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Saladin A&P, 8e Extended Chapter Outline

Chapter 14 The Brain and Cranial Nerves

I. Overview of the Brain (pp. 505–509)

A. The brain has changed a great deal over evolutionary time. In humans, the size of the brain is

proportional to body size, not to intelligence. (p. 505)

B. The brain has been assigned major landmarks as reference points for its study. (pp. 505–507)

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’s 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 occupies the posterior cranial fossa inferior to the cerebrum.

It is the second-largest region of the brain, constituting 10% of its volume but

containing over 50% of the brain’s neurons. (Fig. 14.1b, c)

c. The brainstem is all of the brain except the cerebrum and cerebellum.

i. Its major components, rostral to caudal, are the diencephalon,

midbrain, pons, and medulla oblongata. (Fig. 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. 507)

(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.

b. Deeper masses called nuclei are surrounded by white matter.

Saladin Outline Ch.14

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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. (p. 507) (Fig. 14.3)

1. The nervous system develops from ectoderm.

a. Early in the third week of development, a neural plate forms along the dorsal

midline of the embryo.

b. The neural plate sinks and its edges thicken, forming a neural groove with a

raised neural fold along each side.

c. The neural folds then fuse along the midline, beginning in the cervical (neck)

region and progressing in both directions.

d. By day 26 of development, this process creates a hollow channel called the

neural tube.

e. The neural tube separates from the overlying ectoderm, sinks deeper, and

grows lateral processes that later form motor nerve fibers.

f. The lumen becomes a fluid-filled space that later constitutes the central canal

of the spinal cord and ventricles of the brain.

2. 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 and ganglia and Schwann cells;

and some other structures of the skeletal, integumentary, and endocrine systems.

3. By the fourth week, the neural tube exhibits three primary vesicles: the forebrain

(prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon).

(Fig. 14.4)

4. 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 remains 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. 509–514)

A. The brain is enveloped in three connective tissue membranes, the meninges, which lie between

the nervous tissue and bone. (pp. 509–510)

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 sheath.

b. The dura mater is pressed closely against the cranial bone, but is not attached

except in limited places (around the foramen magnum, the sella turcica, the

crista galli, and the 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. 14.5)

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 the contours

of the brain and sulci.

B. The brain has four internal chambers called ventricles that are filled with cerebrospinal fluid.

(pp. 510–514) (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 choroid

plexus.

a. Ependyma is a type of neuroglia that resembles cuboidal epithelium. It lines

the ventricles and canals, covers the choroids plexuses, and produces

cerebrospinal fluid.

Insight 14.1 Meningitis

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.

c. 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.

Insight 14.2 Hydrocephalus

d. 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 on the brain and

spinal cord surface.


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ii. CSF is reabsorbed by arachnoid granulations protruding through the

dura mater into the superior sagittal sinus.

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.

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

regulates the brain’s chemical environment.

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. 514)

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.

a. A 10-second interruption in blood flow can cause loss of consciousness; an

interruption of 1 to 2 minutes can impair function; and 4 minutes without blood

causes 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 throughout the brain tissue are one point of entry. The

brain 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 formation 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 choroid plexuses are another point of entry. Here, the brain is protected

by the blood–CSF barrier formed by tight junctions between the ependymal

cells.

i. Tight junctions are absent from ependymal cells elsewhere, allowing

exchange between brain and CSF.

3. 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.


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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 human immunodeficiency virus

(HIV).

III. The Hindbrain and Midbrain (pp. 514–520)

A. Beginning caudally, the medulla oblongata of the adult hindbrain differentiates from the

embryonic myelencephalon. (pp. 514–515)

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 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 third-

order fibers that continue to the cerebral cortex.

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 tectospinal tract controls the neck muscles.

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.


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b. Motor functions include chewing, salivation, swallowing, gagging, vomiting,

respiration, speech, coughing, sneezing, sweating, cardiovascular and

gastrointestinal control, and head, neck, and shoulder movements.

