Download the brain and cranial nerves

THE BRAIN AND CRANIAL NERVES
A.
BRAIN
Information processing takes place at several levels within the central nervous
system. Describe where and what types of processing occur at the lowest,
intermediate, and highest levels.
At the lowest level (spinal cord) are reflex responses to sensory input.
There is not consciousness involved.
Lower regions of the brain, such as the brainstem, basal ganglia, and
cerebellum, control most unconscious activities, such as blood pressure,
respiration, muscle tone, coordination, and posture.
At the highest level, the cerebrum integrates conscious activities,
processes and stores information, and provides the neuronal circuitry for
abstract thought processes.
1.
PRINCIPAL PARTS
The adult brain consists of 4 main parts. Briefly describe the location of
each of the following:
Brainstem -- The brain stem consists of the medulla oblongata,
pons, and midbrain. Its lower end is a continuation of the
spinal cord.
Diencephalon -- The diencephalon, sitting atop the midbrain of the
brainstem, consists primarily of the hypothalamus and the
thalamus.
Cerebrum -- The cerebrum spreads over the diencephalon like the
cap of a mushroom and occupies most of the space of the
cranial vault.
Cerebellum -- The cerebellum is inferior to the cerebrum and
posterior to the brainstem.
2.
PROTECTION AND COVERINGS
Give a brief description of the cranial vault and the meninges.
The brain is protected by the cranial bones that form the cranial
vault, and the cranial meninges. The cranial meninges are
continuous with the spinal meninges and surround the brain much
as they do the spinal cord. There are, however, two major
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differences: separations of the dura to form the dural (venous)
sinuses) and folds of the dura to separate the cerebral and
cerebellar hemispheres.
Describe each of the following:
Dural sinus -- The dura mater consists of an inner and an outer
portion. In certain places these layers are separated to form
large venous structures called the dural sinuses (superior
sagittal, inferior sagittal, straight, and transverse (lateral)
sinuses). are the routes of venous blood flow from the
brain.
Falx cerebri -- A second difference is the formation of three large
sheet-like extensions of the dura that separate the larger
parts of the brain. The falx cerebri is a large extension of the
dura that extends between the two cerebral hemispheres.
Falx cerebelli -- The falx cerebelli is a large extension of the dura
mater that extends between the two cerebellar hemispheres.
Tentorium cerebelli -- The tentorium cerebelli separates the
cerebral hemispheres from the cerebellar hemispheres.
3.
CEREBROSPINAL FLUID (CSF)
Describe the flow of cerebrospinal fluid (CSF).
The brain and spinal cord are nourished and protected against
injury by cerebrospinal fluid (CSF).
CSF continuously circulates through the subarachnoid space
around the brain and through the cavities within the brain called the
ventricles and the central canal of the spinal cord.
There are four CSF-filled cavities within the brain called the
ventricles.
Ventricles 1 and 2 are the two lateral ventricles (right and left), one
located within each hemisphere of the cerebrum.
Each lateral ventricle empties via an interventricular foramen (of
Monro) into the third ventricle, a vertical slit located in the midline
between the right and left halves of the thalamus.
The third ventricle drains via the cerebral aqueduct (of Sylvius) into
the fourth ventricle, located between the brain stem and the
cerebellum.
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From the fourth ventricle CSF moves through the median aperture
to enter the central canal of the spinal cord, and through the two
lateral apertures to enter the subarachnoid space.
Cerebrospinal fluid contributes to homeostasis in three ways. Name and
describe them.
Mechanical protection -- The fluid serves as a shock-absorbing
medium to prevent the brain and spinal cord from crashing
against their bony surroundings. In essence, CSF “floats”
the CNS within the bony chambers of the cranial vault and
the vertebral canal.
Chemical protection -- CSF provides an optimal chemical environment for accurate neuronal activity; slight changes in ionic
composition seriously disrupt neuronal homeostasis.
Circulation -- CSF is a medium for exchange of nutrients and
wastes between the blood and the nervous tissue.
Describe the functions of the choroid plexuses, ependymal cells, and the
arachnoid villi (granulations) as concerns cerebrospinal fluid.
CSF is produced by the choroid plexuses, networks of blood
capillaries covered by ependymal cells in the walls of the ventricles
that bulge into the ventricular cavity.
The capillaries become covered by the ependymal cells lining the
ventricles, so that the CSF formed is an ultrafiltrate of the blood.
