Chapter 12 - The Central Nervous System*
I. Introduction
A. The
brain is the largest, most complex portion of the nervous system, containing
100 billion multipolar neurons.
B.
It provides for voluntary movements,
interpretation and integration of sensation, consciousness, and cognitive
function.
C.
It is responsible for processing sensory
information, producing sensations, storing memory, integrating information,
reasoning, controlling visceral activities, providing personality, generating
emotions, and initiating motor activities.
D. Protection
of the brain and spinal cord is provided both by bone and by the membranes called meninges
that surround these structures.
II. Embryonic Development of the Brain
A. Brain structure in
the adult reflects the way the brain forms in the embryo.
B. The brain develops
from the rostral portion of the embryonic neural
tube.
C.
The early brain begins as a part of the
neural tube where there are three cavities at one end:
1.
Forebrain or prosencephalon
(cerebral hemispheres and diencephalon);
2.
Midbrain or mesencephalon
;
3.
Hindbrain or rhombencephalon
(pons, medulla, and cerebellum)
D. The cavities persist as ventricles and the
tissue surrounding them differentiates into the structural and functional
regions of the brain.
E. Cephalization results
in the envelopment of the diencephalon and superior
brain stem by the cerebral hemispheres.
III.Regions and Organization of the Brain
A. The
adult brain is divided into the:
1.
Cerebral hemispheres
a.
Have gray matter nuclei (cell bodies)
surrounded by white matter and an outer cortex of gray matter.
2.
Diencephalon
3.
Brain stem
4.
Cerebellum
a.
Have gray matter nuclei (cell bodies)
surrounded by white matter and
an
outer cortex of gray matter.
IV.
Ventricles of the Brain
1.
The two lateral ventricles are in the
cerebral hemispheres;
2.
The third ventricle is in the diencephalon; and
3.
The fourth ventricle is in the brain stem
and connects with the central canal of the spinal cord.
(*
Human Anatomy & Physiology, 6
Ed.; Marieb, 2004)
V.
The Cerebral Hemispheres
A. Structure of the
Cerebrum
1. The cerebrum is the largest portion of the
mature brain, consisting of two cerebral hemispheres.
2. A deep ridge of nerve fibers called the
corpus callosum connects the hemispheres, and the falx cerebri of the dura mater separates them.
3. The two cerebral hemispheres exhibit gyri, sulci, and fissures.
a.
Gyri
(convolutions) are outward folds of the surface of the cerebral cortex.
b.
Sulci
are shallow furrows on the brain, less deep than fissures.
c. Fissures are deep furrows on the brain.
4. The longitudinal fissure partially
separates the hemispheres
5. Other fissures or sulci
subdivide each hemisphere into lobes.
6. The lobes of the brain are named according to
the bones they underlie and include the frontal lobe, parietal lobe, temporal
lobe, occipital lobe, and insula (
7. Each cerebral
hemisphere consists of the:
a.
Cerebral cortex - A thin layer of gray matter that lies on
the outside of the cerebrum and contains 75% of the cell bodies in the nervous
system.
b.
Cerebral white matter - made up of myelinated nerve fibers connecting the cell bodies of the
cortex with the rest of the nervous system.
c.
Basal nuclei (ganglia)
8.
Each cerebral hemisphere receives sensory
impulses from, and dispatches motor impulses to, the opposite side of the body.
a.
The body is represented in a upside-down
fashion in the sensory and motor cortices.
B. Functions of the Cerebrum
1. The cerebrum provides higher brain functions,
such as interpretation of sensory input, initiating voluntary muscular
movements, memory, and integrating information for reasoning.
2. Functional Regions of the Cerebral Cortex
a.
The functional areas of the brain have been
established using a variety of techniques.
b.
Functional areas of the cerebral cortex:
1)
Motor areas include primary motor and premotor areas of the frontal lobe, the frontal eye field,
and Broca's area in the frontal lobe of one
hemisphere (usually the left).
a) The primary motor areas lie in the frontal
lobes, anterior to the central sulcus and in its anterior wall.
b) Broca's area, anterior to the
primary motor cortex, is usually in the left cerebral hemisphere and
coordinates muscular activity to make speech possible.
(2)
c)
Above Broca's area is the frontal eye field that
controls the voluntary movements of the eyes and eyelids
2)
Sensory Areas:
a.)
The sensory area responsible for receiving
impulses for temperature, pressure, pain, smell, and taste lies posterior to
the central sulcus and in its posterior wall.
b.)
