
classified as a motor nerve.
CSU Chico | Glossary | References | Neuroscience on the Web | SPPA362 Home | Next
Other courses in the Neuroscience on the Web series:
SPPA 336, Neuropathologies of Language and Cognition | SPPA 342 (Neuropathologies of Swallowing and Speech)
Copyright, 1998/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.
The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 10. Upper Motor Neuronal Tracts
In order to reach the muscles of the body, motor commands generated in the central nervous system must travel down
upper motor neurons and lower motor neurons.
Upper motor neurons are a type of first order neuron. They are unable to leave the central nervous system. The
pyramidal tract is a very important upper motor neuron tract. The extrapyramidal tract also consists of upper motor
neurons.
As upper motor neurons must remain inside the neuraxis, they synapse with neurons of another type called lower motor
neurons which can carry messages to the muscles of the rest of the body.
Lower motor neurons, or second order neurons are cranial and spinal nerves. The cell bodies of these neurons are
located in the neuraxis, but their axons can leave the central nervous system and synapse with the muscles of the body.
All lower motor neurons are either spinal or cranial nerves. All spinal nerves have a lower motor neuron component as they
are mixed nerves. However, not all cranial nerves have lower motor neuron components. Some of the cranial nerves
contain only sensory fibers and therefore cannot be classified as lower motor neurons. For example, CN I, the olfactory
nerve, CN II the optic nerve, and CN VIII, the auditory nerve, do not have motor components.
The Pyramidal Tract
This group of fibers carries messages for voluntary motor movement to the lower motor neurons in the brain
stem and spinal cord.
Approximately 80% of the cell bodies of the pyramidal tract are located on the precentral gyrus of the frontal
lobe, which is also known as the motor strip. Particularly large cells located here whose axons are part of
the pyramidal tract are called pyramidal cells. Approximately 20% of the pyramidal tract fibers also originate
in the postcentral gyrus of the parietal lobe, in Brodmann's areas 1, 2, and 3. Regardless of the location of
their cell bodies, pyramidal tract fibers descend from the cortex inside the internal capsule.
This tract is direct and monosynaptic, meaning that the axons of its neurons do not synapse with other
cells until they reach their final destination in the brain stem or spinal cord. These direct connections
between the cortex and the lower motor neurons allow messages to be transmitted very rapidly from the
central nervous system to the periphery.
The fibers of the pyramidal tract that synapse with spinal nerves sending information about voluntary
movement to the skeletal muscles form the corticospinal tract. These axons are among the longest in the
central nervous system, as some of them travel all the way from the cortex to the inferior part of the spinal
cord. As they descend through the brain, they form part of the posterior limb of the internal capsule.
At the pyramids in the inferior part of the medulla, eighty-five to ninety percent of corticospinal fibers
decussate, or cross to the other side of the brain. The remaining ten to fifteen percent continue to descend
ipsilaterally. The fibers that decussate are called the lateral corticospinal tract or the lateral pyramidal
tract. Because they descend along the sides of the spinal cord, the uncrossed or direct fibers that
synapse with spinal nerves on the ipsilateral side of the body are called the direct pyramidal tract. They
may also be referred to as the ventral pyramidal tract or the anterior corticospinal tract since they
travel down the ventral aspect of the spinal cord.
The spinal nerves receive only contralateral innervation from the corticospinal tract. This means that
unilateral pyramidal tract lesions above the point of decussation in the pyramids will cause paralysis of the
muscles served by the spinal nerves on the opposite side of the body. For example, a lesion on the left
pyramidal tract could cause paralysis on the right side of the body.
The fibers of the pyramidal tract that synapse with cranial nerves located in the brain stem form the
corticobulbar tract. Obviously, this is the part of the pyramidal tract that carries the motor messages that
are most important for speech and swallowing. Corticobulbar axons descend from the cortex within the
genu or bend of the internal capsule.
