Download 14: The Brain and Cranial Nerves

14: The Brain and Cranial Nerves
I. An Introduction to the Organization of the Brain, p. 452
Objectives
1. Name the major regions of the brain and describe their functions
2. Name the three primary brain vesicles and indicate which adult structures each
gives rise to.
3. Name the ventricles of the brain and describe their locations and the connections
between them.
•
The human brain ranges in size from 750 cc to 2100 cc and contains 98% of the
body’s neural tissue. The average brain weighs about 1.4 kg (3 lb.).
A Preview of Major Regions and Landmarks, p. 452
Figure 14-1
•
The largest part of the brain is the cerebrum, which controls the higher mental
functions such as thought, memory and conscious movement. The cerebrum is
divided into left and right cerebral hemispheres, and covered by a surface layer of
gray matter or neural cortex (cerebral cortex). The surface is folded to increase
surface area, forming elevated ridges (gyri), shallow depressions, (sulci), and deep
grooves (fissures).
•
The second largest part of the brain is the cerebellum, which coordinates
repetitive body movements. The cerebellum also has 2 hemispheres and is
covered in cerebellar cortex.
•
Underneath the cerebrum and cerebellum is the diencephalon, which links the
cerebrum with the brain stem. The diencephalon is divided into the left thalamus
and right thalamus, which relay and process sensory information, and the
hypothalamus, which is involved in hormone production, emotion and autonomic
function.
•
The hypothalamus is connected to the pituitary gland (a major endocrine gland)
via a small stalk called the infundibulum. The hypothalamus and pituitary gland
are the interface between the nervous system and the endocrine system.
•
The brain stem, which processes information between the spinal cord and the
cerebrum or cerebellum, includes the mesencephalon, the pons, and the medulla
oblongata:
- The mesencephalon (midbrain) processes sight and sound (and their
associated reflexes) and maintains consciousness.
- The pons connects the cerebellum to the brain stem, and is involved in
somatic and visceral motor control.
-
The medulla oblongata connects the brain to the spinal cord. In addition to
relaying information, the medulla oblongata regulates autonomic functions
such as heart rate, blood pressure and digestion.
Embryology of the Brain, p. 452
Table 14-1
• The organization of brain structures is determined by their embryological
development.
•
The origin of the brain is the neural tube, which enlarges into 3 areas called
primary brain vesicles (the prosencephalon, mesencephalon and
rhombencephalon).
•
The prosencephalon and rhombencephalon subdivide to form 5 secondary brain
vesicles. The telencephalon becomes the cerebrum. The metencephalon forms the
cerebellum and pons. The myelencephalon becomes the medulla oblongata.
Ventricles of the Brain, p. 453
Figure 14-2
• The neural tube encloses a fluid-filled cavity called the neurocoel. During
development, the neurocoel expands to form chambers called ventricles, which
are lined with cells of the ependyma.
•
The cerebral hemispheres each contain a large lateral ventricle, separated from
each other by a thin medial partition called the septum pellucidum. The ventricle
of the diencephalon is called the third ventricle. The lateral ventricles
communicate with the third ventricle via the interventricular foramen (foramen of
Monro).
•
The mesencephalon contains a narrow canal called the mesencephalic aqueduct
(cerebral aqueduct), which connects the third ventricle with the fourth ventricle.
The fourth ventricle extends into the medulla oblongata and becomes continuous
with the central canal of the spinal cord.
Key
•
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•
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The brain is a large, delicate mass of neural tissue containing internal
passageways and chambers filled with cerebrospinal fluid.
Each of the five major regions of the brain has specific functions.
As you ascend from the medulla oblongata (which connects to the spinal cord) to
the cerebrum, those functions become more complex and variable.
Conscious thought and intelligence are provided by the neural cortex of the
cerebral hemispheres.
II. Protection and Support of the Brain, p. 455
Objectives
1. Explain how the brain is protected and supported.
2. Discuss the formation, circulation and functions of the cerebrospinal fluid.
•
The tissues of the brain are supported and protected by:
1. the bones of the cranium
2. the cranial meninges
3. cerebrospinal fluid
•
The brain is biochemically isolated from general circulation by the blood-brain
barrier.
