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Saladin 5e Extended Outline
Chapter 14
The Brain and Cranial Nerves
I. Overview of the Brain (pp. 515–520)
A. The brain of vertebrates has changed a great deal over evolutionary time; in average humans,
the size of the brain is proportional to body size, not to intelligence. (p. 515)
B. The brain has been assigned major landmarks as reference points for its study. (pp. 515–518)
1. Two directional terms are rostral (“toward the nose,” or the forehead in upright
humans) and caudal (“toward the tail,” or the spinal cord in humans).
2. The brain can be divided conceptually into the cerebrum, cerebellum, and brainstem.
a. The cerebrum is about 83% of the brain volume and consists of two cerebral
hemispheres. (Fig. 14.1a)
i. Each hemisphere has thick folds called gyri separated by shallow
grooves called sulci.
ii. The deep longitudinal fissure separates the right and left
hemispheres.
iii. At the bottom of this fissure the hemispheres are connected by the
corpus callosum. (Fig. 14.2)
b. The cerebellum, the second largest region of the brain containing over 50% of
the brain’s neurons, occupies the posterior cranial fossa inferior to the cerebrum.
(Fig. 14.1b, c)
c. The brainstem is that which remains of the brain if the cerebrum and
cerebellum are removed.
i. Its major components, rostral to caudal, are the diencephalon,
midbrain, pons, and medulla oblongata. (Figs. 14.1b, 14.2)
ii. It is oriented like a vertical stalk with the cerebrum perched on top in
a living person; postmortem changes give it an oblique angle.
iii. The brainstem ends at the foramen magnum, and the CNS continues
below this as the spinal cord.
C. The brain, like the spinal cord, is composed of gray matter and white matter. (p. 518) (Figs.
14.5, 14.6c)
1. White matter has a bright pearly white color due to myelin around its nerve fibers.
2. Gray matter has little myelin and a duller white color.
a. Gray matter forms a surface layer called the cortex over the cerebrum and
cerebellum.
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b. Deeper masses called nuclei are surrounded by white matter.
c. In most of the brain, the white matter lies deep to the cortical gray matter,
opposite from their relation in the spinal cord.
d. White matter in the brain is composed of tracts, or bundles of axons.
D. Embryonic development of the brain produces the mature brain anatomy consisting of
forebrain, midbrain, and hindbrain.(pp. 518–520) (Fig. 14.3)
1. The nervous system develops from ectoderm; early in the third week of development, a
dorsal streak called the neuroectoderm appears along the embryo’s length and thickens to
form the neural plate.
a. The neural plate gives rise to most neurons and all glial cells except microglia,
which arise from mesoderm.
b. The neural plate sinks and its edges thicken, forming a neural groove with a
raised neural fold.
c. The neural folds then fuse along the midline, beginning in the cervical (neck)
region and progressing in both directions.
2. By four weeks of development, the neural tube has formed and closed, and it separates
from the overlying ectoderm.
a. The neural tube grows lateral processes that later form motor nerve fibers.
b. The lumen becomes a fluid filled space that later constitutes the central canal
of the spinal cord and ventricles of the brain.
3. As the neural tube develops, some ectodermal cells originally along the margin of the
groove separate and form two neural crests on each side of the tube.
a. Neural crest cells give rise to the arachnoid mater and pia mater; most of the
PNS including sensory and autonomic nerves, ganglia, and Schwann cells; and
some other structures of the skeletal, integumentary, and endocrine systems.
4. By the fourth week, the neural tube exhibits three primary vesicles: the forebrain
(prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). (Fig.
14.4)
5. By the fifth week, the neural tube continues to subdivide into five secondary vesicles.
(Fig. 14.4)
a. The forebrain becomes the telencephalon and diencephalon. (Fig. 14.4b)
i. The telenecephalon has a pair of lateral outgrowths that become the
cerebral hemispheres.
ii. The diencephalon has a pair of small cuplike optic vesicles that
become the retinas.
b. The midbrain is undivided and retains the name mesencephalon.
c. The hindbrain becomes the metencephalon and the myelencephalon.
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II. Meninges, Ventricles, Cerebrospinal Fluid, and Blood Supply (pp. 520–524)
A. The brain is enveloped in three connective tissue membranes, the meninges, which lie between
the nervous tissue and bone. (pp. 520–521)
1. The three membranes of the meninges are the dura mater, arachnoid mater, and pia
mater. (Fig. 14.5)
2. In the cranial cavity, the dura mater consists of two layers, the outer periosteal layer
and the inner meningeal layer.
a. Only the meningeal layer continues into the vertebral canal, where it forms
the dural sac.
b. The dura mater is pressed closely against the cranial bone, but is not attached
except in limited places (foramen magnum, sella turcica, crista galli, and
sutures).
c. In some places the two layers of the dura are separated by dural sinuses.
i. The superior sagittal sinus is found just under the cranium along the
median line.
ii. The transverse sinus runs horizontally from the rear of the head
toward each ear.
iii. These sinuses meet like an inverted T at the back of the brain and
ultimately empty into the internal jugular veins.
d. In certain places, the meningeal layer of the dura folds inward to separate
major parts of the brain.
i. The falx cerebri extends into the longitudinal fissure as a wall
between the cerebral hemispheres.
ii. The tentorium cerebelli is like a roof over the posterior cranial fossa
and separates the cerebellum from the cerebrum.
iii. The falx cerebelli partially separates the right and left halves of the
cerebellum.
3. The arachnoid mater and pia mater are similar to those of the spinal cord.
a. The arachnoid mater is a transparent membrane over the brain surface. (Fig.
14.1c)
i. The subarachnoid space separates it from the pia mater below.
ii. In some places a subdural space separates it from the dura above.
b. The pia mater is a very thin, delicate membrane that follows all contours of
the brain and sulci.
Insight 14.1 Meningitis
B. The brain has four internal chambers called ventricles that are filled with cerebrospinal fluid.
(pp. 521–524) (Fig. 14.6)
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1. The largest and most rostral are the lateral ventricles, which form an arc in each
cerebral hemisphere.
2. The lateral ventricles connect to the third ventricle, a median space inferior to the
corpus callosum, via the interventricular foramina.
3. A canal called the cerebral aqueduct leads from the third ventricle to the fourth
ventricle, a triangular chamber between the pons and cerebellum.
4. The fourth ventricle narrows caudally to form the central canal that extends through
the medulla oblongata into the spinal cord.
5. Each ventricle has a mass of blood capillaries on the floor or wall called a choroids
plexus.
a. Ependyma is a type of neuroglia that resembles cuboidal epithelium.
b. It lines the ventricles and canals, covers the choroids plexuses, and produces
cerebrospinal fluid.
