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Chapter 14 – THE BRAIN AND CRANIAL NERVES (figures relate to Tortora/Grabowski 9^th edition of Principles of Anatomy and Physiology)

I. INTRODUCTION

A. The brain is the center for registering sensations, correlating them with one another and with stored information, making decisions, and taking action.

1. It is also the center for intellect, emotions, behavior, and memory.

2. It also directs our behavior towards others.

II. OVERVIEW OF BRAIN ORGANIZATION AND BLOOD SUPPLY

A. The principal parts of the brain are the brain stem, diencephalon, cerebrum,
and cerebellum (Figure 14.1).

B. Protective Covering of the Brain

1. The brain is protected by the cranial bones (Figure 7.2) and the
cranial meninges (Figure 14.2).

2. The cranial meninges are continuous with the spinal meninges and are
named dura mater, arachnoid, and pia mater.

C. Blood Flow and the Blood-Brain Barrier

1. Although the brain comprises only about 2% of the total body weight,
it utilizes about 20% of the oxygen used by the entire body. The brain
is one of the most metabolically active organs of the body, and the
amount of oxygen it uses varies with the degree of mental activity.

2. Any interruption of the oxygen supply to the brain can result in
weakening, permanent damage, or death of brain cells. Interruption of
the mother's blood supply to a child during childbirth before it can
breathe may result in paralysis, mental retardation, epilepsy, or death.

3. Because carbohydrate storage in the brain is limited, the supply of
glucose to the brain must be continuous. Glucose deficiency may
produce mental confusion, dizziness, convulsions, and
unconsciousness.

4. A blood-brain barrier (BBB) protects brain cells from harmful
substances and pathogens by sen ing as a selective barrier to prevent
passage of many substances from the blood to the brain.

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5. An injury to the brain due to trauma, inflammation, or toxins causes a breakdown of the BBB, permitting the passage of normally restricted substances into brain tissue. The BBB may also prevent entry of drugs that could be used as therapy for brain cancer or other CNS disorders, so research is exploring ways to transport drugs past the BBB.

III. CEREBROSPINAL FLUID PRODUCTION AND CIRCULATION IN
VENTRICLES

A. Cerebrospinal fluid (CSF) is a clear, colorless liquid that protects the brain
and spinal cord against chemical and physical injuries and carries oxygen,
glucose, and other needed chemicals from the blood to neurons and neuroglia.

B. There are four CSF filled cavities within the brain called ventricles (Figure 14.3).

C. CSF contributes to hemostasis by providing mechanical protection, chemical
protection, and circulation.

D- CSF is formed by filtration from networks of capillaries called choroid piexuses ( found in the ventricles) and circulates through the subarachnoid space, ventricles, and central canal.

E. Materials entering CSF from the choroid capillaries cannot leak between the
surrounding ependymal cells: these constitute the blood-cerebrospinal fluid
barrier. (Figures 14.4. 4.1).

F. If CSF cannot circulate or drain properly due to some obstruction in the
ventricles or subarachaoid space, a condition called hydrocephalus develops.
Surgically draining the ventricles and diverting the flow of CSF by an
implanted shunt can positively and dramatically affect the individual's
prognosis. (Clinical Application)

IV. THE BRAIN STEM

A. Medulla Oblongata (lowest part of brain stem)

1. The medulla oblongata, or just medulla, is continuous with the upper
part of the spinal cord and contains portions of both motor and sensory
tracts (Figures 14.5,14.1).

2. Functional regions include nuclei that are reflex centers for regulation
of heart rate (Figure 21.14). respiratory rate (Figure 23.24),

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vasoconstriction. swallowing, coughing, vomiting, sneezing, and
hiccuping; the first three are considered vital reflexes. *

3. Injury to the medulla can be fatal or lead to serious problems. (Clin. App, 453)

B. Pons

1. The pons is located superior to the medulla. It connects the spinal cord
with the brain and links parts of the brain with one another by way of
tracts (Figures 14.1, 14.5).

2. It relays nerve impulses related to voluntary skeletal (motor)
movements from the cerebral cortex to the cerebellum.

C. Midbrain

1. The midbrain connects the pons and diencephalon. It conveys motor impulses from the cerebrum to the cerebellum and spinal cord, sends sensory impulses from the spinal cord to the thalamus, and regulates auditory and visual reflexes ( Figures 14.1, 14.5, 14.7).