The Cranial Meninges, p. 455
Figure 14-3
• The cranial meninges are made up of 3 layers (dura mater, arachnoid mater and
pia mater) continuous with the spinal meninges. The distinctive characteristics of
the cranial meninges are:
1. The cranial dura mater has an inner fibrous layer (meningeal layer) and
outer fibrous layer (endosteal layer). The endosteal layer is fused to the
periosteum. Venous sinuses between the 2 layers receive blood from veins
of the brain and deliver it to the jugular veins of the neck.
2. The cranial arachnoid mater covers the brain and is in contact with the
inner epithelial layer of the dura mater.
3. The pia mater is attached to the brain surface by astrocytes. The
subarachnoid space is between the arachnoid mater and the pia mater.
Dural Folds
• The inner layer of the dura mater form dural folds that extend into the cranial
cavity to stabilize and support the brain. The dural folds contain collecting veins
called dural sinuses.
Figure 14-3b
• The 3 largest dural folds are the falx cerebri, the tentorium cerebelli, and the falx
cerebelli.
1. The falx cerebri projects between the cerebral hemispheres. It contains the
superior sagittal sinus and the inferior sagittal sinus.
2. The tentorium cerebelli separates the cerebellum and cerebrum, and
contains the transverse sinus.
3. The falx cerebelli divides the cerebellar hemispheres below the tentorium
cerebelli.
The Protective Function of the Cranial Meninges
• The cranial meninges and CSF cushion and protect the brain from cranial trauma
that results from contact with the bones of the cranium.
Cerebrospinal Fluid, p. 456
• Cerebrospinal fluid (CSF) surrounds all exposed surfaces of the CNS and
interchanges with the interstitial fluid of the brain. The major functions of CSF
are:
1. Cushioning delicate neural structures.
2. Supporting the brain.
3. Transporting nutrients, chemical messengers, and waste products.
Figure 14-4
The Formation of CSF
• The choroid plexus is a combination of specialized ependymal cells and
capillaries that produce cerebrospinal fluid. The ependymal cells secrete CSF into
the ventricles, remove waste products from the CSF, and adjust the composition
of CSF over time.
Circulation of CSF
• The choroid plexus produces about 500 ml of CSF a day, replacing the entire
volume of CSF about every 8 hours.
• CSF circulates from the choroid plexus thorough the ventricles to the central canal
of the spinal cord.
• CSF enters the subarachnoid space through 2 lateral apertures and 1 median
aperture to circulate around the brain, spinal cord and cauda equina.
• Extensions of the subarachnoid space (arachnoid villi) extend through the dura
mater to the superior sagittal sinus.
• Large clusters of villi form arachnoid granulations which absorb CSF into the
venous circulation.
The Blood Supply to the Brain, p. 458
•
The brain has a continuous, high demand for nutrients and oxygen which must be
supplied by blood circulation. Blood is supplied to the brain by the internal
carotid arteries and vertebral arteries, and drained from the dural sinuses by the
internal jugular veins.
•
Disorders that interfere with normal blood circulation to the brain are called
cerebrovascular diseases. A stroke or cerebrovascular accident (CVA) occurs
when the blood supply to a portion of the brain is shut off, and neurons die.
The Blood-Brain Barrier
• Neural tissue in the CNS is isolated from general circulation by the blood-brain
barrier (BBB) formed by an extensive network of tight junctions between
endothelial cells lining the capillaries of the CNS.
•
Only lipid-soluble compounds (e.g. O2, CO2, steroids, and prostaglandins) can
diffuse into the interstitial fluid of the brain and spinal cord. Astrocytes release
chemicals that control the permeability of the endothelium to other substances,
effectively controlling the blood-brain barrier.
•
A blood-CSF barrier is formed by specialized ependymal cells surrounding the
capillaries of the choroid plexus, which is not a part of the neural tissue of the
brain.
•
Transport across the blood-brain and blood-CSF barriers is selective and
directional, limiting the movement of many compounds. As a result, the chemical
composition, pH and concentrations of major ions in blood and CSF are different.
•
The blood-brain barrier is continuous except in 4 specific cases:
1. In portions of the hypothalamus, where hypothalamic hormones enter the
systemic circulation.