Primary somatosensory
cortex, somatosensory association cortex, and the
gustatory (taste) area lie in the parietal lobe
c) Olfactory and auditory areas lie in the
temporal lobe
d) Sensory areas for sight lie within the
occipital lobe.
e) Vestibular area lies in the insula
f) Sensory and motor fibers alike cross over in
the spinal cord or brain stem so centers in the right hemisphere are
interpreting or controlling the left side of the body, and vice versa.
3) Association Areas
a) The various association areas of the brain
analyze and interpret sensory impulses and function in reasoning, judgment,
emotions, verbalizing ideas, and storing memory.
b) A general interpretive area is found at the
junction of the parietal, temporal, and occipital lobe on one side (usually the
left), and plays the primary role in complex thought processing.
c)
Association areas located in the prefrontal cortex in the frontal lobe
d) Language areas, including the lateral
prefrontal cortex, a large portion of the temporal lobe, Broca's
area, Wernicke's areas in one hemisphere (temporal
lobe) only, usually the left, and affective language areas in one hemisphere
(usually the right).
3. Hemisphere Dominance
a. Both cerebral hemispheres function in receiving
and analyzing sensory input and sending motor impulses to the opposite side of
the body.
b. The cerebral hemispheres show lateralization
of cortical function
1)
Most people exhibit left hemisphere
dominance for mathematical skills and
language-related activities of speech, writing, and reading.
a) The left hemisphere is dominant in 90% of the
population, although some individuals have the right hemisphere as dominant,
and others show equal dominance in both hemispheres.
2)
The right hemisphere is more concerned with visual-spatial skills and creative
endeavors.
(3)
4.
Fiber tracts of the cerebral white matter
include:
a.
Commissures;
b.
Association fibers; and
c.
Projection fibers
5. Basal Ganglia
a.
The basal ganglia are masses of gray matter
(subcortical nuclei) located deep within the cerebral
hemispheres that relay motor impulses from the cerebrum and help to control
motor activities (muscular movements) by producing inhibitory dopamine. (Parkinson’s disease occurs when dopamine is
not produced in adequate amounts.)
b.
The paired basal nuclei (also called basal
ganglia) develop from the forebrain and include the:
1)
Lentiform nucleus (globus pallidus and putamen); and
2) Caudate nucleus
c.
Functionally, they are closely associated
with the substantia nigra
of the midbrain.
6. Ventricles and Cerebrospinal Fluid
a.
The ventricles are a series of connected
cavities within the cerebral hemispheres and brain stem.
b.
The ventricles are continuous with the
central canal of the spinal cord, and are filled with cerebrospinal fluid.
c. Choroid plexuses,
specialized capillaries from the pia mater, secrete
cerebrospinal fluid.
1)
Cerebrospinal fluid is a clear fluid that
has nutritive as well as protective (cushioning) functions
2) Specialized
ependymal cells, joined by tight junctions, regulate
the composition of the cerebrospinal fluid.
3) Most
cerebrospinal fluid arises in the lateral ventricles; humans secrete 500
milliliters daily.
4) Cerebrospinal
fluid is reabsorbed through arachnoid granulations.
VI. The Diencephalon
A. The diencephalon lies above the brain stem, encloses the third
ventricle, and
contains the thalamus,
hypothalamus, and epithalamus
1. The thalamus functions in sorting and
directing sensory information arriving from other parts of the nervous system,
performing the services of both messenger and editor.
a.
The thalamus may transmit sensory
information by synchronizing action potentials.
(4)
b.
The thalamus is the major station
for:
1)
Sensory impulses ascending to the sensory cortex;
2)
Inputs from subcortical motor nuclei and the cerebellum traveling to
the cerebral motor cortex; and
3)Impulses
traveling to association cortices from lower centers
2. The hypothalamus maintains homeostasis
by regulating a wide variety of visceral activities and by linking the
endocrine system with the nervous system.
a.
It is an important autonomic nervous system
control center and a pivotal part of the limbic system;
b.
The hypothalamus regulates heart rate and
arterial blood pressure, body temperature, water and electrolyte balance,
thirst, hunger, eating behavior, body weight, movements and secretions of the
digestive tract (gastrointestinal activity), activity of the anterior pituitary
gland, growth and reproduction, and sleep and wakefulness.
3. The epithalamus consists of the:
a.
Pineal gland; and
b.
Choroid
plexus of the third ventricle.
4. Other portions of the
diencephalon are the:
a.
Optic tracts and optic chiasma;
b.
The infundibulum
(attachment for the pituitary);
c.
The posterior pituitary; and
d.
Mammillary
bodies;
5.