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: vestibulocochlear (CN VIII),

glossopharyngeal (CN IX), vagus (CN X), and hypoglossal (CN XII) nerves. (Table 14.1)

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.

(p. 515) (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. 14.8b)

2. The pons has continuations of the reticular formation, medial lemniscus, and

tectospinal 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 and 14.8) (pp. 515–519)

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 brainstem. 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 that connect the cerebrum to the

pons and carry the corticospinal nerve tracts.

4. Surrounding the cerebral aqueduct is the central (periaqueductal) gray matter. 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

brainstem. (pp. 518–519) (Fig. 14.9)

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 for 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. It is also 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 extrathalamic

cortical 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. 519–520) (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 connects to the medulla oblongata.

b. A pair of middle peduncles connects to the pons.

c. A pair of superior peduncles connects to the midbrain.

5. Most spinal input enters by way of the inferior peduncles; most input from the rest of

the brain enters by way of the middle peduncles; and most cerebellar 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 the

prediction of trajectories of moving objects.

c. Cerebellar lesions may impair a person’s ability to judge differences in pitch

between two tones, and language output may be affected.

d. People with cerebellar lesions also have difficulty planning and scheduling

tasks, a tendency to overreact, and have difficulty with impulse control.

i. Many children with attention-deficit/hyperactivity disorder (ADHD)

have abnormally small cerebellums.


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IV. The Forebrain (pp. 521–527)

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. 521)

B. The diencephalon, which encloses the third ventricle, has three major derivatives; the thalamus,

hypothalamus, and epithalamus. (pp. 521–523)

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, 14.17)

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 classified into five main 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 basal nuclei

(deep cerebral motor centers).

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. 14.2a)

a. Each mammillary body contains three to four mammillary nuclei that relay

signals from the limbic system to the thalamus.

b. The pituitary gland 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 endocrine and autonomic

nervous systems 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 are concerned

with labor contractions, lactation, and water conservation.


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ii. Autonomic effects. The hypothalamus is an integrating center for the

autonomic nervous system and influences heart rate, blood pressure,

and other visceral functions.

iii. Thermoregulation. The hypothalamic thermostat consists of a

collection of neurons, mainly in the preoptic nucleus, that monitor body

temperature.

iv. Food and water intake. The hypothalamus regulates sensations of

hunger and satiety. It has receptors for hormones that increase or

decrease hunger and energy expenditure. Its osmoreceptors monitor

blood osmolarity and stimulate behavioral and hormonal changes when

the body is dehydrated.

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 is superior to the optic chiasm

and 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 and sexual response. 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

(which serves as a relay from the limbic system to midbrain), and a thin roof over the third

ventricle. (Fig. 14.2a)

D. The cerebrum develops from the embryonic telencephalon; it is the largest and most

conspicuous part of the human brain. (pp. 523–527)

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 frontal 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.


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ii. The parietal lobe forms the uppermost part of the brain, underlies the

parietal bone, and extends caudally to the parieto–occipital sulcus. It is

involved in receiving and interpreting signals of the general senses and

some visual processing. (Fig. 14.2)

iii. The occipital lobe is at the rear of the head, caudal to the parieto–

occipital 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 the 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. The cerebral 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.

(Fig. 14.14)

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 brainstem and spinal cord.

ii. Superior to the brain stem, the projection tracts form a broad, dense

sheet called the internal capsule, then radiate in a fanlike array,

forming the corona radiata, to specific areas of the cortex.

b. Commissural tracts cross from one hemisphere to the other through

commissures.

i. Most pass through the corpus callosum. (Fig. 14.2)

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.


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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. 14.6c)

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 axon

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. About 90% of the human cerebral cortex is a six-layered tissue called

neocortex because of its recent evolutionary origin (about 600 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 in vertebrate evolution 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 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. 14.16)


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a. Its most prominent components are the cingulate gyrus, which arches over the

corpus callosum; the hippocampus in the medial temporal lobe; and the

amygdala immediately 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; one 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.