2. In the posterior lobe of the pituitary gland, where the hormones ADH and
oxytocin are released into the circulation.
3. In the pineal glands, where pineal secretions enter the circulation.
4. At the choroid plexus, where specialized ependymal cells maintain the
blood-CSF barrier.
Key
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The meninges stabilize the position of the brain within the cranial cavity.
Cerebrospinal fluid provides protection against sudden jolts and shocks.
CSF also provides nutrients and removes wastes generated by active neural
tissues.
The blood-brain barrier and the blood-CSF barrier selectively isolate the brain
from chemicals in blood that might disrupt neural function.
III. The Medulla Oblongata, p. 459
Objectives
1. Describe the anatomical differences between the medulla oblongata and the spinal
cord.
2. List the major components of the medulla oblongata and specify their functions.
Figure 14-5
• The medulla oblongata is continuous with the spinal cord. It is here that the
central canal opens into the fourth ventricle.
Figure 14-6
• The medulla oblongata contains all of the ascending and descending tracts that
allow the brain and spinal cord to communicate, coordinates complex autonomic
reflexes, and controls visceral functions.
•
The medulla oblongata includes 3 groups of nuclei:
1. Autonomic nuclei controlling visceral activities:
- The reticular formation (a mass of gray matter with embedded
nuclei that extends from the medulla oblongata to the
mesencephalon) within the medulla oblongata regulates autonomic
functions.
- There are 2 major groups of reflex centers that control peripheral
systems:
(1) The cardiovascular centers (subdivided into the cardiac
center and the vasomotor center), and
(2) The respiratory rhythmicity centers
2. Sensory and motor nuclei of cranial nerves:
- associated with 5 of the 12 cranial nerves (designated by Roman
numerals VIII, IX, X, XI, and XII)
3. Relay stations along sensory and motor pathways:
- The nucleus gracilis and the nucleus cuneatus pass somatic sensory
information to the thalamus.
- The solitary nucleus receives visceral sensory information.
- The olivary nuclei (olives) relay information about somatic motor
commands.
Table 14-2 summarizes the major components of the medulla oblongata.
IV. The Pons, p. 462
Objective
1. List the main components of the pons and specify their functions.
Figure 14-6c
• The pons links the cerebellum with the mesencephalon, diencephalon, cerebrum
and spinal cord.
•
The pons contains 4 groups:
1. Sensory and motor nuclei of cranial nerves V, VI, VII and VIII.
2. Nuclei involved with the control of respiration:
- The apneustic center and the pneumotaxic center modify the
activity of the respiratory rhythmicity center.
3. Nuclei and tracts that process and relay information heading to or from the
cerebellum.
4. Ascending, descending and transverse tracts
- Transverse fibers (axons) link nuclei of the pons with the
cerebellar hemisphere of the opposite side.
V. The Cerebellum, p. 462
Objective
1. List the main components of the cerebellum and specify their functions.
•
The cerebellum is an autonomic processing center. It has 2 primary functions:
1. Adjusting the postural muscles of the body.
2. Programming and fine tuning movements controlled at the conscious and
subconscious levels.
Table 14-3 shows the components of the cerebellum and their functions.
Figure 14-7
• The surface of the cerebellum is composed of highly folded neural cortex (the
folia).
• The anterior and posterior lobes are separated by the primary fissure.
• At the midline, a narrow band of cortex (the vermis) separates the cerebellar
hemispheres.
• The flocculonodular lobe lies below the fourth ventricle.
• The cerebellar cortex contains large, branched Purkinje cells that receive input
from up to 200,000 synapses.
• The internal white matter of the cerebellum is highly branched, in a formation
called the arbor vitae.
• Cerebellar nuclei embedded in the arbor vitae relay information to the Purkinje
cells.
• The superior cerebellar peduncles, middle cerebellar peduncles, and inferior
cerebellar peduncles are tracts that link the cerebellum with the brain stem,
cerebrum and spinal cord.
• Damage to the cerebellum, or alcohol intoxication, can disturb muscle
coordination (ataxia).