The limbic system consists of numerous
structures that encircle diencephalon,
a.
The limbic system controls emotional
experience and expression;
b.
It is the “emotional-visceral brain”
c.
By generating pleasant or unpleasant
feelings about experiences, the limbic system guides behavior that may enhance
the chance of survival.
d.
It also plays a role in memory.
VII. Brain Stem
A. The
brain stem lies at the base of the cerebrum, and connects the brain to the
spinal cord. The brain stem includes the:
1.
Midbrain;
2.
Pons; and
3.
Medulla Oblongata
(5)
B.
Functions of the brain stem components:
1.
Midbrain
a. The midbrain, located between the diencephalon and pons, contains bundles of myelinated nerve fibers that convey impulses to and from
higher parts of the brain, and masses of gray matter that serve as reflex
centers.
b. The midbrain contains centers for auditory
and visual reflexes as well as reflexes that maintain posture.
c.
The midbrain contains the:
1)
Corpora quadrigemina
(visual and auditory reflex centers);
2) Red
nucleus (subcortical motor centers); and
3) Substantia nigra.
4) The
periaqueductal gray matter elicits the fear response
and contains the motor nuclei of cranial nerves III and IV.
5) The
cerebral peduncles on its ventral face house the pyramidal fiber tracts.
6) The
midbrain surrounds the cerebral aqueduct.
2.
Pons
a.
The pons, lying between the midbrain and
medulla oblongata,
b.
It transmits impulses between the brain and
spinal cord; it is mainly a conduction area;
c. It contains centers that regulate the rate
and depth of breathing; its nuclei (cell bodies) contribute to regulation of
respiration and cranial nerves V-VII.
3. Medulla Oblongata
a. Transmits all ascending and descending
impulses between the brain and spinal cord.
b. It also houses nuclei that control visceral
functions, including the cardiac center that controls heart rate, the
vasomotor center for blood pressure control, and the respiratory center
that works, along with the pons, to control the rate and depth of breathing
(respiratory rhythm).
c.
The olivary nucleus and cough, sneezing, swallowing,
and vomiting centers are in the medulla.
d. Important nuclei in
the medulla serve cranial nerves VIII-XII.
e.
The pyramids (descending corticospinal tracts) form
the ventral face of the medulla oblongata.
f.
These fibers cross over (decussation of the pyramids)
before entering the spinal cord.
(6)
4. Reticular Formation
a. Throughout the brain stem, hypothalamus,
cerebrum, cerebellum, and basal ganglia, is a complex network of nerve fibers
connecting tiny islands of gray matter; this network is the reticular
formation.
b. Decreased activity in the reticular formation
results in sleep;
c. Increased activity results in wakefulness;
thus maintaining the alert state of the cerebral cortex (RAS)
d. The reticular formation filters incoming
sensory impulses.
VIII.
The Cerebellum
A.
The cerebellum is made up of two hemispheres, marked by convolutions, and
connected by a vermis.
B. A thin layer of gray matter lies outside a
core of white matter.
C. The
cerebellum communicates with other parts of the central nervous system through cerebellar peduncles.
D. It
is connected to the brain stem by superior, middle, and inferior peduncles
E. Functions
of the cerebellum
1.
The cerebellum functions to integrate
sensory information about the position of body parts and coordinates skeletal
muscle activity and maintains posture.
2.
It processes and interprets impulses from
the motor cortex and sensory pathways;
3.
It coordinates motor activity so that
smooth, well-timed movements occur.
4.
It also plays a poorly understood role in
cognition.
IX. Higher Mental Functions of the Brain
A.
Brain Wave Patterns and the EEG
1. Brain waves are patterns of electrical activity
of the brain.
a.
An
electroencephalogram (EEG) is a record of brain wave activity.
b.
Brain
wave patterns (identified by their frequencies) include:
1) Alpha waves – indicate a brain that is “idling”
– a calm, relaxed state of
wakefulness
2) Beta waves – occur when
you are awake and mentally alert;
concentrating on some problem or visual stimulus
3) Theta waves – common in
children; however, considered abnormal in adults who are awake
4) Delta waves – seen
during sleep and when the reticular activating system is damped, such as during
anesthesia. In awake adults, delta waves
indicate brain damage.
2. Epilepsy results from abnormal electrical
activity of brain neurons.
a.
Involuntary
muscle contractions and sensory auras are typical during epileptic seizures.
(7)
B. Consciousness
1. Consciousness includes:
a. Sensory
perception;
b. Initiation and control of voluntary movement;
c. Higher mental processing capabilities.
