Chapter 14 - Outline
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.
B.
In
this chapter we will consider the principal parts of the brain, how the brain
is protected and nourished, and how it is related to the spinal cord and to the
12 pairs of cranial nerves.
II. OVERVIEW OF BRAIN ORGANIZATION AND
BLOOD SUPPLY
A. The major parts of the brain are the brain
stem, diencephalon, cerebrum, and cerebellum.
B.
Blood
Flow and the Blood-Brain Barrier
1.
Blood
flows to the brain mainly via blood vessels that branch from the cerebral
arterial circle (circle of Willis) at the base of the brain (Ch. 21); the veins
that return blood from the head to the heart are also seen in Ch. 21.
2.
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.
3.
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.
4.
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.
5.
A
blood-brain barrier (BBB) protects brain cells from harmful
substances and pathogens by serving as a selective barrier to prevent passage of
many substances from the blood to the brain.
6.
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.
C.
Protective
Covering of the Brain
1.
The
brain is protected by the cranial bones and the cranial meninges.
2.
The
cranial meninges are continuous with the spinal meninges and are named dura
mater, arachnoid, and pia
mater.
3.
Three
extensions of the dura mater separate parts of the
brain: the falx cerebri,
falx cerebelli, and the
tentorium cerebelli.
III. CEREBROSPINAL FLUID
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.
C.
CSF
contributes to homeostasis by providing mechanical protection, chemical
protection, and circulation.
D. CSF is formed by filtration from
networks of capillaries called choroid plexuses (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, which permits certain substances to enter the fluid but excludes
others and protects the brain and spinal cord from harmful elements.
F.
Most
of the fluid is absorbed by the arachnoid villi of the superior sagittal
blood sinus; this absorption normally occurs at the same rate at which
CSF is produced in the choroid plexuses, thereby maintaining a relatively constant
CSF volume and pressure.
G. If CSF cannot circulate or drain
properly due to some obstruction in the ventricles or subarachnoid space, a
condition called hydrocephalus develops. The fluid buildup that occurs
causes increased pressure on the brain, either internally or externally,
depending on where the blockage is present. Surgically draining the ventricles
and diverting the flow of CSF by an implanted shunt can positively and dramatically
affect the individual’s prognosis.
IV. THE BRAIN STEM
A. Medulla Oblongata
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.
2.
It
also contains the nuclei of origin for cranial nerves VIII (cochlear and vestibular
branches) through XII.
3.
Structural
regions of the medulla include the pyramids and the inferior olivary nucleus.
a.
Decussation of pyramids results in neurons in the left cerebral cortex
controlling skeletal muscles on the right side of the body and neurons in the
right cerebral cortex controlling skeletal muscles on the left side.
b.
Inferior
olivary neurons relay impulses from proprioceptors to the cerebellum.
4.
Functional
regions include nuclei that are vital reflex centers for regulation of heart
rate and vasoconstriction (cardiovascular center), respiratory rate (medullary rhythmicity center),
and non-vital reflex centers for swallowing, coughing, vomiting, sneezing, and hiccupping.
5.
Injury
to the medulla can be fatal or lead to serious problems.
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.
2.
It
relays nerve impulses related to voluntary skeletal movements from the cerebral
cortex to the cerebellum.
3.
The
pons also contains the pneumotaxic
and apneustic areas, which help control
respiration along with the medullary rhythmicity center in the medulla.
4.
It
contains nuclei for cranial nerves V through VII and the vestibular branch of
VIII.
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.
2.
Structures
within the midbrain include the cerebral peduncles, the tectum (containing the superior and inferior colliculi, the left and right substantia
nigra, and the left and right red nuclei. You
should know the locations and functions of these structures as discussed in
lecture.
3.
It
also contains nuclei of origin for cranial nerves III and IV.
D. Reticular Formation and Medial Lemniscus run through all 3 parts of the brainstem.
1. Reticular Formation
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.
2. Medial Lemniscus
a.
The axons of the cuneate and gracile
fasciculi ascend through the brain stem to the thalamus as a band of white matter
called the medial lemniscus.
V. THE CEREBELLUM
A. The cerebellum occupies the
inferior and posterior aspects of the cranial cavity and consists of two hemispheres and
a central, constricted vermis.
B.
It
is attached to the brain stem by three pairs of cerebellar
peduncles - inferior attach to
medulla, middle to the pons, and superior to the
midbrain.
C.
The
cerebellum functions in the coordination of skeletal muscle contractions and in
the maintenance of normal muscle tone, posture, and balance. Refer to our
discussion of cerebellar functions in your lecture
notes.