This relationship of the blood capillaries with ependyma creates the
blood-cerebrospinal fluid barrier, permitting certain substances to
pass into the CSF, but excluding others that may be harmful to the
CNS.
From the subarachnoid space around the brain, the CSF is
reabsorbed into the blood by finger-like projections of the arachnoid
membrane called arachnoid villi or granulations that project into the
superior sagittal sinus.
Normally, CSF is reabsorbed into the blood at the same rate it is
made by the choroid plexuses (about 20 mL/hour). What would
occur if production exceeded reabsorption?
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4.
BRAIN STEM
a.
MEDULLA
Describe the medulla oblongata:
Location -- The medulla oblongata is continuous with the
inferior spinal cord and the superior pons. It lies just
inside the cranial vault, superior to the foramen
magnum.
Gray matter -- The gray matter of the medulla consists of
nuclei that function in the regulation of processes
necessary for immediate survival: blood pressure,
heart rate, respiration, swallowing, coughing,
vomiting, etc.
White matter -- The medulla contains all ascending and
descending tracts that communicate between the
brain and spinal cord. These tracts form the white
matter of the medulla.
Cranial nerves -- Also found in the medulla gray matter are
the motor nuclei for cranial nerves XII, XI, X, IX, and
part of VIII. The sensory neuron cell bodies for these
cranial nerves are located outside the medulla in
various ganglia.
b.
PONS
Describe the pons:
Location -- The pons lies directly superior to the medulla,
anterior to the cerebellum, and inferior to the
midbrain.
Gray matter -- The gray matter of the pons consists of two
nuclei that are involved in the alteration of basic
respiratory pattern.
White matter -- The pons consists mostly of white matter,
tracts passing between the lower and higher CNS
structures, as well as communicating the cerebellum
with the rest of the CNS.
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Cranial nerves -- In addition to the centers, the motor nuclei
of cranial nerves part of VIII, VII, VI, and V are also
located in the pontine gray matter. The sensory
neuronal cell bodies are located in ganglia outside the
pons.
c.
MIDBRAIN
Describe the midbrain:
Location -- The midbrain extends from the superior portion of
the pons to the inferior border of the diencephalon.
Dorsal portion -- The dorsal portion of the midbrain, called
the tectum (roof), is divided into four rounded
eminences (a natural elevation) of gray matter called
the corpus quadrigemina. There are two superior
colliculi and two inferior colliculi
The two upper elevations are the superior colliculi,
gray matter nuclei associated with reflex movements
of the eyes, head, and neck in response to visual and
other stimuli.
The two lower elevations, the inferior colliculi, consists
of gray matter nuclei involved in reflex movements of
the head and trunk in response to auditory stimuli).
Ventral portion -- The ventral portion of the midbrain is all
white matter, consisting of two groups of fiber tracts:
the cerebral peduncles and the medial lemniscus.
The cerebral peduncles pass motor fibers from the
cerebrum through the brain stem. They also carry
some sensory information passing to the cerebrum
from the spinal cord.
The medial lemniscus is a band of sensory fiber tracts
passing from the lower CNS to the thalamus. In
particular, the medial lemniscus carries impulses for
discriminative touch, proprioception, pressure, and
vibrations.
Cranial nerves -- Also located in the tectum are the motor
neurons for cranial nerves IV and III; the sensory
neuron cell bodies associated with these cranial
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nerves are located in the ganglia outside the
midbrain.
5.
DIENCEPHALON
a.
THALAMUS
Describe the thalamus of the diencephalon.
The thalamus (“inner chamber”) is an oval structure above
the midbrain, forming 4/5 of the diencephalon.
The thalamus consists of paired masses of gray matter
organized into nuclei that form the lateral walls of the third
ventricle.
The two halves of the thalamus, right and left, are joined by
the massa intermedia, a bridge of gray matter that crosses
the midline through the third ventricle.
The thalamus is the principal relay station for sensory
impulses, except smell, that reach the cerebral cortex from
the spinal cord, brain stem, and other parts of the cerebrum.
The neurons of the thalamus provide some crude
appreciation of some sensations (pain, temperature,
pressure) and have some role in emotions and memory.
Each pair of nuclei within the thalamus has a specific role in
this relay function in that each is specific for a specific
modality (sensation).
b.
HYPOTHALAMUS
Describe the hypothalamus of the diencephalon.
The hypothalamus is small portion of the diencephalon
(about 1/5) lying below the thalamus.