Almost all of the cranial nerves receive bilateral innervation from the fibers of the pyramidal tract. This
means that both the left and right members of a pair of cranial nerves are innervated by the motor strip areas
of both the left and right hemispheres.
This redundancy is a safety mechanism. If there is a unilateral lesion on the pyramidal tract, both sides of
body areas connected to cranial nerves will continue to receive motor messages from the cortex. The
message for movement may not be quite as strong as it was previously but paralysis will not occur.
The two exceptions to this pattern are the portion of CN XII that provides innervation for tongue protrusion
and the part of CN VII that innervates the muscles of the lower face. These only receive contralateral
innervation from the pyramidal tract. This means that they get information only from fibers on the opposite
side of the brain. Therefore, a unilateral upper motor neuron lesion could cause a unilateral facial droop or
problems with tongue protrusion on the opposite side of the body. For example, a lesion on the left
pyramidal tract fibers may cause the right side of the lower face to droop and lead to difficulty in protruding
the right side of the tongue. The other cranial nerves involved in speech and swallowing would continue to
function almost normally as both members of each pair of nuclei still receives messages from the motor
strip.
Because most cranial nerves receive bilateral innervation, lesions of the upper motor neurons of the
pyramidal tract must be bilateral in order to cause a serious speech problem. (The effects of the inability to
protrude the tongue and of paralysis of the lower face on speech are negligible.)
On the other hand, unilateral lesions of the lower motor neurons may cause paralysis. This occurs because
the lower motor neurons are the final common pathway for neural messages traveling to the muscles of the
body. At the level of the lower motor neurons, there is no alternative route which will allow messages from
the brain to reach the periphery. Muscles on the same side of the body as the lesion will be affected.
Lesions on the cranial nerve nuclei located in the brain stem are called bulbar lesions. The paralysis that
they produce is called bulbar palsy.
Lesions to the axons of the cranial nerves are called peripheral lesions.
As cranial nerves are lower motor neurons, both bulbar and peripheral lesions are lesions of the final
common pathway.
When bilateral lesions of the upper motor neurons of the pyramidal tract occur, they produce a paralysis
resembles that which occurs in bulbar palsy. For this reason, the condition is known as pseudo-bulbar
palsy.
If a lesion occurs in the brain stem and damages both the nucleus of a cranial nerve and one side of the
upper motor neurons of the pyramidal tract, a condition known as alternating hemiplegia may result. This
involves paralysis of different structures on each side of the body. The lesioning of the nucleus of the cranial
nerve will cause a paralysis of the structures served by that nerve on the same side of the body as the
injury. Because the pyramidal tract provides only contralateral innervation to the spinal nerves, damage to
the upper motor neurons will meanwhile cause a paralysis of different structures on the other side of the
body. For example, a lesion that affected the right nucleus of the trigeminal cranial nerve and the right side
of the pyramidal tract would cause paralysis of the right side of the jaw and of part of the left side of the
body.
Both the corticospinal and corticobulbar tracts send some axons to the pontine nuclei in the pons as they
descend to synapse with lower motor neurons. These fibers that end in the pons form the corticopontine
tract. This pathway carries information about the type and strength of the motor impulses generated in the
cortex to the cerebellum. While the corticopontine fibers actually end in the pontine nuclei, second order
neurons carry their message to the cerebellum via the middle cerebellar peduncle. This tract may be
considered to be a part of the extrapyramidal system rather than a component of the pyramidal tract since
it does not synapse directly with lower motor neurons.
The Extrapyramidal Tract
This system is involved in automatic motor movements, and in gross rather than fine motor movement. It
works with the autonomic nervous system to help with posture and muscle tone and has more influence
over midline structures than over those in the periphery. Facial expression is one important communicative
behavior that is mediated by the extrapyramidal tract. In contrast to pyramidal tract, the extrapyramidal tract
is an indirect, multisynaptic tract.