6. Cerebrospinal fluid (CSF) is a clear, colorless liquid that fills the ventricles and canals
of the CNS and bathes its external surface.
a. The brain produces about 500 mL of CSF per day, but it is constantly
reabsorbed and only 100 to 160 mL is normally present at one time.
b. CSF production begins with filtration of blood plasma through the brain’s
capillaries.
i. Ependymal cells modify this filtrate so that CSF has more sodium
and chloride, but less potassium, calcium, and glucose and very little
protein.
b. CSF is circulated through the CNS by its own pressure, by the beating of cilia
on the ependymal cells, and by rhythmic pulsations of the brain produced by the
heartbeat.
i. CSF secreted in the lateral ventricles flows through the
interventricular foramina into the third ventricle and then down the
cerebral aqueduct to the fourth ventricle. (Fig. 14.7)
ii. The third and fourth ventricles add more CSF.
c. A small amount of CSF fills the central canal of the spinal cord, but all of it
escapes through three pores in the walls of the fourth ventricle: a median
aperture and two lateral apertures.
i. These apertures lead into the subarachnoid space.
ii. CSF is reabsorbed in this space by the arachnoid villi.
7. CSF serves three purposes.
a. Buoyancy. The brain and CSF are similar in density; this buoyancy allows the
brain to attain considerable size without being impaired by its own weight.
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b. Protection. CSF helps prevent the brain from striking the cranium when the
head is jolted; however, severe jolts may still be damaging, as in shaken baby
syndrome and concussions from car accidents, boxing, etc.
c. Chemical stability. The flow of CSF rinses metabolic wastes away and
homeostatically regulates the brain’s chemical environment.
Insight 14.2 Hydrocephalus
C. The blood supply to the nervous system is critically important, and the brain barrier system
protects the brain from harmful agents in the blood. (p. 524)
1. The brain is only 2% of the adult weight, but it receives 15% of the blood and
consumes 20% of the oxygen and glucose of the body.
a. A 10-second interruption in blood flow can cause loss of consciousness; 1 to 2
minutes, impairment of function; and 4 minutes irreversible brain damage.
2. The brain barrier system regulates what substances can get from the bloodstream into
the tissue fluid of the brain.
a. The blood capillaries through the brain tissue is one point of entry, and it is
protected by the blood–brain barrier (BBB) consisting of tight junctions between
endothelial cells that form the capillary walls.
i. During development, astrocytes induce development of the tight
junctions in these endothelial cells.
ii. Anything leaving the blood must therefore pass through the cells and
not between them.
b. The choriod plexuses are another point of entry, and this is protected by the
blood–CSF barrier formed by tight junctions between ependymal cells.
i. Tight junctions are absent from ependymal cells elsewhere, allowing
exchange between brain and CSF.
2. The BBS is highly permeable to water, glucose, and lipid-soluble substances such as
oxygen, carbon dioxide, alcohol, caffeine, nicotine, and anesthetics; it is slightly
permeable to sodium, potassium, chloride, and waste produces urea and creatinine.
a. The BBS is an obstacle to delivery of medications such as antibiotics and
cancer drugs.
b. Trauma and inflammation sometimes damage the BBS, allowing pathogens to
enter the brain tissue.
c. In the third and fourth ventricles, circumventricular organs (CVOs) lack the
barrier, and the blood has direct access to the brain.
i. CVOs allow the brain to monitor and respond to blood variables, but
they also afford a route of invasion by HIV.
III. The Hindbrain and Midbrain (pp. 524–531)
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A. Beginning caudally, the medulla oblongata of the adult hindbrain differentiates from the
embryonic myelencephalon. (pp. 524–525)
1. The medulla begins at the foramen magnum and extends about 3 cm rostrally, ending
at a groove between the medulla and pons. (Figs. 14.2, 14.8)
a. Externally, the anterior surface has a pair of ridges called the pyramids, which
are wider at the rostral end, taper caudally, and are separated by the anterior
median fissure.
b. Lateral to each pyramid is a bulge called the olive.
c. Posteriorly, the gracile fasciculi and cuneate fasciculi of the spinal cord
continue as two pairs of ridges on the medulla.
2. All nerve fibers connecting the brain to the spinal cord pass through the medulla.
a. The ascending fibers include first-order sensory fibers of the two fasciculi,
which end in the gracile and cuneate nuclei. (Fig. 14.9c)
i. These nuclei synapse with second-order fibers that decussate and
form the medial lemniscus on each side.
ii. The second-order fibers rise to the thalamus, synapsing with thirdorder fibers that continue to the cerebral cortex.
iii. Near the cuneate nucleus, a continuation of the spinal posterior
spinocerebellar tract carries sensory signals to the cerebellum.
b. The largest group of descending fibers is the pair of corticospinal tracts filling
the pyramids on the anterior surface.
i. These carry motor signals from the cerebral cortex to the spinal cord,
ultimately to stimulate skeletal muscles.
ii. About 90% of these fibers cross over at the pyramidal decussation
near the caudal end of the pyramids; muscles below the neck are
therefore controlled contralaterally. (Fig. 14.8a)
iii. A smaller bundle of descending fibers, the tectospinal tract,
originates in the midbrain, passes through the medulla, and controls
muscles of the neck.
3. The medulla contains neural networks involved in many sensory and motor functions.
a. Sensory functions include the sense of touch, pressure, temperature, taste and
pain
b. Motor functions include chewing, salivation, swallowing, gagging, vomiting,
respiration, speech, coughing, sneezing, sweating, cardiovascular and
gastrointestinal control, and head, neck, and shoulder movements.
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4. Signals enter and leave the medulla not only via the spinal cord but also through four
pairs of cranial nerves that begin or end there: glossopharyngeal (CN IX), vagus, (CN X),
accessory (CN XI), and hypoglossale (CN XII) nerves. (Table 14.1)
a. The trigeminal nerve (CN V), belongs to the pons but has parts extending into
the medulla.
5. The wavy inferior olivary nucleus is a major relay center.
6. The reticular formation is a loose network of nuclei extending throughout the medulla,
pons, and midbrain.
a. In the medulla, it includes a cardiac center, a vasomotor center, two
respiratory centers, and other nuclei involved in motor functions.
B. The embryonic metencephalon develops into two structures, the pons and the cerebellum; the
pons is about 2.5 cm long and appears as a broad anterior bulge rostral to the medulla. (pp. 525–
527) (Figs. 14.2, 14.8)
1. Posteriorly, the pons consists of two pairs of thick stalks called cerebellar peduncles
that connect the pons and midbrain. (Fig. 14.9b)
2. The pons has continuations of the reticular formation, medial lemniscus, and
tectospinal tract, as well as extensions from the spinal cord of the anterolateral system
and anterior spinocerebellar tract.
3. The anterior pons has tracts of white matter, including transverse fascicles that
decussate and connect the cerebellar hemispheres, and longitudinal fascicles that carry
sensory and motor signals. (Fig. 14.9b)
4. Cranial nerves V to VIII begin or end in the pons; their functions include sensory roles
and motor roles. (Table 14.1)
5. The reticular formation in the pons contains additional nuclei concerned with sleep,
respiration, and posture.