VI. The Mesencephalon, p. 464
Objective
1. List the main components of the mesencephalon and specify their functions.
Table 14-4 summarizes the components and functions of the mesencephalon or midbrain.
Figure 14-8
• Structures of the tectum (the roof of the mesencephalon posterior to the
mesencephalic aqueduct) include 2 pairs of sensory nuclei called the corpora
quadrigemina:
1. the superior colliculus receives visual input
2. the inferior colliculus receives auditory input
•
Structures of the tegmentum (anterior to the mesencephalic aqueduct) include:
- the red nucleus (many blood vessels)
- the substantia nigra (pigmented gray matter)
•
The cerebral peduncles (nerve fiber bundles on the ventrolateral surfaces of the
mesencephalon) contain descending fibers to the cerebellum, and fibers carrying
motor commands.
VII. The Diencephalon, p. 465
Objective
1. List the main components of the diencephalon and specify their functions.
Figure 14-5
• The diencephalon integrates conscious and unconscious sensory information and
motor commands. It contains the epithalamus, the thalamus, and the
hypothalamus.
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The posterior part of the epithalamus contains the pineal gland, which secretes the
hormone melatonin.
The Thalamus, p. 466
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The thalamus filters ascending sensory information for the primary sensory
cortex, and relays information between the basal nuclei and cerebral cortex.
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The left thalamus and right thalamus are separated by the third ventricle. A
projection of gray matter called the intermediate mass extends into the ventricle
from each side.
Functions of the Thalamic Nuclei
Figure 14-9
• Each thalamus consists of a rounded mass of thalamic nuclei which relay sensory
information to the basal nuclei and cerebral cortex.
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There are 5 major groups of thalamic nuclei: anterior, medial, ventral, posterior
and lateral
1. the anterior group includes the anterior nuclei, part of the limbic system
involved with emotion.
2. the medial group provides awareness of emotional states
3. the ventral group relays sensory information
4. the posterior group includes the pulvinar and genicular nuclei
- the pulvinar integrates sensory information
- the lateral geniculate nucleus receives visual information
- the medial geniculate nucleus relays auditory information
5. the lateral group affects emotional states and integrates sensory
information
Table 14-5 lists the 5 major groups of thalamic nuclei and their functions.
The Hypothalamus, p. 467
Figure 14-10
• The hypothalamus lies below the thalamus. Its main features include the:
1. mamillary bodies, which process olfactory and other sensory information,
and controls reflex movements of the mouth involved in eating.
2. the infundibulum, a narrow stalk connecting the hypothalamus to the
pituitary gland.
3. the tuberal area, between the infundibulum and mamillary bodies, which
helps control pituitary gland function.
Functions of the Hypothalamus
• The hypothalamus has 8 major functions:
1. Subconscious control of skeletal muscle contractions.
2. Control of autonomic function.
3. Coordination of activities of the nervous and endocrine systems.
4. Secretion of 2 hormones:
- antidiuretic hormone (ADH) secreted by the supraoptic nucleus
- oxytocin (OT) secreted by the paraventricular nucleus
5. Production of emotions and behavioral drives, such as:
- the feeding center produces the sensation of hunger
- the thirst center produces the sensation of thirst
6. Coordination between voluntary and autonomic functions
7. Regulation of body temperature, coordinated by the preoptic area of the
hypothalamus.
8. Control of circadian rhythms (day-night cycles) coordinated by the
suprachiasmatic nucleus.
Table 14-6 shows the major areas of the hypothalamus and their functions.
VIII. The Limbic System, p. 469
Objective
1. List the main components of the limbic system and specify their locations and
functions.
•
The limbic system is a functional grouping that includes:
1. establishing emotional states
2. linking conscious functions of the cerebral cortex with unconscious,
autonomic functions of the brain stem
3. facilitating memory storage and retrieval
Figure 14-11
• The components of the limbic system include:
- the amygdaloid body interfaces between the limbic system, the
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cerebrum and the sensory systems.
the limbic lobe of the cerebral hemisphere, including:
• the cingulate gyrus
• the dentate gyrus
• the parahippocampal gyrus
• the hippocampus (important in long-term memory)
the fornix, a tract of white matter connecting the hippocampus with
the hypothalamus
the anterior nucleus of the thalamus relays information from the
mamillary body of the hypothalamus to the cingulate gyrus.
stimulation or inhibition of the reticular formation affects a number
of emotions (rage, fear, pain, sexual arousal, pleasure).