Components of the extrapyramidal tract include the basal ganglia, the red nucleus, the substantia nigra,
the reticular formation and the cerebellum. All of these structures send information to the lower motor
neurons in an indirect fashion.
Some sources, including the text by Love and Webb, 1992, consider the basal ganglia to be the sole
constituent of the extrapyramidal system, saying that the other structures listed above synapse with the
extrapyramidal tract but are not part of it.
The basal ganglia acts to inhibit the release phenomenon, or the rapid firing of motor neurons. It is aided
in this function by the substantia nigra of the midbrain. The muscles most often affected by this inhibitory
functions are those controlling the head, the hands, and the fingers.
The neurotransmitters involved in the inhibitory function of the basal ganglia include dopamine, which is
produced by the substantia nigra, acetylcholine, and GABA (gamma amino butric acid), which is a
glutamate. Dopamine is an especially powerful inhibitor.
Extrapyramidal Projections to Lower Motor Neurons
The extrapyramidal tract has an important role in motor movement. it has projections that carry autonomic motor impulses
to voluntary muscles in the body, including the muscles for speech and swallowing. During speech, muscles are receiving
input from both the pyramidal and extrapyramidal systems. it is involved in gross motor movement rather than fine. It is
responsible for facial expression such as sadness, irony and happiness.
The rubrospinal tract passes through the red nucleus. The cerebellum sends messages to the spinal
nerves along this tract. Information flows from the superior cerebellar peduncle to the red nucleus and finally
to the spinal nerves. This information is very important for somatic motor, or skeletal muscle control and the
regulation of muscle tone for posture.
The reticulospinal tract runs from the reticular nuclei of the pons and medulla to the spinal nerves. It is
involved in somatic motor control like the rubrospinal tract and also plays an important role in the control of
autonomic functions.
The tectospinal tract has points of origin throughout the brain stem, but especially in the midbrain area,
and ends in the spinal nerves. It is involved in the control of neck muscles and also in visual and auditory
reflexes. So, when you jump after hearing a noise or duck when you see something coming toward you, this
tract helps to mediate these reactions.
The vestibulospinal tract runs from the vestibular nuclei located in the lower pons and medulla to the
spinal nerves. It is involved in balance.
(Note that all of these tracts receive input from the cerebellum.)
Extrapyramidal Diseases and Syndromes Affecting Communication/Swallowing
Lesions in the extrapyramidal tract cause various types of diskinesias or disorders of involuntary
movement.
The problems most commonly affecting the extrapyramidal tract include degenerative
diseases, encephalitis, and tumors.
Parkinson's Disease, which is a degenerative disease, is probably the most frequently occurring illness
that results from extrapyramidal tract lesions. It occurs when the dopaminergic neurons of the substantia
nigra are destroyed. Its symptoms include:
Tremor
Festinating movements, especially a festinating gait. (Festinating movements are
movements which become increasingly rapid and uncontrolled).
Hypokinetic dysarthria
Weak Voice
Mask-like facial expression
Diseases associated specifically with lesions of the basal ganglia include Huntington's Chorea and
Sydenham's Chorea. The term "chorea" comes from the Greek "khoros" which means dance. Both of
these diseases are associated with jerky, uncontrolled movements of the limbs. Sydenham's chorea was
probably the cause of the malady that was known as St. Vitus' Dance during the middle ages. Huntington's
Chorea is an inherited degenerative disease. Sydenham's tends to clear up spontaneously.
Essential Tremor Syndrome, which is associated with Spastic Dysphonia may also be the result of basal
ganglia lesions.
Lesions of the basal ganglia will also cause hyperkinetic dysarthria.
Note that not only is the definition of the extrapyramidal system controversial, but also many sources say that it is very
difficult to make functional distinctions between the extrapyramidal and pyramidal systems. When upper motor neuron
lesions occur, it is often difficult to determine which tract has been damaged.