VI. THE DIENCEPHALON
A. Thalamus
1.
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. There are seven major
groups of thalamic nuclei on each side.
2.
It
also registers conscious recognition of pain and temperature and some awareness
of light touch and pressure.
3.
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, controls many body
activities, and is one of the major regulators of homeostasis.
2.
The
hypothalamus has a great number of functions.
a.
Controls
the autonomic nervous system.
b.
Produces
hormones.
c.
It
functions in regulation of emotional and behavioral patterns.
d.
It
regulates eating and drinking through the feeding center, satiety center, and
thirst center.
e.
It
aids in controlling body temperature.
f.
It
regulates circadian rhythms and states of consciousness.
C.
Epithalamus
1.
The
epithalamus lies superior and posterior
to the thalamus and contains the pineal gland and the habenular nuclei.
2.
The
pineal gland secretes melatonin to influence diurnal cycles in
conjunction with the hypothalamus.
3.
The
habenular nuclei are involved in
olfaction, especially emotional responses to odors.
D. Circumventricular Organs
1. These parts of the diencephalon lie in the
wall of the third ventricle and monitor chemical changes in the blood because
they lack a blood-brain barrier.
2.
They coordinate homeostatic activities of the endocrine and nervous systems and
include part of
the hypothalamus, pineal gland, pituitary and other nearby structures.
VII. THE CEREBRUM
A. The cerebrum is the largest
part of the brain.
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 (convolutions), deep
grooves called fissures, and shallower sulci.
3.
Deep
to the cortex lies the cerebral white matter, tracts
that connect parts of the brain with itself and other parts of the nervous
system.
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.
3.
A
fifth part of the cerebrum, the insula, lies deep to the parietal, frontal, and temporal lobes and
cannot be seen in an external view of the brain.
D. White Matter
1.
The
white matter is deep to the cortex and consists of myelinated
axons running in three principal directions.
2.
Association fibers connect and transmit nerve impulses between gyri
in the same hemisphere.
3.
Commissural fibers connect gyri in one cerebral hemisphere to the
corresponding gyri in the opposite hemisphere.
4.
Projection fibers form ascending and descending tracts that transmit impulses from the
cerebrum to other parts of the brain and spinal cord.
E.
Basal
Ganglia
1.
The
basal ganglia are paired masses of gray matter in each cerebral hemisphere.
2.
They
are responsible for helping to control subconscious muscular movements,
regulating muscle tone, and helping regulate the initiation and termination of
movements.
F.
Limbic
System
1.
The
limbic system is found in the cerebral hemispheres and diencephalon.
2.
It
functions in emotional aspects of behavior and memory, and is associated with
pleasure and pain. It is also involved in olfaction which explains why memories
are easily triggered by smells associated with the original stimulus.
A. Specific types of sensory, motor,
and integrative signals are processed in certain cerebral regions.
1.
Sensory
Areas
a.
The
sensory areas of the cerebral cortex are concerned with the reception
and interpretation of sensory impulses. The size of the cortical area receiving
impulses from a particular part of the body corresponds to the number of
receptors present there.
b.
Some
important sensory areas include the primary somatosensory
area, primary visual area, primary auditory area, and primary gustatory area
and primary olfactory area. You should refer to your lecture notes for our
discussion of the functions of these areas.
2.
Motor
Areas
a.
The
motor areas are the regions that govern muscular movement.
b.
Important
motor areas are the primary motor area and Broca’s
speech area. You should refer to your lecture notes for our discussions of the
functions of these areas.
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. You should refer to your
lecture notes for our discussion of the functions of these areas.
B.
Hemispheric
Lateralization
1.
The
two hemispheres of the cerebrum are not bilaterally symmetrical, either
anatomically or functionally, with the functional asymmetry called
hemispheric lateralization.
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.
2.
Brain
waves generated by the cerebral cortex are recorded as an electroencephalogram
(EEG).
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).
IX. CRANIAL NERVES
A. Twelve pairs of cranial nerves
originate from the brain.
B.
The
pairs are named primarily on the basis of distribution and numbered by order of
attachment to the brain.
C.
Some
cranial nerves (I, II, VIII) contain only sensory fibers and are called sensory
nerves. Others contain only motor fibers (III, IV, VI, XI, and XII) and are
called motor nerves.
All the rest contain both sensory and
motor fibers and are called mixed nerves.
**You should know the name, number, and function of all cranial nerves
and whether the nerve is a sensory, motor or mixed nerve.**