6. The basal nuclei are masses of cerebral gray matter buried in the white matter, lateral

to the thalamus. (Fig. 14.17)

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. They are collectively called the corpus striatum.

ii. The putamen and globus pallidus are collectively called the lentiform

nucleus.

V. Integrative Functions of the Brain (pp. 527–538)

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 involve

interactions between the cerebral cortex and other regions such as the basal nuclei, brainstem, and

cerebellum. (p. 527)

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. (p. 527–529) (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.


15

b. They are suppressed during sensory stimulation and mental tasks, and are

absent during deep sleep.

3. Beta (β) waves have a frequency of 14 to 30 Hz and occur in the frontal to parietal

region.

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 can be defined as a temporary state of unconsciousness from which one can awaken

when stimulated. (pp. 529–530)

1. Sleep is one of many bodily functions that occur in cycles called circadian rhythms.

a. Sleep 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 those 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.


16

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.

a. The parasympathetic nervous system is very active during REM sleep,

causing constriction of the pupils and erection of the penis or clitoris, 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. Some nerve fibers from the eyes go to the suprachiasmatic nucleus (SCN),

which uses the input to synchronize body rhythms with the external rhythm of

night and day.

i. The SCN does not induce sleep or waking, but regulates the time of

day that a person sleeps.

b. The lateral and posterior hypothalamus produce neuropeptides called orexins

that act as an important “sleep switch.” They stimulate wakefulness and elevate

the metabolic rate. Orexin levels are low or absent in narcolepsy.

6. Scientists know little about the purposes of sleep and dreaming, except that non-REM

sleep seems to have a restorative effect on the body and sleep deprivation can lead to

death in experimental animals.

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.

(pp. 530–531)

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).


17

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.

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. (p. 531)

1. Information management by the brain entails learning, memory proper, and

forgetting. a. Forgetting is important in that people with a 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. 14.16)

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 and 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 resides

in the superior temporal lobe, and 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.

F. Emotional feelings and memories are not exclusively cerebral functions, but result from an

interaction between areas of the prefrontal cortex and diencephalon. (pp. 531–532)

1. Emotional control centers of the brain have been identified by studying people with

brain lesions, but interpretation of results is controversial.

2. The prefrontal cortex is the seat of judgment, intent, and control over the expression of

emotions, but emotions and emotional memories arise from deeper regions, especially

the hypothalamus and amygdala.


18

3. The amygdala is a major component of the limbic system and receives 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 drawing our attention to novel stimuli.

b. Output from the amygdala goes in two directions of interest:

i. Some goes to the hypothalamus and lower brainstem, where it

influences somatic and visceral motor systems.

ii. Other input goes 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 parental 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 suffering from incurable schizophrenia, pain, or epilepsy

and that have 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 or a relaxed feeling.

G. Much of the cerebrum, most of the cortex of the insula and of the parietal, occipital, and

temporal lobes, is concerned with the senses. (pp. 532–533)

1. Regions called primary sensory cortex are the 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. 14.20)

a. Vision. Visual signals are received by the primary visual cortex in the

posterior region of the occipital lobe.


19

i. The visual cortex is bordered anteriorly by the visual association area

that includes 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 the temporal lobe

inferior to the primary auditory cortex and deep within the lateral

sulcus.

c. Equilibrium. Signals from the inner ear for equilibrium project mainly to the

cerebellum and several brainstem nuclei that are 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 the lower end of the central

sulcus, which perceive body movements and orientation in space.

d. Taste and smell. The primary gustatory cortex and the primary olfactory

cortex receive signals for taste and smell, respectively.

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 (somatosensory, somesthetic, or somatic) senses are distributed over the

entire body and include touch, pressure, stretch, movement, heat, 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. 14.21)

c. The cerebral cortex of the postcentral gyrus is called the primary

somatosensory cortex.

i. Adjacent to this is the somatosensory association area. (Fig. 14.20)

d. Because of the decussation, the right postcentral gyrus receives input from the

left side of the body and vice versa. The primary somatosensory cortex is like


20

an upside-down sensory map of the contralateral side of the body, traditionally

diagrammed as a sensory homunculus. (Fig. 14.21)

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 the 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. The intention to contract a skeletal muscle begins in the motor association (premotor) area of

the frontal lobes. (pp. 533–535) (Fig. 14.20)

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. 14.22a)

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. 14.22b)

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,

the repetitive hip and shoulder movements of walking, and highly practiced

learned behaviors like writing 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 the primary visual and

auditory cortices, send signals to the basal nuclei.