CSU Chico | Glossary | References | Neuroscience on the Web | SPPA362 Home | Next
Other courses in the Neuroscience on the Web series:
SPPA 336, Neuropathologies of Language and Cognition | SPPA 342 (Neuropathologies of Swallowing and Speech)
Copyright, 1998/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.
The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 11. The Blood Supply
The Blood Supply
The Blood Supply
The
Blood Supply Medial View
The Blood Supply Lateral View
Blood transports oxygen and other nutrients necessary for the health of neurons, so a constant flow of
blood to the brain must be maintained.
According to Love and Webb,1992, the brain uses approximately twenty percent of the body's blood
and needs twenty-five percent of the body's oxygen supply to function optimally. Blood flow in a
healthy person is 54 milliliters per 1000 grams of brain weight per minute. There are 740 milliliters of
blood circulating in the brain every minute. 3.3 milliliters of oxygen are used per minute by every 1000
grams of brain tissue. This means that approximately 46 milliliters of oxygen are used by the entire
brain in one minute. During sleep, blood flow to the brain is increased, but the rate of oxygen
consumption remains the same.
Subclavian Artery
The main artery of the body is called the aorta. It supplies blood to all parts of the body with the
exception of the lungs. The aorta ascends from the heart and forms an arch, from which arise two
subclavian arteries. Each subclavian has two main branches, the common carotid and the
vertebral. Both of these carry blood to the brain.
Each common carotid divides into an external carotid artery, which supplies blood to the face and
an internal carotid artery which supplies the brain with blood.
The external carotid is a fairly straight artery, so it is not prone to blockages due to the build up of
cholesterol. Even if a blockage does occur, it would obviously not cause a stroke as this artery does
not carry blood to the brain.
The Internal Carotid
Each internal carotid artery ascends along one side of the neck. They pass behind the ear into the
temporal lobe and enter the subarachnoid space. Then, they run posteriorly to the medial end of the
fissure of Sylvius where they bifurcate into two main branches, the anterior cerebral artery and the
middle cerebral artery.
As the internal carotids have many twists and turns, there are many places where plaque can build
up, causing a blockage. Such blockages can be identified by sonogram (non-invasive), or by
angiograms (invasive). Also, a sound called a bruit can sometimes be heard via stethoscope when a
blockage exists.
The anterior cerebral artery goes above the optic chiasm to the medial surface of the
cerebral hemispheres. It arches around the genu (horn) of the corpus callosum
(FitzGerald, 1996). It supplies blood to the medial cortex, including the medial aspect of
the motor strip and the sensory strip. This means that damage to the anterior cerebral
artery can cause sensory and motor impairment in the lower body. For example, a
patient who has had a stroke affecting this artery may be incontinent or have unilateral
paralysis from the hips on down.
The anterior cerebral artery also delivers blood to some parts of the frontal lobe and
corpus striatum. So a blockage in this artery can affect cognition and cause motoric
problems due to damage to fibers in the internal capsule or to the basal ganglia.
The other main branch of the internal carotids is the middle cerebral artery. This large
artery has tree-like branches that bring blood to the entire lateral aspect of each
hemisphere. This means that this artery supplies blood to the cortical areas involved in
speech, swallowing and language, including the lateral motor strip, lateral sensory strip,
Broca's area, Wernicke's area, Heschl's gyrus, and the angular gyrus. In addition, it
provides most of the blood supply to the corpus striatum.
If a patient has a blockage in the middle cerebral artery, it is probable that s/he will have aphasia. S/
he will probably also have impaired cognition and corticohyposthesia, or numbness, on the opposite
side of the body. Problems with hearing and the sense of smell may also result from damage to this
artery because it supplies the lateral surface of the temporal lobe.
The central branches of the middle cerebral are the medial and lateral striata arteries. The striata
supply the basal ganglia, internal capsule, and thalamus (FitzGerald, 1996). Because they are the
main blood supply to the internal capsule, they are called by some the arteries of stroke. When
something happens to these arteries, the bottleneck of fibers within the internal capsule can be
damaged, causing many disabilities. The striata are very thin arteries and blood pressure within it high.