Table 14-7 summarizes the structures and functions of the limbic system.
IX. The Cerebrum, p. 470
Objectives
1. Identify the major anatomical subdivisions of the cerebrum.
2. Locate the motor, sensory, and association areas of the cerebral cortex, and
discuss their functions.
3. Discuss the origin and significance of the major categories of brain waves seen in
an electroencephalogram.
•
The cerebrum is the largest part of the brain. It controls all conscious thoughts and
intellectual functions, as well as processing somatic sensory and motor
information.
•
Gray matter is located in the cerebral cortex and basal nuclei. White matter is
found deep to the basal cortex and around the basal nuclei.
The Cerebral Cortex, p. 470
Figure 14-12
• The gyri of the neural cortex increase its surface area, which increases the number
of cortical neurons.
•
Some important structural features of the cerebrum include:
- the longitudinal fissure which separates the two cerebral
hemispheres
- each hemisphere is divided into lobes:
- the central sulcus divides the anterior frontal lobe from the
posterior parietal lobe
- the lateral sulcus separates the frontal lobe from the temporal lobe
- the parieto-occipital sulcus separates the parietal lobe from the
occipital lobe
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the insula (island) of cortex, medial to the lateral sulcus
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Each lobe contains functional regions that do not have clear structural boundaries.
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There are 3 important guidelines concerning the function of the cerebral lobes:
1. Each cerebral hemisphere receives sensory information from, and sends
motor commands to, the opposite side of the body.
2. The 2 hemispheres have different functions (although their structures are
the same).
3. The correspondence between a specific function and a specific region of
the cerebral cortex is not precise.
The White Matter of the Cerebrum, p. 472
Figure 14-13
• The axons (white matter) inside the cerebrum may be classified as association
fibers, commissural fibers or projection fibers.
1. Association fibers interconnect areas of neural cortex within the same
hemisphere:
- arcuate fibers are short, from one gyrus to another
- longitudinal fasciculi are longer bundles, connecting the frontal lobe with
other lobes of the same hemisphere
2. Commissural fibers are bands of fibers connecting the 2 cerebral hemispheres:
- corpus callosum
- anterior commissure
3. Projection fibers pass through the diencephalon to link the cerebral cortex with
the diencephalon, brain stem, cerebellum and spinal cord.
- the internal capsule is made up of all ascending and descending projection
fibers
The Basal Nuclei, p. 472
Figure 14-14
• Basal nuclei (cerebral nuclei) are masses of gray matter embedded in white matter
of the cerebrum, deep to the floor of the lateral ventricle within each hemisphere.
Basal nuclei direct subconscious activities such as processing sensory information
and subconscious motor commands.
•
The major structures of the basal nuclei include:
- the caudate nucleus (with a slender tail)
- the lentiform nucleus (consisting of the globus pallidus and the putamen)
•
The amygdaloid body (limbic system) lies anterior to the tail of the caudate
nucleus and inferior to the lentiform nucleus.
Functions of the Basal Nuclei
- subconscious control of skeletal muscle tone
- coordination of learned movement patterns (e.g. walking, lifting)
Motor and Sensory Areas of the Cortex, p. 474
Figure 14-15
• The central sulcus separates the motor and sensory areas of the cortex. Major
structures include:
- the precentral gyrus of the frontal lobe directs voluntary movements:
♣ the surface of the precentral gyrus is the primary motor cortex
♣ neurons of the primary motor cortex (pyramidal cells)
♣ specific motor neurons activate specific skeletal muscles
- the postcentral gyrus of the parietal lobe receives somatic sensory
information from touch, pressure, pain, vibration, taste and temperature
receptors:
♣ the surface of the postcentral gyrus is the primary sensory cortex
- the visual cortex receives information from sight receptors
- the auditory cortex receives information from sound receptors
- the olfactory cortex receives information from odor receptors
- the gustatory cortex receives information from taste receptors
Association Areas
• Association areas interpret incoming information or coordinate motor responses.