Like the thalamus, the hypothalamus is also primarily gray
matter dividing into pairs of nuclei.
Each nucleus pair is responsible for the control of a
particular homeostatic function. The hypothalamus is,
therefore, one of the major controllers of homeostasis.
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Functions:
1) integration between the nervous and endocrine
systems
2) control of the autonomic nervous system
3) regulation of rage and aggression
4) regulation of body temperature
5) regulation of food intake
6) regulation of thirst
7) regulation of sleep-wake patterns
8) regulation of sex drive.
6.
CEREBRUM
Describe the cerebrum:
Location -- Supported on the diencephalon and brain stem and
forming the bulk of the brain, thus filling most of the cranial
vault, is the cerebrum.
Cortex -- The surface of the cerebrum is composed of gray matter
2-4 mm thick called the cerebral cortex. It consists of
approximately 6 billion neuronal cell bodies and their
associated dendritic zones.
White matter -- Beneath the cerebral cortex is the cerebral white
matter consisting of axons traveling to and from the cortex.
It is often referred to as the internal capsule.
Function -- The neurons of the cerebral cortex are the seat of
intelligence, consciousness, and where we “see,” “hear,”
“smell,” “taste,” and “feel.”
Gyrus/sulcus -- During development, as the gray matter outgrows
the white matter, the gray matter is thrown into rolls and it
folds upon itself. Each shallow fold is a gyrus and each
groove between the gyri is a sulcus.
Fissures -- The deep grooves of the cerebrum are called fissures.
The three most prominent fissures are the longitudinal
fissure, separating the two cerebral hemispheres, the lateral
fissure, lying between the temporal and parietal lobes, and
the transverse fissure, between the cerebrum and the
cerebellum.
Corpus callosum -- The two cerebral hemispheres are connected
internally by a large bundle of whiter matter called the corpus
callosum.
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a.
LOBES
Name the lobes of the cerebrum named for the bones which cover
them.
Each cerebral hemisphere is further subdivided into lobes by
sulci or fissures. The lobes are named for the cranial bones
that cover them: frontal, parietal, temporal, and occipital.
What is the insula?
A fifth portion of the cerebrum, the insula, cannot be seen
from the exterior of the cerebrum. It lies deep within the
lateral fissure, under the parietal, temporal, and frontal lobes.
b.
WHITE MATTER
Describe the following types of fibers found in the cerebral white
matter:
Association -- Association fibers transmit impulses between
the gyri of one hemisphere, associating information
between different inputs and outputs.
Commissural -- Commissural fibers transmit impulses from
the gyri one of the hemispheres to the gyri of the
other hemisphere, passing through either the corpus
callosum, the anterior commissure, or the posterior
commissure.
Projection -- Projection fibers form descending or ascending
tracts that communicate the cerebral cortex with the
spinal cord and other lower brain structures.
c.
GRAY MATTER
What are the basal ganglia?
The basal ganglia (cerebral nuclei) consist of several groups
of paired nuclei found within the white matter of the
cerebrum. The nuclei are interconnected by many nerve
fibers with each other, and receive numerous inputs from the
cerebral cortex, thalamus, and hypothalamus.
What are their functions?
The basal ganglia function in the control of large, learned,
semi-voluntary skeletal muscle movements, such as
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swinging the arms when walking, laughing, etc., and in the
regulation of muscle tone.
d.
LIMBIC SYSTEM
What is the limbic system?
The limbic system is a ring of structures surrounding the
inner border of the cerebrum and the floor of the
diencephalon, encircling the midbrain. It is formed by gyri of
the insular lobe, nuclei associated with the basal ganglia,
hypothalamus, and the thalamus, the olfactory bulbs, and
bundles of interconnecting white matter.
What is its function?
The limbic system, often called the emotional brain,
functions in emotional aspects of behavior related to
survival, memory, smell, pleasure and pain, rage and
aggression, docility and tameness, affections, sexual desire,
etc.
e.
FUNCTIONAL AREAS OF THE CEREBRAL CORTEX
What are the functional areas of the cerebral cortex?
Specific types of sensory, motor, and integrative signals are
processed in specific regions of the cerebral cortex called
functional areas. In general, the areas are divided into three
types: primary sensory, primary motor, and association.
Describe the following:
Sensory areas -- Sensory areas receive sensory input, and
integrate and interpret the sensations. There are at
least 20 different specific areas for each of the
individual modalities.