For this reason, they are cobsidered by many to be more vulnerable to hemorrhages than to
blockages, although FitzGerald says that occlusion of one of these areteries is the major cause of of
classical stroke where pyramidal tract damage results in contralateral hemiplegia.
Other arteries which arise from the internal carotid arteries include the anterior communicating
artery and the posterior communicating arteries.
The anterior communicating artery joins the anterior cerebral arteries of each
hemisphere together.
The posterior communicating arteries join the middle cerebral arteries to the
posterior cerebral arteries, which are part of the basilar artery system.
The Vertebral Artery
Both of the vertebral arteries ascend through the spinal column and enter the brain through the
magnum foramen. Once in the brain, they continue to ascend, traveling beside the brain stem. At the
lower border of the pons the two vertebral arteries join together to form the basilar artery or vertebrobasilar artery.
The vertebral arteries and the basilar are straight arteries and therefore not as subject to blockages
due to the build up of cholesterol as are the internal carotids.
The posterior inferior cerebellar not only supply the cerebellum but take blood to the lateral medulla.
Anterior and posterior spinal arteries the ventral and dorsal medulla, respectively (FitzGerald 1996).
The three arteries are branches of the vertebral.
The side of the pons and the cerebellum receive blood from the anterior inferior cerebellar artery and
the superior cerebellar artery. These arteries are branches of the basilar. The anterior inferior
cerebellar artery also has a branch, the labyrinthine artery, that supplies the inner ear. The basilar also
gives off about twelve pontine arteries that supply the medial pons (FitzGerald, 1996).
At the superior border of the pons, the basilar artery divides to form the two posterior cerebral
arteries.
Before the basilar artery divides, several other arteries arise from it. These include the anterior,
inferior, and posterior cerebellar arteries as well as pontine branches. So, the cerebellum and
pons are supplied by branches of the basilar.
The posterior cerebral arteries supply the part of the brain found in the posterior
fossa of the skull, including the medial area of the occipital lobes and the inferior
aspects of the temporal lobes. They also supply yhe midbrain and deliver blood to the
thalamus and some other subcortical structures. Blockages in this artery can affect the
sense of smell, and cause cranial nerve damage, as well as visual problems, including
visual agnosia, hemianopsia and alexia.
The choroidal arteries, which arise both from the divisions of the internal carotid arteries and from
the basilar system, supply blood to the choriod plexuses and also to the hippocampus. Blockages in
these arteries can affect the production of cerebrospinal fluid and can also cause memory problems.
The Circle of Willis
The Circle of Willis or the Circulus Arteriosus is the main arterial anastomatic trunk of the brain.
According to Bhatnagar and Andy, 1995, anastomosis occurs when blood vessels bring blood to one
spot from which it is then redistributed. The Circle of Willis is a point where the blood carried by the
two internal carotids and the basilar system comes together and then is redistributed by the anterior,
middle, and posterior cerebral arteries.
The anterior cerebral arteries of the two hemispheres are joined together by the anterior
communicating artery. The middle cerebral arteries are linked to the posterior cerebral arteries by the
posterior communicating arteries. This anastamosis or communication between arteries make
collateral circulation which Love and Webb, 1995, define as "the flow of blood through an alternate
route" (p. 40) possible. This is a safety mechanism, allowing brain areas to continue receiving
adequate blood supply even when there is a blockage somewhere in an arterial system. The blood
streams of the internal carotid system and the basilar system meet in the posterior communicating
arteries. If there are no problems in either system, the pressure of the streams will be equal and they
will not mix. However, if there is a blockage in one of them blood will flow from the intact artery to the
damaged one, preventing a cerebral vascular accident.