- sensory association areas monitor and interpret information coming into
the sensory areas of the cortex
♣ the somatic sensory association area monitors and interprets input
to the primary sensory cortex (e.g. recognizes and responds to a
touch on the skin).
♣ the visual association area interprets activity in the visual cortex
♣ the auditory association area monitors the auditory cortex
- somatic motor association area (premotor cortex) coordinates learned
movements
Integrative Centers
• Integrative centers (located in the lobes and cortical areas of both cerebral
hemispheres) receive information from association areas and direct complex
motor or analytical activities.
- the general interpretive area (Wernicke’s area), present in only 1
hemisphere, receives information from all sensory association areas, and
coordinates access to complex visual and auditory memories.
- the speech center (associated with the general interpretive area)
coordinates all the functions used in vocalization.
- the prefrontal cortex of the frontal lobe integrates information from
sensory association areas and performs abstract intellectual activities such
as predicting the consequences of different actions.
•
A procedure called prefrontal lobotomy was once used to treat mental illnesses by
causing patients to lose all concerns about the consequences of their actions.
•
Brodmann’s areas describe patterns of cellular organization within the cerebral
cortex, some of which correspond with functional areas such as the speech center.
Hemispheric Lateralization
Figure 14-16
• Each cerebral hemisphere is responsible for certain functions that are not
normally performed by the opposite hemisphere (hemispheric lateralization).
•
In most people, the left brain (the dominant hemisphere) controls the reading,
writing, mathematical, decision-making, speech and language skills.
•
The right cerebral hemisphere relates to the senses of touch, smell, sight, taste and
feel, such as recognizing faces and inflections of voice.
Monitoring Brain Activity: The Electroencephalogram
• The electrical activity of the brain can be monitored to assess brain activity by
placing electrodes on the skull and printing out a pattern of the electrical activity
(or brain waves) called the electroencephalogram (EEG).
Figure 4-17
• Typical brain waves are divided into 4 categories:
1. alpha waves are found in healthy, awake adults at rest with their
eyes closed
2. beta waves, of higher frequency, are seen when the person is
concentrating or mentally stressed
3. theta waves are found in children or intensely frustrated adults, or
may indicate a brain disorder in adults
4. delta waves are seen during sleep, or in adults with brain damage
•
A pacemaker mechanism synchronizes electrical activity between the 2
hemispheres. Brain damage can cause the hemispheres to become
unsynchronized.
•
During a temporary cerebral disorder called a seizure, the electroencephalogram
changes significantly. Other symptoms may occur, depending upon the regions of
the brain affected.
X. Focus: Cranial Nerves, p. 480
Figure 14-18
• Twelve pairs of cranial nerves (Roman numerals I-XII) connect directly to the
brain, near associated sensory or motor nuclei.
•
Cranial nerves are classified by their primary function:
1. Sensory nerves carrying somatic sensory information (touch,
pressure, vibration, temperature, pain)
2. Special sensory nerves carrying sensations of smell, sight, hearing
or balance.
3. Motor nerves: axons of somatic motor neurons.
4. Mixed nerves: a mixture of motor and sensory fibers.
•
Most cranial nerves also have important secondary functions, such as distributing
autonomic fibers to peripheral ganglia.
•
For each of the 12 cranial nerves, we will summarize its primary function, origin,
pathway and destination.
The Olfactory Nerves (I)
Figure 14-19
Primary function: special sensory (smell)
Origin: receptors of olfactory epithelium
Pathway: olfactory foramina in cribriform plate of ethmoid
Destination: olfactory bulbs
•
The olfactory nerves are made up of about 20 bundles of sensory neurons in
epithelium covering the roof of the nasal cavity. The bundles enter the olfactory
bulbs on either side of the crista galli. The olfactory tracts are axons of
postsynaptic neurons leading to the cerebrum.
The Optic Nerves (II)
Figure 14-20
Primary function: special sensory (vision)
Origin: retina of eye
Pathway: optic canals of sphenoid
Destination: diencephalon via the optic chiasm
•
The optic nerves contain about 1 million sensory nerve fibers that converge at the
optic chiasm where fibers cross over to the opposite side of the brain. The optic
tracts are reorganized axons leading toward the lateral geniculate nuclei.