Motor areas -- Motor areas contain the neuronal cell bodies
of origin for practically all motor activities. They are
divided into at least 5 different areas associated with
muscular activity.
Association areas -- Association areas make up several
main areas that serve to connect the sensory and
motor areas. They provide integrative and
interpretive functions, such as memory, emotion,
relationships between sensations, etc.
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7.
CEREBELLUM
Describe the cerebellum.
The cerebellum is located in the inferior and posterior aspect of the
cranial cavity, inferior to the occipital lobes of the cerebrum and
posterior to the brain stem.
The cerebellum is separated from the cerebrum by a fold of dura
known as the tentorium cerebelli.
The cerebellum is designed like the cerebrum, with 2 cerebellar
hemispheres divided into 3 lobes each, a cerebellar cortex, an inner
white matter, and several cerebellar nuclei.
The white matter of the cerebellum serves to connect the cerebellar
cortex and the cerebellar nuclei with each other and with all of the
other pertinent CNS structures.
The cerebellar gray matter functions to compare intended bodily
movement with what is actually happening, to help in regulating
smooth, sequential skeletal muscle contractions, and to control
muscle tone, balance, and posture.
B.
CRANIAL NERVES
Describe the cranial nerves. How many are there? From where do they
originate? How do they exit the skull?
There are 12 pairs of cranial nerves, 10 of which originate in the
brainstem. All pass through specific foramina of the skull to leave the
cranial cavity.
Where are the cell bodies of origin of the motor and sensory components of the
cranial nerves?
Motor neuron cells bodies are in the gray matter of the brainstem. The
sensory neuron cell bodies are all found in ganglia located outside the
brain.
How are the cranial nerves designated?
Roman numerals indicate the order in which they arise from the brain,
anterior to posterior. The names indicate their distribution or function.
Name the cranial nerves, describe there origin from the brainstem, give their
sensory function(s) (if applicable), and the motor function(s) (if applicable).
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I
Olfactory
origins -- arises from the olfactory epithelium within the nasal
cavity
sensory functions -- sense of smell
motor function - none
II
Optic
origins -- arises from retinal epithelium of the eye
sensory functions -- sense of sight
motor functions -- none
III
Oculomotor
origins -- arises from the midbrain
sensory functions -- proprioceptive to muscles served
motor functions -- levator palpebrae superioris, superior
rectus, medial rectus, inferior rectus, and inferior
oblique muscles of eyeball; parasympathetic to ciliary
muscle of eye for focusing lens and to sphincter and
dilator pupillae muscle of iris for regulating pupillary
diameter
IV
Trochlear
origins -- arises from the midbrain
sensory functions -- proprioceptive to muscle served
motor functions -- superior oblique muscle of eyeball
V
Trigeminal
origins -- arises from the midbrain
sensory functions -- touch, pain, and temperature from skin
of front half of the head, eyeball, nasal cavity, oral
cavity (not taste), part of pharyngeal mucosa
motor functions -- muscles of mastication (chewing)
(masseter, temporalis, lateral and medial pterygoids)
VI
Abducens
origins -- arises from the pons
sensory functions -- proprioception to muscle served
motor functions -- lateral rectus muscle of eyeball
VII
Facial
origins -- arises from the pons
sensory functions -- anterior 2/3 of tongue for taste,
proprioceptive for muscles of facial expression
motor functions -- muscles of facial expression, lacrimal and
salivary glands secretion.
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VIII
Auditory (vestibulocochlear)
origins -- arises from the cochlear and vestibular portions of
the inner ear
sensory functions -- senses of equilibrium and hearing
motor functions -- none
IX
Glossopharyngeal
origins -- arises from the medulla
sensory functions -- posterior 1/3 of tongue for taste, carotid
sinus and body related to blood pressure control,
proprioceptive for muscles of swallowing
motor function -- muscles of swallowing and salivary glands
X
Vagus
origins -- arises from the medulla
sensory functions -- aortic sinus and body related to blood
pressure control, mucosa of respiratory and digestive
systems
motor functions -- muscles of the bronchial tree, digestive
tract, pancreas, liver, and glands of the GI system
XI
Accessory
origins -- arises from the medulla
sensory functions-- proprioceptive for the muscles served
motor functions -- muscles of the pharynx for swallowing,
larynx, soft palate, sternocleidomastoid, and
trapezius.
XII
Hypoglossal
origins -- arises from the medulla
sensory functions -- proprioceptive for the muscles served
motor functions -- intrinsic and extrinsic muscles of the
tongue
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