C. The embryonic mesencephalon becomes just one adult brain structure, the midbrain; it connects
the hindbrain and forebrain. (Figs. 14.2, 14.8) (p. 528)
1. The midbrain contains the cerebral aqueduct, continuations of the medial lemniscus
and reticular formation, and motor nuclei for the oculomotor (CN III) and trochlear (CN
IV) nerves that control eye movements.
2. The part of the midbrain posterior to the cerebral aqueduct is the rooflike tectum,
which exhibits four bulges, the corpora quadrigemina.
a. The upper pair, the superior colliculi, controls vision and eye-related functions
(visual tracking, blinking, focusing, etc.).
b. The lower pair, the inferior colliculi, receives signals from the inner ear and
relays them to other parts of the brain, especially the thalamus.
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3. Anterior to the cerebral aqueduct, the midbrain consists mainly of the two cerebral
peduncles that anchor the cerebrum to the brain stem; each peduncle has three main
components: tegmentum, substantia nigra, and cerebral crus.
a. The tegmentum is dominated by the red nucleus, whose fibers form the
rubrospinal tract in most mammals, but in humans go to and from the
cerebellum to collaborate in fine motor control.
b. The substantia nigra is a nucleus pigmented with melanin; it is a motor center
that relays inhibitory signals to the thalamus and basal nuclei.
i. Degeneration of neurons in the substantia nigra is responsible for the
tremors of Parkinson disease.
c. The cerebral crus is a bundle of nerve fibers connecting the cerebrum to the
pons and carrying corticospinal nerve tracts.
4. Surrounding the cerebral aqueduct is the central (periaqueductal) gray matter, an
arrowhead-shaped body; it is involved with the reticulospinal tracts in controlling
awareness of pain.
D. The reticular formation is a web of gray matter that runs vertically through all levels of the
brain stem. (Fig. 14.9) (pp. 528–529)
1. The reticular formation occupies much of the space between the white fiber tracts and
the brainstem nuclei, and connects with many areas of the cerebrum. (Fig. 14.10)
2. It consists of more than 100 small neural networks that include five functions.
a. Somatic motor control. Some motor neurons of the cerebral cortex send axons
to reticular formation nuclei, which give rise to the reticulospinal tracts of the
spinal cord; these adjust muscle tone, balance, and posture during movement
i. The reticular formation also relays signals from the eyes and ears to
the cerebellum so that this information can be integrated with motor
coordination.
ii. Other motor nuclei include gaze centers and central pattern
generators.
b. Cardiovascular control. The reticular formation includes the cardiac and
vasomotor centers of the medulla oblongata.
c. Pain modulation. The reticular formation is one route for pain signals to the
cerebral cortex; is also is the origin of the descending analgesic pathways that
block pain signal transmission.
d. Sleep and consciousness. The reticular formation plays a central role in states
such as alertness and sleep; injury to the reticular formation can result in
irreversible coma.
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e. Habituation. This process allows the brain to ignore repetitive,
inconsequential stimuli via the reticular activating system or
extrathalamiccortical modulatory system.
E. The cerebellum is the largest part of the hindbrain and consists of right and left cerebellar
hemispheres connected by a wormlike bridge, the vermis. (pp. 529–530) (Fig. 14.11)
1. Each hemisphere exhibits parallel folds called folia separated by shallow sulci..
2. The cerebellum has a surface cortex of gray matter and a deeper layer of white matter.
a. The white matter exhibits a fernlike pattern called the arbor vitae.
b. Each hemisphere has four masses of gray matter called deep nuclei embedded
in the white matter.
c. All input to the cerebellum goes to the cortex and all output comes from the
deep nuclei.
3. The cerebellum is 10% of the brain’s mass but has 60% of the surface area of the
cerebral cortex and contains more than half of all brain neurons.
a. Its tiny granule cells are the most abundant type of neuron in the brain.
b. The unusually large Purkinje cells are the most distinctive; they have a
tremendous profusion of dendrites compressed into a single plane like a flat tree.
(Fig. 12.5)
4. The cerebellum is connected to the brain stem by three pairs of stalks, the cerebellar
peduncles. (Fig. 14.8b)
a. A pair of inferior peduncles connect to the medulla oblongata.
b. A pair of middle peduncles connect to the pons.
c. A pair of superior peduncles connect to the midbrain.
5. Most spinal input enters by way of the inferior peduncles; most input from the rest of
the brain by way of the middle peduncles; and most output travels by way of the superior
peduncles.
6. Cerebellar lesions cause deficits in coordination and locomotor ability, and also in
sensory, linguistic, emotional, and other nonmotor functions.
a. The cerebellum is highly active in tactile exploration and in spatial perception.
b. The cerebellum is a timekeeping center involved in rhythm and in prediction
of trajectories of moving objects.
c. Cerebellar lesions may impair a person’s ability to judge differences in pitch
of sounds, and language input and output may be affected.
d. People with cerebellar lesions also have difficulty planning and scheduling
tasks, tend to overreact, and have difficulty with impulse control.
i. Many children with ADHD have abnormally small cerebellums.
IV. The Forebrain (pp. 531–537)
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A. The forebrain consists of the diencephalon, which is the most rostral part of the brainstem, and
the telencephalon, which develops chiefly into the cerebrum. (p. 531)
B. The diencephalon, which encloses the third ventricle, has three major derivatives; the thalamus,
hypothalamus, and epithalamus.
1. Each side of the brain has a thalamus, an ovoid mass at the superior end of the
brainstem beneath the cerebral hemisphere. (Figs. 14.6c, 14.8, 14.16)
a. The thalami constitute about four-fifths of the diencephalon, protruding
medially into the third ventricle and laterally into the lateral ventricles.
b. In about 70% of people, the thalami are joined medially by a narrow
intermediate mass.
c. The thalamus is composed of 23 nuclei classed into five main functional
groups: anterior, posterior, medial, lateral, and ventral. (Fig. 14.12a)
d. The thalamus is the “gateway to the cerebral cortex” in that nearly all input
passes through synapses in the thalamic nuclei.
e. The thalamus plays a key role in motor control by relaying signals from the
cerebellum to the cerebrum.
f. It provides feedback loops between the cerebral cortex and the deep cerebral
motor centers (the basal nuclei).
g. The thalamus is involved in the memory and emotional functions of the
limbic system.
2. The hypothalamus forms part of the walls and floor of the third ventricle and extends
anteriorly to the optic chiasm and posteriorly to the mammillary bodies. (Fig. 14.2)
a. Each mamillary body contains three to four mammillary nuclei that relay
signals from the limbic system to the thalamus.
b. The pituitary glad is attached to the hypothalamus by a stalk (infundibulum)
between the optic chiasm and mammillary bodies.
c. The hypothalamus is the major control center of the autonomic nervous
system and endocrine system and is concerned with a variety of visceral
functions. (Fig. 14.12b)
i. Hormone secretion. Hormones secreted by the hypothalamus control
the anterior pituitary gland to regulate growth, metabolism,
reproduction, and stress response; the hypothalamus also produces
hormones that are stored in the posterior pituitary that have to do with
labor contraction, lactation, and water balance.
ii. Autonomic effects. The hypothalamus is an integrating center for the
autonomic nervous system and influences heart rate, blood pressure,
and other visceral functions.