The Oculomotor Nerves (III)
Figure 14-21
Primary function: motor (eye movements)
Origin: mesencephalon
Pathway: superior orbital fissures of sphenoid
Destination:
- somatic motor: superior, inferior and medial rectus muscles; inferior
•
oblique muscle; levator palpebrae superioris muscle
visceral motor: intrinsic eye muscles
In addition to controlling 4 of the 6 muscles that move the eye, the oculomotor
nerve delivers preganglionic autonomic fibers to neurons of the ciliary ganglion
which control intrinsic muscles of the iris and lens.
The Trochlear Nerves (IV)
Figure 14-21
Primary function: motor (eye movements)
Origin: mesencephalon
Pathway: superior orbital fissure of sphenoid
Destination: superior oblique muscle
•
The smallest cranial nerve, the trochlear nerve controls muscles that turn the eye
down and to the side.
The Abducens Nerves (VI)
Figure 14-21
Primary function: motor (eye movements)
Origin: pons
Pathway: superior orbital fissures of sphenoid
Destination: lateral rectus muscle
•
The abducens nerves control muscles that turn the eye to the side.
The Trigeminal Nerves (V)
Figure 14-22
Primary function: mixed (sensory and motor) to face
Origin:
- ophthalmic branch (sensory): orbital structures, nasal cavity, skin of
forehead, upper eyelid, eyebrow, part of nose
- maxillary branch (sensory): lower eyelid, upper lip, gums and teeth,
cheek, nose, palate and part of pharynx
- mandibular branch (sensory): lower gums, teeth, lips, palate and part of
tongue
- mandibular branch (motor): motor nuclei of pons
Pathway:
- ophthalmic branch: superior orbital fissure
- maxillary branch: foramen rotundum
- mandibular branch: foramen ovale
Destination:
- sensory nerves: sensory nuclei in pons
- motor nerves of mandibular branch: muscles of mastication
•
The trigeminal nerves are the largest cranial nerves, with 3 major branches. The
semilunar ganglion contains the cell bodies of the sensory neurons. Tic
douloureux is a painful disorder of the maxillary and mandibular branches.
The Facial Nerves (VII)
Figure 14-23
Primary function: mixed (sensory and motor) to face
Origin:
- sensory: taste receptors on anterior 2/3 of tongue
- motor: motor nuclei of pons
Pathway: internal acoustic canals to facial canals (stylomastoid foramina)
Destination:
- sensory: sensory nuclei of pons
- somatic motor: muscles of facial expression
- visceral motor: tear and nasal mucous glands; submandibular and
sublingual salivary glands
•
The facial nerves split to form the temporal, zygomatic, buccal, mandibular and
cervical branches. The sensory neurons monitor parts of the face and the anterior
1/3 of the tongue. The cell bodies of the sensory neurons are located in the
geniculate ganglia, and the motor nuclei are in the pons. Postganglionic fibers
from the pterygopalatine ganglia innervate lacrimal, nasal cavity, and pharynx
glands. The submandibular ganglia innervate the salivary glands. An
inflammation of the facial nerve causes Bell’s palsy.
The Vestibulocochlear Nerves (VIII)
Figure 14-24
Primary function: special sensory
- vestibular branch: balance and equilibrium
- cochlear branch: hearing
Origin: receptors of the inner ear
Pathway: internal acoustic canals of temporal bones
Destination: vestibular and cochlear nuclei of pons and medulla oblongata
•
The vestibulocochlear nerves (auditory nerves) have 2 distinct bundles of sensory
fibers:
- the vestibular branch originates at receptors of the vestibule (balance), and
connects to the vestibular nuclei of the pons and medulla oblongata.
- the cochlear branch originates at sensors of the cochlea (hearing), and
connects with the cochlear nuclei of the pons and medulla oblongata.