2. A large ponion of the brain stem is called the reticular formation (Figure 14.7).

a. It consists of small areas of gray matter interspersed among
fibers of white matter and has both sensory and motor
functions.

b. It helps regulate muscle tone, alerts the cortex to incoming
sensory signals (reticular activating system, or RAS) and is
responsible for maintaining consciousness and awakening from
sleep.

D. The functions of the brain stem are summarized in Table 14.1.

V. THE CEREBELLUM - 2^nd largest

A. The cerebellum occupies the inferior and posterior aspects of the cranial
cavity and consists of two hemispheres (Figures 14.1, 14.4, 14.8).

B. The cerebellum functions in the coordination of skeletal muscle contractions
and in the maintenance of normal muscle tone, posture, and balance

VI. THE DIENCEPHALON

A. Thalamus

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The thalamus is located superior to the midbrain and contains nuclei
that serve as relay stations for all sensory impulses, except smell, to the cerebral cortex (Figure 14.9).

1. It also registers conscious recognition of pain and temperature and
some awareness of light touch and pressure.

2. It plays an essential role in awareness and the acquisition of
knowledge, which is termed cognition.

B. Hypothalamus

1. The hypothalamus is found inferior to the thalamus, has four major regions, controls many body activities, and is one of the major regulators of homeostasis (Figure 14.10, Table 14.1 ). 2. The hypothalamus has a great number of functions.

a. It functions in regulation of emotional and behavioral patterns. b. It regulates eating and drinking through the feeding center. satiety center, and thirst center.

c. it aids in controlling body temperature.

d. It regulates circadian rhythms and states of consciousness.

C. Table 14.l summarizes the functions of the parts of the diencephalon.
D. Circumventricular Organs

1. Parts of the diencephalon, called circumventricular organs (CVOs).
can monitor chemical changes in the blood because they lack a blood-
brain barrier.

2. They are also thought to be the site of entry into the brain of HIV.
VII. THE CEREBRUM

A. The cerebrum is the largest part of the brain (Figure 14.11).

1. The surface layer, the cerebral cortex, is 2-4 mm thick and is
composed of gray matter. The cortex contains billions of neurons.

2. The cortex contains gyri (covolutions), deep grooves called fissures,
and shallower sulci.

3. Beneath the cortex lies the cerebral white matter, tracts that connect
parts of the brain with itself and other parts of the nervous system.

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B. The cerebrum is nearly separated into right and left halves, called
hemispheres, by the longitudinal fissure. Internally it remains connected by
the corpus callosum, a bundle of transverse white fibers.

C. Lobes

1. Each cerebral hemisphere is further subdivided into four lobes by sulci

or fissures. 2. The cerebral lobes are named the frontal, parietal, temporal, and

occipital.

D. White Matter

1 . The white matter is under the cortex and consists of myelinated axons

E. Basal Ganglia

1 . The basal ganglia are paired masses of gray matter in each cerebral

hemisphere, in which many nerve fibers interconnect the nuclei (Fig. 14.13). 2. They are responsible helping to control muscular movements.

F. Limbic System

1 . The limbic system is found in the cerebral hemispheres and diencephalon

(Figure 14.14).

2 . It functions in emotional aspects of behavior and memory, and is associated

with pleasure and pain.

G- Brain Injuries

1 . Lapse in memory is one of many effects resulting from brain injuries; brain injuries are commonly associated with head injuries and result, in part, from displacement and distortion of neuronal tissue at the moment of impact and in part from the release of disruptive chemicals from injured brain cells.

2. Various degrees of brain injury are described by the terms concussion, contusion, and laceration.

VIII. FUNCTIONAL ASPECTS OF THE CEREBRAL CORTEX

A. Specific types of sensory, motor, and integrative signals are processed in certain cerebral regions (Figure 14.15). 1 . Sensory Areas

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a. The sensory areas of the cerebral cortex are concerned with the
reception and interpretation of sensory impulses.

b. Some important sensory areas include the primary visual,
auditory, and gustatory areas.

2. Motor Areas

a. The motor areas are the regions that govern muscular movement.

b. Two important motor areas are the primary motor area and
Broca's speech area.