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iii. Thermoregulation. The hypothalamic thermostat, a collection of
neurons mainly in the preoptic nucleus, monitor body temperature and
adjust it.
iv. Food and water intake. The hunger and satiety centers of the
hypothalamus monitor blood glucose levels, and osmoreceptors
monitor the salt concentration of the blood; these stimulate behavioral
and hormonal changes.
v. Sleep and circadian rhythms. The caudal part of the hypothalamus is
part of the reticular formation and regulates the rhythm of sleep and
waking; the suprachiasmatic nucleus superior to the optic chiasm
controls circadian rhythms.
vi. Memory. The mammillary nuclei lie in the pathway of signals from
the hippocampus, a memory center, to the thalamus; lesions to the
mammillary nuclei cause memory deficits.
vii. Emotional behavior. Hypothalamic centers are involved in anger,
aggression, fear, pleasure, sexual drive, copulation, and orgasm.
C. The epithalamus is a very small mass of tissue composed of the pineal gland, the habenula that
serves as a relay from the limbic system to midbrain, and a thin roof over the third ventricle. (p.
533) (Fig. 14.2a)
D. The cerebrum develops from the embryonic telencephalon; it is the largest and most
conspicuous part of the human brain. (pp. 533–537)
1. In terms of gross anatomy, the cerebrum has two cerebral hemispheres separated by the
longitudinal fissure but connected by the corpus callosum.
a. The conspicuous gyri of each hemisphere are separated by grooves called
sulci; the folding into gyri allows a greater amount of cortex to fit into the
cranial cavity.
b. Some gyri have consistent anatomy while others vary between individuals.
c. Certain prominent sulci divide each hemisphere into five distinct lobes. (Fig.
14.13)
i. The front lobe lies behind the frontal bone, superior to the eyes, and
extends caudally to the central sulcus; it is involved in voluntary motor
functions and higher mental functions.
ii. The parietal lobe forms the uppermost part of the brain, underlying
the parietal bone, and extends caudally to the parieto-occipital sulcus; it
is involved in general sense, taste, and some visual processing. (Fig.
14.2)
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iii. The occipital lobe is at the rear of the head, caudal to the parietooccipital sulcus and underlying the occipital bone; it is the principle
visual center.
iv. The temporal lobe is a lateral, horizontal lobe deep to the temporal
bone and separated from fthe parietal lobe by a deep lateral sulcus; it is
concerned with hearing, smell, learning, memory, and some aspects of
vision and emotion.
v. The insula is a small mass of cortex deep to the lateral sulcus and
only visible by retracting or cutting away some of the cerebrum; it has
roles in language, sense of taste, and integrating visceral sensory
information. (Figs. 14.1c, 14.6c, 14.13)
2. White matter makes up most of the volume of the cerebrum and is composed of glia
and myelinated nerve fibers organized into three kinds of tracts.
a. Projection tracts extend vertically between higher and lower brain and spinal
cord centers.
i. For example, corticospinal tracts carry motor signals from the
cerebrum to the brain stem and spinal cord.
ii. Superior to the brain stem, the projection tracts form a broad, dense
sheet, the internal capsule, and the radiate in a fanlike array, the corona
radiate, to specific cortical centers.
b. Commissural tracts cross from one hemisphere to the other through
commisures.
i. Most pass through the corpus callosum, which forms the floor of the
longitudinal fissure.
ii. A few pass through the much smaller anterior and posterior
commissures.
c. Association tracts connect different regions within the same hemisphere.
i. Long association fibers connect different lobes within the same
hemisphere, whereas short association fibers connect different gyri
within a single lobe.
ii. Among other roles, association tracts link perceptual and memory
centers.
3. Neural integration is carried out in the cerebral gray matter, found in the cerebral
cortex, basal nuclei, and limbic system.
4. The cerebral cortex is a layer covering the surface of the hemispheres, constituting
40% of the mass of the brain and containing 14 to 16 billion neurons. (Fig. 14.6)
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a. The cerebral cortex contains two principle types of neurons, stellate cells and
pyramidal cells. (Fig. 14.15)
b. Stellate cells have spheroidal somas with dendrites projecting short distances
in all directions; they are concerned with sensory input and processing
information locally.
c. Pyramidal cells are tall and conical with their apex pointing toward the brain
surface.
i. They have a thick dendrite with many branches, and small knobby
dendritic spines.
ii. The base gives rise to horizontally oriented dendrites and an axons
that passes into the white matter.
iii. Pyramidal cells include the output neurons of the cerebrum and are
the only neurons whose fibers leave the cortex and connect with other
parts of the CNS.
d. 90% of the human cerebral cortex is a six-layered tissue called neocortex
from its recent evolutionary origin about 60 million years ago. (Fig. 14.15)
i. Layer thickness, composition, and connections vary in different
regions.
ii. All axons that leave the cortex and enter the white matter arise from
layers III, V, and VI.
e. Some regions of the cerebral cortex have fewer than six layers.
i. The earliest type of cortex to appear was the paleocortex, limited in
humans to part of the insula and certain areas of the temporal lobe
concerned with smell.
ii. The next to evolve was the archicortex, found in the human
hippocampus.
iii. The neocortex was the last to evolve.
5. The basal nuclei are masses of cerebral gray matter buried in the white matter, lateral
to the thalamus. (Fig. 14.16)
a. They are often called basal ganglia, although ganglion is best restricted to
clusters of neurons outside the CNS.
b. Three brain centers are classified as basal nuclei: the caudate nucleus,
putamen, and globus pallidus.
i. The putamen and globus pallidus are collectively called the lentiform
nucleus.
ii. The putamen and caudate nucleus are collectively called the corpus
striatum.
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c. The basal nuclei receive input from the substantia nigra of the midbrain and
motor areas of the cerebral cortex.
6. The limbic system is an important center of emotion and learning and consists of a ring
of structures on the medial side of the cerebral hemisphere, encircling the corpus
callosum and thalamus. (Fig. 14.17)
a. Its most prominent components are the cingulate gyrus that arches over the
corpus callosum; the hippocampus, in the medial temporal lobe; and the
amygdala, rostral to the hippocampus in the temporal lobe.
b. Other components include the mammillary nuclei and other hypothalamic
nuclei; some thalamic nuclei; parts of the basal nuclei; and parts of the frontal
cortex.
c. Limbic system components are interconnected through a complex loop of
fiber tracts allowing for feedback; the structures are bilaterally paired in each
hemisphere.
d. The limbic system has significant roles in emotion and memory and contains
structures for both gratification and aversion.
i. Gratification centers dominate some structures, such as the nucleus
accumbens, while aversion centers dominate others, such as the
amygdala.