As long as the Circle of Willis can maintain blood pressure at fifty percent of normal, no infarction or
death of tissue will occur in an area where a blockage exists. If collateral circulation is good, no
permanent effects may result from a blockage.
Sometimes, an adjustment time is required before collateral circulation can reach a level that
supports normal functioning; the communicating arteries will enlarge as blood flow through them
increases. In such cases, a transient ischemic attack may occur, meaning that parts of the brain
are temporarily deprived of oxygen.
Some people lack one of the communicating arteries that form the Circle of Willis. In this case, if a
blockage develops, collateral circulation will be impeded and the collateral blood supply will be
compromised, causing brain damage to occur.
There are some watershed areas in the brain located at the ends of the vascular systems. Problems
with blood supply are particularly likely to occur here, especially in those who have hardening of the
arteries. Blockages in the water shed areas can cause transcortical aphasia.
Extraneural Factors Affecting the Blood Supply to the Brain
Low or High Blood Pressure
Abnormally low blood pressure can cause brain damage. This may occur as a result of
surgical shock which involves blood pressure as low as 70 milliliters per kilogram of
tissue.
Hypertension, or blood pressure that remains high regardless of activity level, can
cause arteries to narrow over time.
Cerebrovascular Resistance
Cerebrovascular resistance makes collateral circulation more difficult. It can be caused
by an arterial spasm (remember that arteries are lined with muscles). Another potential
cause of resistance is increased viscosity of the blood, which can result from leukemia
or from high levels of tri-glycerides in the blood, or other factors such as increased red
blood cells, often seen in chronic obstructive pulmonary disease (COPD). Increased
cerebrospinal fluid pressure can also lead to high levels of cerebrovascular resistance.
Atherosclerosis
This is hardening of the arteries which often occurs with old age but can also happen in
young people.
An occluded artery may cause a stroke due to one of the extraneural factors listed
above which can compromise the overall integrity of the cerebrovascular system. If an
individual does not have one of these problems and has a sufficient number of
communicating arteries, a blockage may not have a significant effect on blood supply
throughout the brain.
The Blood-Brain Barrier
Many substances present in the blood supply are unable to pass through the meninges into the cells
of the central nervous system. The blood brain barrier includes two components, the blood/
cerebrospinal fluid barrier and the arachnoid barrier layer.
Cerebrospinal fluid is a filtrate of blood by the choroid plexuses (capillary networks) of
the ventricles which are formed by fusion of the pia mater and the ependyma (ventricular
lining). In the course of this process, not all components of blood are allowed to enter
the brain. According to Webster, 1999, only clear plasma passes through, leaving blood
cells behind.
The arachnoid barrier layer is a part of the arachnoid meningeal layer. It is formed by
tight junctions between the endothelial cells of cerebral capillaries in the arachnoid
mater.
Glucose diffuses across the blood-brain barrier through a process that is
like selective osmosis.
CSU Chico | Glossary | References | Neuroscience on the Web | SPPA362 Home | Next
Other courses in the Neuroscience on the Web series:
SPPA 336, Neuropathologies of Language and Cognition | SPPA 342 (Neuropathologies of Swallowing and Speech)
Copyright, 1998/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.
The Neuroscience on the Web Series:
SPPA 362, Neuroanatomy of Speech, Swallowing and Language
CSU, Chico, Patrick McCaffrey, Ph.D
Unit 12. Neurochemistry
The Nerve Cell
Image of a Nerve Cell
The nerve cells of the central and peripheral nervous systems are called neurons. Most neurons have
three parts; an axon, a cell body or soma, and dendrites. All neurons have one soma and one axon,
but while some neurons have many dendrites, others have none.
Neurons vary in size; the smallest have a diameter of 5 microns while the largest are approximately 100
microns in width. (A micron is one one-thousandth of a millimeter).
The soma of a neuron contains the cell's nucleus and cytoplasm, a jelly-like substance that
surrounds the nucleus.