2. Consciousness is
described clinically on a continuum from alertness to drowsiness
to stupor and finally to coma.
3. Human consciousness is
thought to involve holistic information processing, which is:
a. Not localizable;
b. Superimposed on other
types of neural activity; and
c. Totally interconnected.
4. Fainting (syncope) is a
temporary loss of consciousness that usually reflects inadequate blood delivery
to the brain.
5. Coma is loss of
consciousness in which the victim is unresponsive to stimuli.
C. Sleep and
Sleep-Awake Cycles
1. Sleep is a state of
altered consciousness from which one can be aroused by stimulation.
2. The two major types of
sleep are:
a. Slow wave or Non-Rapid Eye Movement (NREM)
sleep; and
b. Rapid eye movement
(REM) sleep.
3. NREM
Sleep:
a.
Slow-wave sleep (NREM) occurs for 70 to 90 minutes when the person is
very tired, and results from decreasing activity in the reticular formation.
b. During stages 1-4,
brain waves become more irregular; and
c. Brain waves increase in amplitude until delta
wave sleep (stage 4) is achieved.
d. Slow-wave sleep (stage
4 of NREM) appears to be restorative.
e. Time spent in slow-wave sleep declines
steadily throughout life.
4. REM
Sleep:
a. During REM
sleep, certain areas of the brain are active and dreaming; eye movements and
irregular heart and breathing rates occur.
b. During REM, the eyes
move rapidly under the lids.
c. REM sleep is important
for emotional stability.
d. REM occupies half of an
infant's sleep time and then declines to about 25% of sleep time by the age of
ten years.
(8)
5.
NREM
and REM sleep alternate throughout the night.
6.
Narcolepsy
is involuntary lapses into sleep that occur without warning during waking
periods.
7.
Insomnia
is a chronic inability to obtain the amount or quality of sleep needed to
function adequately.
8.
Sleep
apnea is a temporary cessation of breathing during sleep due to hypoxia (a
deficiency of oxygen reaching the tissues of the body).
D. Memory
1. Memory
is the ability to recall one's thoughts
a. It is essential for
learning;
b. It is part of
consciousness.
2. Memory storage has two stages:
a. Short-term memory (STM)
which is mostly electrical in nature; and
b. Long-term memory (LTM).
1)
In long-term memory, synaptic connections between neurons are established and
remain unchanged for years.
2) According to the long-term synaptic potentiation theory, memory consolidation (conversion of
short-term memory into long-term) occurs as repeated stimulation of the same
synapses strengthens them.
3. Transfer of information
from STM to LTM takes minutes to hours, but more time is required for LTM
consolidation.
4.
Fact
memory is the ability to learn and consciously remember information.
a. Fact (declarative) memory appears to involve
the following:
1) Hippocampus;
2) Amygdala;
3) Diencephalon;
4) Basal forebrain; and
5) Prefrontal cortex.
5.
Skill
memory is the learning of motor skills, which are then performed without
conscious thought.
a. Skill (declarative)
memory pathways are mediated by the corpus striatum.
6.
The
nature of memory traces in the human brain is not fully known, but NMDA
receptors (essentially calcium channels), activated sequentially by
depolarization and glutamate binding, play a major role in long-term potentiation (LTP).
a. The calcium influx that
follows NMDA receptor activation mobilizes enzymes that mediate events
necessary for memory consolidation.
(9)
X.
Protection of the Brain
1.Meninges:
a.
The meninges from superficial to deep are the:
1)
Dura mater;
2)
Arachnoid mater; and
3)
Pia mater.
b.
The meninges
enclose the brain and spinal cord and their blood vessels.
c.
Functions of the various meninges:
1)
The outermost meninx is made up of tough, white dense
connective tissue, contains many blood vessels, and is called the dura mater.
a)
It forms the inner periosteum of the skull bones.
b) In some areas, the dura mater forms partitions between lobes of the brain, and
in others, it forms dural sinuses.
c)The
sheath around the spinal cord is separated from the vertebrae by an epidural space.
d)
Inward folds of the inner layer of the dura mater
secure the brain to the skull.
2)
The middle meninx, the arachnoid mater, is
thin and lacks blood vessels.
a)
It does not follow the convolutions of the brain.
b)
Between the arachnoid and pia
maters is a subarachnoid space containing
cerebrospinal fluid.
3)
The innermost pia mater is thin and contains many
blood vessels and nerves.
a)
It is attached to the surface of the brain and spinal cord and follows their
contours.