V. Integrative Functions of the Brain (pp. 538–549)
A. Higher brain functions such as sleep, memory, cognition, emotion, sensation, motor control,
and language are associated with the cerebral cortex, but not exclusively; they involved
interactions between the cerebral cortex and other regions such as the basal nuclei, brainstem, and
cerebellum. (p. 538)
B. The brain’s surface electrical activity, or brain waves, can be recorded as an
electroencephalogram (EEG), which can be useful in studying both normal and abnormal brain
functions. (pp. 538–539) (Fig. 14.18)
1. Four types of brain waves can be distinguished based on differences in amplitude (mV)
and frequency (Hz): Alpha, beta, theta, and delta waves.
2. Alpha (α) waves have a frequency of 8 to 13 Hz and are recorded especially in the
parieto-occipital area.
a. They dominate the EEG when a person is awake and resting with the mind
wandering.
b. They are suppressed during sensory stimulation and mental tasks, and are
absent during sleep.
3. Beta (β) waves have a frequency of 14 to 30 Hz and occur in the frontal to parietal
region.
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a. They are accentuated during mental activity and sensory stimulation.
4. Theta (θ) waves have a frequency of 4 to 7 Hz.
a. They are normal in children and in drowsy or sleeping adults, but a
predominance in awake adults suggests emotional stress or brain disorders.
5. Delta (δ) waves are high-amplitude “slow waves” with a frequency of less than 3.5 Hz.
a. Infants exhibit delta waves when awake, and adults exhibit them in deep
sleep.
b. A predominance of delta waves in awake adults indicates serious brain
damage.
C. Sleep is one of many bodily functions that occur in cycles called circadian rhythms. (pp. 539–
540)
1. Sleep can be defined as a temporary state of unconsciousness from which one can
awaken when stimulated.
a. It is characterized by a stereotyped posture (lying down with eyes closed) and
inhibition of muscular activity (sleep paralysis).
b. It resembles coma and hibernation, except that individuals cannot be aroused
from these states by sensory stimulation.
2. Sleep occurs in four distinct stages recognizable from changes in EEG. (Fig. 14.19a)
a. Stage 1 includes feeling drowsy, closing the eyes, and starting to relax; the
EEG is dominated by alpha waves.
b. Stage 2 is light sleep during which the EEG declines in frequency but
increases in amplitude, occasionally exhibiting sleep spindles from interactions
between thalamus and cerebral cortex.
c. Stage 3 is moderate to deep sleep, typically beginning about 20 minutes after
stage 1. Theta and delta waves appear and vital signs fall.
d. Stage 4 is also called slow-wave sleep (SWS) because the EEG is dominated
by delta waves; vital signs are at their lowest levels and a person is difficult to
awaken.
3. About five times a night, a sleeper backtracks to stage 2 and exhibits rapid eye
movement (REM) sleep. (Fig. 14.19b)
a. The eyes oscillate back and forth as though watching a movie.
b. It is also called paradoxical sleep because the EEG resembles the waking
state, yet the sleeper is harder to arouse than at any other stage.
c. Vital signs increase, and sleep paralysis is especially strong during REM
sleep.
4. Dreams occur during both REM and non-REM sleep, but REM dreams tend to be
longer, more vivid, and more emotional.
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a. The parasympathetic nervous system is very active during REM sleep,
causing constriction of the pupils and erection of the penis or clitoris.
b. In men, penile erection accompanies 80% to 95% of REM sleep but is seldom
associated with sexual dream content.
5. The cycle of sleep and waking is controlled by complex interactions between cerebral
cortex, thalamus, hypothalamus, and reticular formation.
a. Nuclei in the upper reticular formation near the junction of the pons and
midbrain induce arousal, whereas nuclei below the pons induce sleep.
b. Sleep is also induced by a ventrolateral preoptic nucleus in the hypothalamus,
which inhibits arousal neurons in the upper reticular formation.
c. The suprachiasmatic nucleus (SCN), just above the optic chiasm, is another
important control center for sleep.
i. Some nerve fibers from the eyes go to the SCN, which uses the input
to synchronize body rhythms with the external rhythm of night and day.
ii. The SCN does not induce sleep or waking, but regulates the time of
day that a person sleeps; destruction of the SCN in an animal results in
sleeping at random times although for the same number of hours per
day.
6. Scientists know little about the purposes of sleep and dreaming, except that sleep
deprivation can lead to death.
a. One hypothesis is that energy sources such as glycogen and ATP are
replenished during sleep.
b. Another idea is that sleep may have evolved to motivate animals to find a safe
place and remain inactive during dangerous times of day.
c. Some researchers suggest that REM sleep is a period in which the brain either
consolidates and strengthens memories or purges superfluous information from
memory.
D. Cognition is the range of mental processes by which we acquire and use knowledge. (p. 541)
1. Cognitive functions are widely distributed over regions of the cerebral cortex called
association areas, which make up 75% of brain tissue.
2. Much of what we know has come from studies of patients with brain lesions; more
recently PET scans and fMRI scans have yielded more sophisticated insights.
a. Parietal lobe lesions can cause people to become unaware of objects, or even
their own limbs, on the opposite side of the body (contralateral neglect
syndrome).
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b. Temporal lobe lesions often result in agnosia, the inability to recognize,
identify, and name familiar objects; prosopagnosia is the inability to remember
familiar faces.
c. Frontal lobe lesions affect qualities we think of as personality and responses
to stimuli.
i. The prefrontal cortex (frontal association area) is the most rostral part
of the frontal lobe and is well developed only in primates, particularly
humans.
ii. Lesions here may produce personality disorders and socially
inappropriate behaviors.
E. Memory is one of the major cognitive functions. (pp. 541–542)
1. Information management by the brain entails learning, memory proper, and forgetting.
a. Forgetting is important in that people with pathological inability to forget
trivial information have difficulty in reading comprehension and separation of
important information from nonimportant.
2. Brain-injured people may be unable to store new information (anterograde amnesia) or
recall things known before the injury (retrograde amnesia)
a. Amnesia refers to defects in declarative memory (recounting) not procedural
memory (actions).
3. The hippocampus of the limbic system is an important memory forming center (Fig.
14.17)
a. The hippocampus does not store memories, but organizes sensory and
cognitive experiences into a unified long-term memory.
b. It learns from sensory input during an experience but is thought to play the
memory repeatedly to the cerebral cortex, a process called memory
consolidation.
c. Long-term memories are stored in different cortical areas: vocabulary in the
superior temporal lobe, plans and social roles in the prefrontal cortex.
d. Lesions of the hippocampus can cause profound anterograde amnesia.
4. Other parts of the brain involved in memory include the cerebellum and the amygdala.
Insight 14.3 The Seat of Personality—A Lesson from an Accidental Lobotomy (Fig. 14.20)
F. Emotional feelings and memories are not exclusively cerebral functions but result from an
interaction between areas of the prefrontal cortex and diencephalon. (pp. 542–543)
1. Emotional control centers of the brain have been identified by studying people with
brain lesions, but interpretation of results is controversial.
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2. The prefrontal cortex is the seat of judgment, intent, and control over expression of
emotions, but emotions and emotional memories arise from deeper regions, expecially
the hypothalamus and amygdala.