21

iii. The basal nuclei process these signals and issue output to the

thalamus, which relays signals back to the midbrain and cerebral

cortex.

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. It aids in learning motor

skills, maintains muscle tone and posture, coordinates eye and body movements,

and helps to coordinate 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 muscles and joints about performance.

(Fig. 14.23)

iii. The Purkinje cells compare the two, and if there is a discrepancy,

they signal the deep cerebellar nuclei. These nuclei then issue signals to

the thalamus and lower brainstem, and ultimately to the motor

association area of the cerebrum and the reticulospinal and

vestibulospinal tracts of the spinal cord. (Fig. 14.23)

iv. Output from these areas corrects the muscle performance to match

the intent.

v. Lesions of the cerebellum can results in a clumsy, awkward gait

(ataxia).

I. Language includes several abilities—reading, writing, speaking, signing, and understanding

words—assigned to different regions of the cerebral cortex. (pp. 535–537) (Fig. 14.24)

1. The Wernicke area is responsible for the recognition of spoken and written language.

a. It lies posterior to the lateral sulcus, usually in the left hemisphere, at the

crossroad between visual, auditory, and somatosensory areas.

2. The angular gyrus, part of the parietal 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)


22

b. The Broca area generates a motor program for the muscles in the larynx,

tongue, cheeks, and lips to produce speech and transmits it to the primary motor

cortex.

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

result in flat, emotionless speech (aprosody).

b. The cortex opposite Wernicke’s area is concerned 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 choosing

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 normally 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. 540) (Fig. 14.25)

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 and 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 4%.

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 if

one hemisphere is damaged.

VI. The Cranial Nerves (pp. 539–549)


23

A. The brain communicates with the rest of the body via 12 pairs of cranial nerves, in addition to

the major input and output via the spinal cord. (p. 539)

B. The cranial nerves are numbered I to XII starting with the most rostral pair, and each nerve also

has a descriptive name. (p. 539) (Fig. 14.26)

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. 539)

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 brainstem and ipsilateral receptors and

effectors. The exceptions are the optic nerve (II), where half of the fibers decussate, and

the trochlear nerve (IV), in which all efferent fibers lead to a muscle of the contralateral

eye.

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. 539)

1. Other sensory cranial nerves contain both afferent and efferent fibers and therefore are

mixed nerves.

2. Those traditionally classified as motor not only stimulate muscle but also contain

sensory fibers of proprioception.

3. CN VIII, concerned with hearing and equilibrium, is traditionally classified as sensory,

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 role in facial expressions.

E. The cranial nerves are described individually below. (pp. 539–549) (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. 14.27)

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. 14.28)

a. Its origin is the retina.


24

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.

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. 14.29)

a. Its origin is the midbrain.

b. Its termination is in somatic fibers to muscles of the eye, with autonomic

fibers entering the eyeball and leading 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. It can be tested by looking for

difference in pupil size and shape, response to light, and ability to track moving

objects.

4. The trochlear nerve (CN IV), predominantly a motor nerve, controls a muscle that

directs the vision slightly downward and rotates the eyeball medially. It passes through

the superior orbital fissure. (Fig. 14.30)

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 and a

weakened ability to look downward. The eye points superolaterally, resulting in

a tendency to tilt one’s head toward the affected side. It can be tested by asking

the subject to tilt the head toward one shoulder; the affected eye will show

upward deviation when the head is tilted toward that side.

5. The trigeminal nerve (CN V) is the largest and most important sensory nerve of the

face and has three divisions. (Fig. 14.31)

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 the eyeball,

lacrimal gland, superior nasal mucosa, and the 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).