Chromosomes, which consist of molecules of DNA (deoxyribonucleic acid) are found in the nucleus
of the neuron. RNA (ribonucleic acid) molecules are also located within the nucleus.
Nissl Bodies or Nissl Substances, which also contain RNA, and Golgi Apparati are found in the
cytoplasm.
DNA forms the genetic code that determines the cell's function. As DNA cannot pass through the
nuclear membrane, its commands are carried to the cytoplasm by messenger RNA which can travel
outside of the nucleus. These RNA molecules link up with the Nissl substances, connecting to them
as a key would fit into a lock. This combination causes the cell to work, or use glucose. The waste
products generated by this process are removed from the cell by the Golgi apparati.
The axon allows the neuron to send messages to other nerve cells. Each neuron has only one axon,
but this may have numerous branches which connect the cell to many other.
Axons vary in length; the axons of some pyramidal cells in the precentral gyrus are long enough to
travel all the way down to the end of the spinal cord but other axons are very short.
Cells can be classified based on the length of their axons. Golgi Type I neurons have long axons
while those of Golgi Type II cells are short.
Axons arise from an area on the cell body of the neuron called the axon hillock. Most axons form many
branches as they extend away from the soma. At its end, each axonal branch divides into a number of
telodendria. The boutones terminaux or boutones de passage, which contain neurotransmitters, are
located on the telodendria.
Myelin
Many of the axons in the central and peripheral nervous systems are
covered at regular intervals with a fatty insulating substance called myelin.
The segments of the axon which lie between areas of myelin and are
therefore in direct contact with extracellular fluid are called the Nodes of
Ranvier.
Myelin coating increases the speed with which an axon can transmit
messages. The neural impulse travels by a process known as saltatory
conduction, jumping from one unmyelinated segment to the next. This
means that the impulse does not have to be propagated through the entire
area of the axon.
Dendrites
The dendrites of a neuron receive messages from the axons of other nerve
cells. There are two types of dendrites, apical dendrites and basilar
dendrites.
Apical dendrites have stalks filled with cytoplasm that appear to be part of
the soma of the neuron to which they are attached. Most of these
dendrites are found in the cerebral cortex.
Basilar dendrites do not have a stalk. They are more numerous than
apical dendrites.
Links with relevant information about nerve cells:
Image of a Nerve Cell
For more information on Nerves and nerve cells visit the following web sites
Neural Definitions: http://psych.hanover.edu/Krantz/neural/neurldef.html
A Self-Quiz: http://psych.hanover.edu/Krantz/neural/struct3.html
The Transmission of Neural Messages
The Action Potential
The messages conducted along axons are electrochemical in nature.
Four different types of ions, or electrically charged atoms, are involved in the
transmission of neural impulses; chloride ions (Cl -), sodium ions (Na+), potassium
ions (K+), and organic anions (A-).
When a neuron is at rest, there are high concentrations of A- and K+ within the cell, while
most Na+ and Cl- ions are located outside its membrane. The resting potential of the
neuron is -70 millivolts, meaning that the electrical charge of the cell is slightly negative in
comparison to that of the extracellular fluid surrounding it.
This arrangement is due to the selective permeability of the neural membrane. Cl- and K+
can pass through the membrane, but Cl- does not enter the cell in great quantities
because both it and the interior of the neuron are negatively charged. Organic anions
cannot move through the membrane due to their large size. As Na+ is positively charged
and the interior of the nerve cell has a negative electrical potential, these ions should
pass into the cell. However, sodium cannot readily pass through the membrane and most
of the Na+ ions that do enter the cell are extruded by the sodium-potassium pump.
This is the name given to a group of molecules located in the cellular membrane which
push Na+ out of the cell and draw K+ ions inside.