3. The amygdala is a major component of the limbic system and received processed
information from the general senses and from vision, hearing, taste, and smell.
a. It is especially important in the sense of fear, but also plays roles in food
intake, sexual behavior, and novel stimuli.
b. Output from the amygdala goes in two directions of interest: (1) to the
hypothalamus and lower brainstem, where it influences somatic and visceral
motor systems; (2) to areas of the prefrontal cortex that mediate conscious
control of emotions.
4. Many important aspects of personality, such as expressions of anger, fear, pleasure,
pain, love, sexuality, and affection, as well as aspects of learning, memory, and
motivation depend on an intact, functional amygdala and hypothalamus.
5. Much of human behavior is shaped by learned associations between stimuli, our
responses, and the results.
a. Certain nuclei in the hypothalamus are involved in feelings of reward and
punishment; one which has been studied in animals is the median forebrain
bundle (MFB).
i. Mammals that can press a pedal to cause electrical stimulation of the
MFB will do so repeatedly even to the point of neglecting food and
water.
ii. Humans with incurable schizophrenia, pain, or epilepsy with
electrode implants that stimulate the MFB also will press a button to
cause stimulation, but do not report feelings of joy or ecstasy—rather a
relief from tension, a relaxed feeling, or no feeling at all.
G. Much of the cerebrum is concerned with the senses: most of the cortex of the insula and of the
parietal, occipital, and temporal lobes. (pp. 543–544)
1. Regions called primary sensory cortex are sites where sensory input is first received
and one becomes conscious of a stimulus.
2. Adjacent to these are association areas where the sensory input is interpreted.
a. Some association areas are multimodal, receiving input from multiple senses;
an example is the orbitofrontal cortex, which receives taste, smell, and visual
input to form an impression of a food.
3. The special senses are limited to the head: vision, hearing, equilibrium, taste, and
smell. (Fig. 14.21)
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a. Vision. Visual signals are received by the primary visual cortex in the
posterior region of the occipital lobe.
i. The visual cortex is bordered anteriorly by the visual association area
that makes up the remainder of the occipital lobe, some of the posterior
parietal lobe, and much of the inferior temporal lobe.
b. Hearing. Auditory signals are received by the primary auditory cortex in the
superior region of the temporal lobe and in the nearby insula.
i. The auditory association area occupies areas of temporal lobe inferior
to the primary audiotyr cortex and deep within the lateral sulcus.
c. Equilibrium. Signals from the inner ear for balance and the sense of motion
project mainly to the cerebellum and several brainstem nuclei concerned with
head and eye movements and visceral functions.
i. Some fibers, however, are routed through the thalamus to association
areas in the roof of the lateral sulcus and lower end of the central
sulcus, which perceive body movements and orientation in space.
d. Taste and smell.
i. Gustatory (taste) signals are received by the primary gustatory cortex
in the inferior end of the postcentral gyrus of the parietal lobe and an
anterior region of the insula.
ii. Olfactory (smell) signals are received by the primary olfactory
cortex in the medial surface of the temporal lobe and inferior surface of
the frontal lobe.
iii. The orbitofrontal cortex is a multimodal association area for both
these senses.
4. The general senses (somesthetic, somatosensory, or somatic senses) are distributed
over the entire body and include touch, pressure, stretch, movement, heat and cold, and
pain.
a. Coming from the head, such signals reach the brain via certain cranial nerves,
notably the trigeminal nerve; from the rest of the body, the signals ascend
sensory tracts of the spinal cord.
i. Both routes decussate to the contralateral thalamus.
b. The thalamus processes the input and selectively relays signals to the
postcentral gyrus. (Fig. 14.22a)
c. The cerebral cortex of the postcentral gyrus is called the primary somesthetic
cortex.
i. Adjacent to this is the somesthetic association area (Fig. 14.21)
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d. Because of the decussation, the primary somesthetic cortex is like an upsidedown sensory map of the contralateral side of the body, traditionally
diagrammed as a sensory homunculus. (Fig. 14.22b)
i. Receptors in the lower limb project to superior and medial parts of
the gyrus, and receptors in the face project to the inferior and lateral
parts.
ii. Point-for-point correspondence between an area of the body and an
area of th CNS is called somatotopy.
iii. Relative sizes of body parts in the sensory homunculus correspond
to the amount of innervation and sensitivity of each part.
H. Motor control involves first the intention to contract a skeletal muscle, which begins in the
motor association (premotor) area of the frontal lobes. (pp. 544–546) (Fig. 14.21)
1. The program for action is then transmitted from the premotor area to neurons of the
precentral gyrus (primary motor area), the most posterior gyrus of the frontal lobe,
immediately anterior to the central sulcus. (Fig. 14.23a)
a. Neurons in the precentral gyrus send signals to the brainstem and spinal cord,
which ultimately results in muscle contraction.
b. The precentral gyrus, like the postcentral one, exhibits somatotopy and can be
mapped as a motor homunculus. (Fig. 14.23b)
i. Relative sizes of body parts in the motor homunculus correspond to
the number of muscles and motor units in each part.
2. The pyramidal cells of the precentral gyrus are called upper motor neurons.
a. Their fibers project caudally with about 19 million fibers ending in nuclei of
the brainstem and 1 million forming the corticospinal tracts.
b. Most of these fibers decussate in the lower medulla oblongata and form the
lateral corticospinal tract on each side of the spinal cord.
c. A smaller number of fibers pass through the medulla without decussation and
form the anterior corticospinal tracts, which cross over lower in the spinal cord.
3. In the brainstem or spinal cord, fibers from the upper motor neurons synapse with
lower motor neurons, the axons of which innervate the skeletal muscles. (Fig. 13.6)
4. The basal nuclei and cerebellum are other important muscle control areas.
a. The basal nuclei determine the onset and cessation of intentional movements;
walking; and highly practiced learned behaviors like typing or tying shoes.
i. The basal nuclei lie in a feedback circuit from the cerebrum to the
basal nuclei to the thalamus and back to the cerebrum.
ii. Nearly all areas of cerebral cortex, except for primary visual and
auditory cortices, send signals to the basal nuclei.
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iii. The basal nuclei process these signals and output to the thalamus,
which relays signals back to the cerebral cortex, notably to motor areas.
iv. Lesions of basal nuclei cause dyskinesias, such as seen in the rigid
movements of Parkinson disease and the exaggerated movements of
Huntington disease.
b. The cerebellum is important in motor coordination, aids in learning motor
skills, maintains muscle tone and posture, coordinates eye and body movements,
and the motions of different joints.
i. Through the middle peduncles, the cerebellum receives information
from the upper motor neurons of the cerebrum about movement
intentions and information about body movement from the eyes and
inner ears.
ii. Through the inferior peduncles, it receives information from
proprioceptors in the muscle and joints about performance. (Fig. 14.24)
iii. The Purkinje cells compare the two, and if there is a discrepancy,
they signal the deep cerebellar nuclei, which issue signals to the
thalamus and lower brainstem and ultimately the motor association area
of the cerebrum and tracts of the spinal cord. (Fig. 14.24)
iv. Output from these areas correct the muscle performance.
v. Lesions of the cerebellum can results in a clumsy, awkward gait
(ataxia).