25

b. The maxillary division (V2) has the same functions as V1, but lower on the

face. It passes through the foramen rotundum and the infraorbital foramen.

i. Its origin is the middle region of the face, the nasal mucosa, maxillary

sinus, palate, and the upper teeth and gums.

ii. Its termination is the pons.

iii. If it is damaged, sensation is lost from the middle of the face. It can

be tested by challenging the senses of touch, pain, and temperature.

c. The mandibular division (V3) is a mixed nerve, with sensory functions 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, the anterior two-thirds

of the tongue (but not taste buds), lower teeth and gums, floor of the

mouth, and the 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. Motor functions can be assessed by palpating the masseter

and temporalis while the subject clenches teeth to test the ability to

move the mandible from side to side and open the mouth against

resistance.

Insight14.3 Some Cranial Nerve Disorders

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. 14.32)

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. It can be tested by assessing lateral eye movement.

7. The facial nerve (CN VII), a mixed nerve, has sensory function in taste, is the major

motor nerve of the facial muscles, and also functions in the secretion of tears, saliva, and

nasal and oral mucus. It has five prominent branches (temporal, zygomatic, buccal,

mandibular, and cervical). It passes through the internal acoustic meatus and the

stylomastoid foramen. (Fig. 14.33)

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

digastric muscle, stapedius muscle of the middle ear, and muscles of facial


26

expression, with autonomic fibers to the submandibular and sublingual 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. Substances such as sugar, salt, vinegar, and quinine

can be used to test the anterior two-thirds of the tongue; ammonia fumes can test

the response of the tear glands; and control of facial muscles can be tested by

assessing the 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 it also has motor fibers that tune the sense of hearing. It

passes through the internal acoustic meatus. (Fig. 14.34)

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). It

can be tested by assessing hearing, balance, and the ability to walk a straight

line; also 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. 14.35)

a. Its sensory origins include the pharynx, middle and outer ear, posterior one-

third of the 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. It can be tested by assessing the gag reflex, the ability to swallow and

cough, and the presence of any speech impediments; sour and bitter substances

can be used to test the posterior third of the tongue.

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. 14.36)


27

a. Its sensory origin includes the thoracic and abdominopelvic viscera, the root

of the tongue, the pharynx, larynx, epiglottis, outer ear, and the dura mater; its

motor origin is the medulla oblongata.

b. It sensory termination is the medulla oblongata; its motor terminations include

the tongue, palate, 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 if both vagus nerves are

damaged. It can be tested by examining palatal movement during speech, and

checking for abnormalities of swallowing, the presence of a weak hoarse voice,

the inability to cough forcefully, and the absence of the 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 from the brain but from the

cervical spinal cord. It passes through the jugular foramen. (Fig. 14.37)

a. Its origin is spinal cord segments C1 to C6.

b. Its terminations are in the palate, pharynx, trapezius, and sternocleidomastoid

muscles.

c. If it is damaged, a patient exhibits impaired movement of the head, neck, and

shoulders, difficulty shrugging on affected side, and paralysis of the

sternocleidomastoid, causing the head to turn toward the injured side. It can be

tested by assessing the ability to rotate the head and shrug the shoulders against

resistance.

12. The hypoglossal nerve (CN XII), predominantly a motor nerve, controls tongue

movements. It passes through the hypoglossal canal. (Fig. 14.38)

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, the effects include impaired speech and swallowing, an

inability to protrude the tongue if both left and right nerves are damaged, and

deviation of the tongue toward the injured side, with atrophy, if only one nerve

is damaged. It can be tested by assessing deviations of the tongue as it is

protruded and retracted.

F. Due to the great number of parts, the nervous system is highly subject to malfunctions.

(Table 14.2)

Insight 14.4 Images of the Mind

Cross ReferencesAdditional information on topics mentioned in Chapter 14 can be found in the chapters listed below.


28

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

saladin 5e extended outline · web viewand that have electrode implants that stimulate the mfb,...

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