3. Association Areas

a. The association areas are concerned with complex integrative
functions such as memory, emotions, reasoning, will,
judgment, personality traits, and intelligence.

b. Association areas include the somatosensory association area,
visceral association area, auditory association area. Wernicke's
(posterior language) area, common integrative area, premotor
area, frontal eye field area, and language areas.

4. Table 14.1 summarizes the various functions of the cerebrum.

5. Injury to the motor speech or association areas results in aphasia, an

inability to use or comprehend words, respectively. (Clin. App.)

B. Hemispheric Lateralization

1. Two hemispheres of the cerebrum are not bilaterally symmetrical, either anatomically or functionally, with the functional asymmetry called hemispheric lateralization(Figure 14.16).

2. The left hemisphere is more important for right-handed control, spoken and written language, and numerical and scientific skills.

3. The right hemisphere is more important for left-handed control, musical and artistic awareness, space and pattern perception, insight, imagination, and generating mental images of sight, sound, touch, taste, and smell.

C. Brain Waves

1. Electrical potentials generated by brain cells are called brain waves.

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2. Brain waves generated by the cerebral cortex are recorded as an
electroencephalogram (EEG Figure 14.17).

3. An EEG may be used to diagnose epilepsy and other seizure disorders,
infectious diseases, tumors, trauma, hematomas, metabolic
abnormalities, degenerative diseases, and periods of unconsciousness
and confusion; it may also provide useful information regarding sleep
and wakefulness.

4. An EEG may also be one criterion in confirming brain death (complete
absence of brain waves in two EEGs taken 24 hours apart).

5. Figure 14.17 shows four kinds of brain waves that can be recorded
from normal individuals.

IX. CRANIAL NERVES

A. Twelve pairs of cranial nerves originate from the brain. (Figure 14.5)

B. Table 14.2 presents a summary of cranial nerves, including clincal
applications related to their dysfunction.

X. AGING AND THE NERVOUS SYSTEM

A. Age-related effects involve loss of neurons and decreased capacity for sending nerve impulses to and from the brain; processing of information also diminishes.

B. Other effects include decreased conduction velocity, slowing of voluntary

motor movements, and increased reflex time.

C. Degenerative changes and disease states involving the sense organs can alter

vision, hearing, taste, smell, and touch.

XII. DISORDERS: HOMEOSTATIC IMBALANCES

A. The most common brain disorder is a cerebrovascular accident (CVA or

Stroke).

1. CVAs are classified into two principal types: ischemic ( the most
common type), due to a decreased blood supply (and therefore O2), or
hemorrhagic. due to a blood vessel in the brain that bursts.

2. Common causes of CVAs are intracerebral hemorrhage, emboli, and
atherosclerosis.

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3. CVAs are characterized by abrupt onset of persisting neurological symptoms that arise from destruction of brain tissue (infarction).

B. A transient ischemic attack (TIA) is an episode of temporary cerebral
dysfunction caused by impaired blood flow to the brain, (and therefore, low o2)

1. Symptoms include dizziness, weakness, numbness, or paralysis in a
limb or in half of the body: drooping of one side of the face; headache;
slurred speech or difficulty understanding speech; or a partial loss of
vision or double vision.

2. Onset is sudden and a TIA usually persists for only a few minutes,
rarely lasting as long as 24 hours.

3. Causes of the impaired blood flow include blood clots, atherosclerosis,
and certain blood disorders; TIAs commonly are forerunners of future
CVAs.

C. Alzheimer 's disease (AD) is a disabling neurological disorder that afflicts
about 11% of the population over age 65.

1. Its causes are unknown, its effects are irreversible and devastating, and
it has no cure at the present time.

2. It involves widespread intellectual impairment, personality changes,
sometimes delirium and culminates in dementia, the loss of reason
and ability to care for oneself.

3. A person with AD usually dies of some complication that affects
bedridden patients, such as pneumonia.

4. Brains of AD victims show three distinct structural abnormalities:

a. Great loss of neurons in specific regions (e.g.. hippocampus
and cerebral cortex).

b. Plagues of abnormal proteins deposited outside neurons
(amyloid plaques) .

c. Tangled protein filaments within neurons (neurofibrillary
tangles).