I. Language includes several abilities—reading, writing, speaking, and understanding words—
assigned to different regions of the cerebral cortex. (pp. 546–548) (Fig. 14.25)
1. The Wernicke area is responsible for the recognition of spoken and written language.
a. It is a multimodal association area that lies posterior to the lateral sulcus,
usually in the left hemisphere, at the crossroad between visual, auditory, and
somesthetic areas.
2. The angular gyrus, part of the pariental lobe caudal and superior to the Wernicke area,
is important in the ability to read and write.
3. When we intend to speak, the Wernicke area formulates phrases and transmits a plan to
the Broca area, located in the inferior prefrontal cortex in the same hemisphere.
a. PET scans show a rise in metabolic activity of the Broca area as we prepare to
speak (Fig. 14.40)
b. The Broca area generates a motor program from muscles in the larynx,
tongue, cheeks, and lips to produce speech and transmits it to the primary motor
cortex.
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4. The emotional aspect of language is controlled by regions in the opposite hemisphere
that mirror the Wernicke and Broca areas.
a. The affective language area lies opposite the Broca area; lesions to this area
results in flat, emotionless speech (aprosody).
b. The cortex opposite Wernicke’s area is concernd with recognizing the
emotional content of another person’s speech; lesions here can result in
problems understanding speech intent, such as a joke.
5. Aphasia is any language deficit resulting from lesions in the hemisphere, usually the
left, containing the Wernicke and Broca areas.
a. Nonfluent (Broca) aphasia results in slow speech, difficulty in chooswing
words, or use of words that only approximate the correct word.
b. Fluent (Wernicke) aphasia results in a person speaking normally and
sometimes excessively, but using jargon and invented words that make little
sense.
c. In anomic aphasia, a person can speak noirmally and understand speech, but
cannot identify written words or pictures.
J. The two cerebral hemispheres look roughly identical but have a number of differences in
function, a difference termed cerebral lateralization. (p. 548)
1. One hemisphere, usually the left, is specialized for spoken and written language and
for sequential and analytical reasoning; it is termed the categorical hemisphere.
2. The other hemisphere perceives information in an integrated, holistic way and is the
seat of imagination, insight, musical/artistic skills, spatial relationships and patterns, and
comparisons of special senses; it is termed the representational hemisphere.
3. Cerebral lateralization is highly correlated with handedness.
a. The left hemisphere is categorical in 96% of right-handed people, and the
right hemisphere in 3%.
b. Among left-handed people, the right hemisphere is categorical in 15% and the
left in 70%; in the remaining 15% neither hemisphere is distinctly specialized.
4. Lateralization develops with age, and in children, one hemisphere can often take over
the functions of the other if it is damaged.
5. Adult males exhibit more lateralization than females and suffer more functional loss of
one hemisphere is damaged.
VI. The Cranial Nerves (pp. 549–560)
A. The brain communicates via 12 pairs of cranial nerves in addition to the major input and output
via the spinal cord. (p. 549)
B. The cranial nerves are numbered I to XII starting with the most rostral pair, and each nerve also
has a descriptive name. (p. 549) (Fig. 14.27)
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C. In terms of cranial nerve pathways, most motor fibers of these nerves begin in nuclei of the
brainstem and lead to glands and muscles. (p. 549)
1. Cranial nerve sensory fibers begin in receptors located mainly in the head and neck and
lead mainly to the brainstem.
2. Sensory fibers for proprioception begin in muscles innervated by motor fibers of the
cranial nerves, but they often travel to the brain in a different nerve than the one
supplying motor innervation.
3. Most cranial nerves carry fibers between the brain stem and ipsilateral receptors and
effectors; the exceptions are the optic nerve (II), where have the fibers decussate, and the
trochlear nerve (IV) in which all efferent fibers lead contralaterally.
D. Cranial nerves are traditionally classified as sensory (I, II, and VIII), motor (III, IV, VI, XI, and
XII), or mixed (V, VII, IX, and X), but in reality, only CN I and CN II (smell and vision) are
purely sensory. (p. 550)
1. Other sensory cranial nerves contain both afferent and efferent fibers and therefore are
mixed.
2. Those traditionally classified as motor not only stimulate muscle but also contain
afferent fibers of proprioception.
3. CN VIII, hearing and equilibrium, is traditionally classified as sensoyr but also has
motor fibers that return signals to the inner ear for fine tuning.
4. The mixed nerves have sensory functions quite unrelated to motor functions, such as
CN VII (facial nerve) has a sensory role in taste and a motor roll in facial expression.
E. The cranial nerves are described individually below. (pp. 550–559) (Table 14.1)
1. The olfactory nerve (CN I), the sensory nerve for the sense of smell, consists of several
separate fascicles that pass independently through the cribiform plate of the ethmoid
bone; these fascicles are severed when the brain is removed from the skull. (Fig. 14.28)
a. Its origin is the olfactory mucosa in the nasal cavity.
b. Its termination is at the olfactory bulbs on each side.
c. If damaged, the sense of smell is impaired; it can be tested by determining
whether a patient can sense aromatic substances.
2. The optic nerve (CN II), the sensory nerve for vision, passes out of the cranium via the
optic foramen. (Fig. 14.29)
a. Its origin is the retina.
b. Its termination is in the thalamus and midbrain.
c. If damaged, blindness occurs in part or all of the visual field; it can be tested
by inspecting the retina and testing peripheral vision and visual acuity.
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3. The oculomotor nerve (CN III), predominantly a motor nerve, controls muscles that
move the eyeball up, down, and medially, as well as those that control the iris, lens, and
upper eyelid; it passes out of the cranium via the superior orbital fissure. (Fig. 14.30)
a. Its origin is the midbrain.
b. Its termination is in somatic fibers to muscles of the eye, with autonomic
fibers entering the eyeball to muscles of the iris and lens.
c. If the nerve is damaged, a patient may have a drooping eyelid, dilated pupil,
inability to move the eye in some directions, tendency of eye to move laterally at
rest, double vision, and/or difficulty focusing; test by looking for difference in
pupil size, response to light, and ability to track moving objects.
4. The trochlear nerve (CN IV), predominantly a motor nerve, controls a muscle that
rotates the eyeball medially and slightly depresses the eyeball when the head turns; it
passes through the superior orbital fissure. (Fig. 14.31)l
a. Its origin is in the midbrain.
b. Its termination is the superior oblique muscle of the eye.
c. If CN IV is damaged, the patient may experience double vision, an inability to
rotate the eye inferolaterally, the eye pointing superolaterally, and may tend to
tilt the head toward the affected side; test the ability of the eye to rotate
inferolaterally.