The resting potential of a neuron changes when messages are received from other nerve
cells. Inhibitory impulses cause the electrical charge of the neuron to become even more
negative, decreasing its ability to fire, or send messages to other nerve cells. When
excitatory messages are received, however, the permeability of the cell membrane
changes, allowing Na+ ions to enter the neuron. This influx of positively charged ions
causes the cell's electrical potential to temporarily become positive, peaking at +40
millivolts. This change of the electrical potential of the cell from negative to positive is the
action potential, which ultimately causes the cell to fire.
When the charge of the cell reaches its positive peak, K+ ions are forced out of the neuron because
they are positively charged. The exit of these ions causes the potential of the cell to become negative
again, temporarily dipping below -70 millivolts before returning to resting potential.
After the cell fires, it goes through a refractory period during which it will not fire again. The refractory
period may be divided into two phases, the absolute refractory period and the relative refractory
period.
During the absolute refractory period, the neuron will not fire again, no matter how strong the excitatory
messages that it receives.
The cell will fire during the relative refractory period, but only if it receives a very strong stimulus.
Neurotransmitters
When a neuron fires, it communicates with other nerve cells through the release of chemicals called
neurotransmitters.
Neurotransmitters are found in the boutons terminaux located on the teledendria of the axons. They are
stored in circular or oval-shaped capsules called synaptic vesicles.
When an excitatory impulse of sufficient strength reaches the teledendria, the synaptic vesicles fuse
with the axonal membrane and open, spilling the chemicals they contain into the extracellular fluid. The
neurotransmitters then travel across a small space called the synaptic cleft to attach to the neuron
that will receive their message.
After a message has been sent, excess quantities of the neurotransmitter that remain in the synaptic
cleft must be cleared away in order to allow further communication between the cells. In some cases,
the left-over chemicals are recycled; they are picked up and repackaged in new synaptic vesicles to be
used in future transmissions. Other types of neurotransmitters are destroyed by enzymes when they
remain in the synaptic cleft.
Acetylcholine (ACh) is one neurotransmitter that has been well-studied. It is the major
neurotransmitter of the peripheral nervous system and is also present in the central nervous system. It
carries messages controlling voluntary muscle movement, as the nerve fibers located in muscles and in
the spinal and cranial nerves are acetylcholinergic. After messages have been transmitted, ACh is
broken down in the synaptic cleft by an enzyme called acetylcholinesterase.
An insufficient supply of acetylcholine, whether due to excess quantities of acetylcholinesterase or
resulting from inadequate synthesis of the chemical, causes Myasthenia Gravis. In this disorder, the
strength of neural impulses is attenuated, causes the voluntary movement of muscles, including those
involved in articulation, voicing and respiration, to be weakened.
Myasthenia Gravis is distinguished from other disorders like ALS by the administration of a derivative of
curare. This drug will temporarily improve the strength of someone suffering from Myasthenia Gravis,
but will have no effect in cases of ALS.
Myasthenia Gravis should not be confused with Myasthenia Laryngis, which is a localized weakness
in the larynx and is not the result of a neurological condition.
Other known neurotransmitters include two groups of chemicals called the monoamines and the
peptides. The monoamines, which include dopamine, norepinephrin, and serotonin, are all
synthesized from proteins called amino acids. The peptides, including enkephalin, endorphins, and
substance P, are large molecules which may be involved in blocking sensations of pain.
Consequences of Neuronal Damage
When cells in the rest of the body are injured, they can regenerate and repair themselves. Neurons, on
the other hand, do not have this capability. If an axon is damaged, the soma of the cell may also
degenerate. Also, when a cell body is injured or an axon is severed from the soma, Wallerian
Degeneration, or death of the axon occurs.
When a neuron dies, it is ingested by one of the support cells in the nervous system. This process is
called phagocytosis.
CSU Chico | Glossary | References | Neuroscience on the Web | SPPA362 Home | Next
Other courses in the Neuroscience on the Web series:
SPPA 336, Neuropathologies of Language and Cognition | SPPA 342 (Neuropathologies of Swallowing and Speech)
Copyright, 1998/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.