The Glossopharyngeal Nerves (IX)
Figure 14-25
Primary function: mixed (sensory and motor) to head and neck
Origin:
- sensory: posterior 1/3 of tongue, part of the pharynx and palate, carotid
arteries
- motor: motor nuclei of medulla oblongata
Pathway: jugular foramina between the occipital bone and the temporal bones
Destination:
- sensory: sensory nuclei of medulla oblongata
- somatic motor: nerves involved in swallowing
- visceral motor: parotid salivary gland
•
The glossopharyngeal nerves innervate the tongue and pharynx. The sensory
neurons are in the superior ganglion and inferior ganglion. Visceral motor fibers
synapse in the otic ganglion.
The Vagus Nerves (X)
Figure 14-26
Primary function: mixed (sensory and motor): thorax and abdomen
Origin:
- sensory: part of pharynx, auricle and external acoustic canal, diaphragm,
visceral organs of thoracic and abdominopelvic cavities
- motor: motor nuclei in medulla oblongata
Pathway: jugular foramina between the occipital bone and temporal bones
Destination:
- sensory: sensory nuclei and autonomic centers of medulla oblongata
- visceral motor: palate, pharynx, digestive, respiratory and cardiovascular
systems in thoracic and abdominal cavities
•
The vagus nerves branch and radiate extensively. Sensory neurons are located in
the jugular ganglion and the inferior nodose ganglion. Preganglionic autonomic
motor fibers affect the heart, smooth muscle and glands.
The Accessory Nerves (XI)
Figure 14-27
Primary function: motor to muscles of neck and upper back
Origin: motor nuclei of spinal cord and medulla oblongata
Pathway: jugular foramina between occipital bone and temporal bones
Destination:
- internal branch: voluntary muscles of palate, pharynx, larynx
- external branch: sternocleidomastoid and trapezius muscles
•
The accessory nerves (spinal accessory nerves) each have some motor fibers that
originate in the anterior gray horns of the first 5 cervical segments of the spinal
cord, forming the spinal root. Motor fibers of the cranial root originate in the
medulla oblongata. The 2 roots combine in the cranium, then divide into 2
branches as they leave the cranium: The internal branch joins the vagus nerve; the
external branch controls muscles of the neck and back.
The Hypoglossal Nerves (XII)
Figure 14-27
Primary function: motor (tongue movements)
Origin: motor nuclei of medulla oblongata
Pathway: hypoglossal canals of occipital bone
Destination: muscles of the tongue
•
The hypoglossal nerve provides voluntary control over tongue movement.
Table 14-9 summarizes the basic distribution and function of each cranial nerve.
XI. Cranial Reflexes, p. 490
Objective
1. Describe representative examples of cranial reflexes that produce somatic
responses or visceral responses to specific stimuli.
•
Cranial reflexes are monosynaptic and polysynaptic reflex arcs that involve the
sensory and motor fibers of cranial nerves. Cranial reflexes are clinically useful
for checking the condition of cranial nerves, nuclei and tracts in the brain.
Table 14-10 lists examples of cranial reflexes and their functions
Key
•
•
•
There are 12 pairs of cranial nerves.
They are responsible for the special senses of smell, sight, and hearing/balance;
for control over the muscles of the eye, jaw face and tongue; and for control over
the superficial muscles of the neck, back and shoulders.
Cranial nerves also provide sensory information from the face, neck and upper
chest, and autonomic (parasympathetic) innervation to organs in the thoracic and
abdominopelvic cavities.
SUMMARY
In chapter 14 we learned about:
- the 6 regions in the adult brain (cerebrum, cerebellum, diencephalon, mesencephalon,
pons and medulla oblongata)
- the embryology of the brain (telencephalon, metencephalon, myelencephalon)
- the ventricles of the brain
- the cranial meninges
- the function and production of cerebrospinal fluid
- the blood-brain barrier
- the structures and functions of:
- the medulla oblongata
- the pons
- the cerebellum
- hemispheres and peduncles
- the mesencephalon
- the diencephalon
- epithalamus, thalamus, and hypothalamus
- the limbic system (gyri and fornix)
- the cerebrum
- the cerebral cortex
- the white matter
- the basal nuclei
- motor and sensory cortex
- association areas
- functional centers
- the electroencephalogram
- the 12 pairs of cranial nerves (sensory, motor and mixed)
- their primary functions, origins, pathways and destinations
- cranial reflexes