5. The trigeminal nerve (CN V) is the largest and most important sensory nerve of the
face and has three divisions. (Fig. 14.32)
a. The ophthalmic division (V1) has sensory function in touch, temperature, and
pain sensations from the upper face; it passes through the superior orbital
fissure.
i. Its origin is the superior region of face, surface of eyeball, lacrimal
gland, superior nasal mucosa, and frontal and ethmoid sinuses.
ii. Its termination is the pons.
iii. If this nerve is damaged, sensation is lost from the upper face; it can
be tested by evoking the corneal reflex (blink in response to light touch
to eyeball).
b. The maxillary division (V2) has the same functions as V1, but lower on the
face; it passes through the foramen rotundum and infraorbital foramen.
i. Its origin is the middle region of the face, the nasal mucosa, maxillary
sinus, palate, and upper teeth and gums.
ii. Its termination is the pons.
iii. If it is damaged, sensation is lost from the middle of the face; test
the sense of touch, pain, and temperature.
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c. The mandibular division (V3) is a mixed nerve, with sensory function the
same as V1 and V2 but still lower on the face, and motor function in mastication;
it passes through the foramen ovale.
i. Its sensory origin is the inferior region of face, anterior two0-thirds of
the tongue (but not taste buds), lower teeth and gums, floor of the
mouth, and dura mater; its motor origin is the pons.
ii. Its sensory termination is the pons; its motor termination is the
anterior belly of the digastric, masseter, temporalis, mylohyoid,.and
pterygoid muscles and the tensor tympani muscle of the middle ear.
iii. If it is damaged, the patient may have loss of sensation and impaired
chewing; assess motor functions by palpating masseter and temporalis
while subject clenches teeth and test ability to move mandible side to
side and open mouth.
6. The abducens nerve (CN VI), predominantly a motor nerve, controls a muscle that
turns the eyeball laterally; it passes through the superior orbital fissure. (Fig. 14.33)
a. Its origin is the inferior pons.
b. Its termination is the lateral rectus muscle of the eye.
c. If it is damaged, the patient is not able to turn the eye laterally, and at rest the
eye turns medially; test for lateral eye movement.
7. The facial nerve (CN VII), a mixed nerve, has sensory function in taste and is the
major motor nerve of the facial muscles. It has five prominent branches (temporal,
zygomatic, buccal, mandibular, and cervical); it passes through the internal acoustic
meatus and stylomastoid foramen. (Fig. 14.34)
a. Its sensory origin is the taste buds of the anterior two-thirds of the tongue, and
its motor origin is the pons.
b. Its sensory termination is the thalamus, and its motor terminations are in the
muscles of the jaw, middle ear, and facial muscles, with autonomic fibers to the
salivary glands, tear glands, and nasal and palatine glands.
c. If it is damaged, the patient may have an inability to control facial muscles
and facial sagging due to loss of muscle tone, plus a distorted sense of taste,
especially for sweet flavor; test taste with substances such as sugar, salt, vinegar,
and quinine, test response of tear glands, and test subject’s ability to smile,
frown, etc.
8. The vestibulocochlear nerve (CN VIII), predominantly a sensory nerve, is the nerve of
hearing and equilibrium but also has motor fibers that tune the sense of hearing; it passes
through the internal acoustic meatus. (Fig. 14.35)
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a. Its sensory origin is the cochlea, vestibule, and semicircular ducts of the inner
ear, and its motor origin is the pons.
b. Its sensory terminations consist of fibers for hearing that end in the medulla,
and those for equilibrium that end at the junction of the medulla and pons; its
motor termination is the outer hair cells of the cochlea of the inner ear.
c. If it is damaged, the patient experiences nerve deafness, dizziness, nausea,
loss of balance, and nystagmus (involuntary oscillation of eyes side to side); test
for hearing, balance, and ability to walk, and check for nystagmus.
9. The glossopharyngeal nerve (CN IX) is a complex mixed nerve with numerous sensory
and motor functions in the head, neck, and thoracic regions; it passes through the jugular
foramen. (Fig. 14.36)
a. Its sensory origins include the pharynx, middle and outer ear, posterior onethird of tongue including taste buds, and the internal carotid artery; its motor
origin is in the medulla oblongata.
b. Its sensory termination is the medulla oblongata; its motor terminations
include the parotid gland, glands of the posterior tongue, and the
stylopharyngeal muscle.
c. If it is damaged, a person has impaired swallowing and loss of bitter and sour
tastes; test for the gag reflex, ability to swallow and cough, and note any speech
impediments; also test posterior third of tongue with sour and bitter substances.
10. The vagus nerve (CN X), a mixed nerve, has the most extensive distribution of any
cranial nerve, supplying organs in the head, neck, and most viscera of the thoracic and
abdominopelvic cavities; it passes through the jugular foramen. (Fig. 14.37)
a. Its sensory origin includes the thoracic and abdominopelvic viscera, the root
of the tongue, the pharynx, larynx, epiglottis, outer ear, and dura mater; its
motor origin is the medulla oblongata.
b. It sensory termination is the medulla oblongata; its motor terminations include
the tongue, pharynx, larynx, lungs, heart, liver, spleen, digestive tract, kidney,
and ureter.
c. If it is damaged, a patient may exhibit hoarseness or loss of voice, impaired
swallowing and gastrointestinal motility, and even death of both vagus nerves
are damaged; test palatal movement during speech, abnormalities of swallowing,
weak, hoarse voice, and absence of gag reflex.
11. The accessory nerve (CN XI), predominantly a motor nerve involved in head, neck,
and shoulder actions, is unusual in that it does not arise entirely from the brain but in part
from the cervical spinal cord; it passes through the jugular foramen. (Fig. 14.32)
a. Its origin is the medulla oblongata and spinal cord segments C1 to C6.
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b. Its terminations are in the palate, pharynx, trapezius, and sternocleicomastoid
muscles.
c. if it is damaged, a patient exhibits impaired movement of head, neck, and
shoulders, difficulty shrugging on affected side, and paralysis of
sternocleidomastoid causing the head to turn toward injured side; test ability to
rotate head and shrug shoulders.
12. The hypoglossal nerve (CN XII), predominantly a motor nerve, controls tongue
movements; it passes through the hypoglossal canal. (Fig. 14.39)
a. Its origin is the medulla oblongata.
b. Its termination is in the intrinsic and extrinsic muscles of the tongue.
c. If it is damaged, speech and swallowing are impaired, with deviation of the
tongue toward injured side, with atrophy, or an inability to protrude the tongue if
both left and right nerves are damaged; test movement of tongue.
Cross References
Additional information on topics mentioned in Chapter 14 can be found in the chapters listed below.
Chapter 12: The memory process
Chapter 13: Innervation of skeletal muscles
Chapter 16: Control of pain awareness
Chapter 16: Role of the thalamus in motor and sensory circuits
Chapter 16: Nerve pathways for the special senses
Chapter 17: Interaction of the hypothalamus and pituitary gland
Chapter 17: The pineal gland
Chapter 20: Sinuses of the brain
Chapter 20: Blood vessels that supply and drain the brain
Chapter 26: Temperature regulation mechanisms