2. Cerebrospinal
Fluid (CSF)
a. Is formed by the choroid plexuses from blood plasma;
b. Circulates through
the ventricles and into the subarachnoid space;
c. Returns to the dural venous sinuses via the arachnoid
villi;
d. Supports and cushions
the brain and spinal cord; and
e. Helps to nourish the brain and spinal cord.
(10)
3. The
Blood-Brain Barrier
a.
Reflects the relative impermeability of the epithelium of capillaries of the
brain.
b.
Allows water, respiratory gases, essential nutrients, and fat soluble molecules
to enter the neural tissue, but
c. Prevents entry of other water-soluble,
potentially harmful substances.
B. Homeostatic Imbalances of the Brain
1.
Head
trauma may cause brain injuries called concusions (reversible damage) or contusions
(irreversible damage).
a.
When the brain stem is affected,
unconsciousness (temporary or permanent) occurs.
b. Trauma-induced brain injuries may be
aggravated by intracranial hemorrhage or cerebral edema, both of which compress
brain tissue.
2.
Cerebrovascular accidents (strokes) result when blood circulation to
brain neurons is impaired and brain tissue dies.
a.
The result may be hemiplegia (paralysis of one side
of the body), sensory deficits, or speech impairment.
3.
Alzheimer's
disease is a degenerative brain disease in which beta amyloid peptide deposits and neurofibrillar
tangles appear.
a.
Marked by a deficit of acetylcholine (ACh),
b.
Results in slow, progressive loss of memory and motor control and increasing
dementia.
4.
Neurodegenerative
disorders of the basal nuclei involve abnormalities of the neurotransmitter
dopamine (too little or too much secreted) and are characterized by abnormal
movements:
a. Parkinson's disease
b.
Huntington's disease.
XI.
Spinal Cord
1.
The spinal cord develops from the neural
tube.
2.
Its gray matter forms from the alar and basal plates.
3.
Fiber tracts form the outer white matter.
4.
The neural crest forms the sensory (dorsal
root) ganglia.
1.
The spinal cord is a two-way impulse conduction
pathway and a reflex center.
2.
It resides within the vertebral column
3.
It is protected by meninges
and cerebrospinal fluid.
(11)
4.
It extends from the base of the brain
(foramen magnum) and extends as a slender cord to the intervertebral disk
between the first and second lumbar vertebrae.
5.
The spinal cord consists of 31 segments,
each of which gives rise to a pair of spinal nerves.
6.
The cord is enlarged in the cervical and
lumbar regions.
a.
A cervical enlargement gives rise to nerves
leading to the upper limbs,
b.
A lumbar enlargement gives rise to nerves
innervating the lower limbs.
7. The spinal cord ends at the conus medullaris and continues as
a thin cord of connective tissue called the filum terminale.
1.
Two deep longitudinal grooves (anterior
median fissure and posterior median sulcus) divide
the cord into right and left halves.
2.
The central gray matter of the
spinal cord is H shaped.
a.
Composition of the horns of the gray
matter:
1) Anterior horns mainly contain somatic
motor neurons.
2) Lateral horns contain visceral
(autonomic) motor neurons.
3) Posterior
horns contain interneurons.
b.
Axons of motor neurons
of the lateral and anterior horns emerge in common from the spinal cord via the
ventral roots.
c.
Axons of sensory neurons
(with cell bodies located in the dorsal root ganglion) enter the posterior
aspect of the cord and from the dorsal roots.
3.
White matter, made up of bundles of myelinated nerve fibers, surrounds a butterfly-shaped core
of gray matter of the spinal cord housing interneurons.
a.
Each side of the white matter of the spinal
cord has posterior, latera, and anterior columns (funiculi);
b.
Each funiculus
contains a number of ascending and descending tracts.
c.
All tracts are paired and most decussate
(cross over)
d.
A central canal contains cerebrospinal
fluid.
1.
The spinal cord has two major functions:
a.
To transmit impulses to and from the brain
via ascending and descending nerve tracts, and
b.
To house spinal reflexes.
(12)
2. Ascending
tracts include:
a.
Fasciculi
gracilis and fasciculi cuneatus conduct sensory impulses from the skin, muscles,
tendons, and joints to the brain.
b. Spinothalamic
tracts transmit impulses concerning pain, temperature, touch, and pressure.
c.
Spinocerebellar
tracts carry impulses from the lower limbs and help coordinate muscular
movements.
3. Descending tracts include:
a. The pyramidal tracts (anterior and lateral corticospinal tracts) conduct impulses to skeletal muscles
to control voluntary movements.
b. Reticulospinal
tracts control muscular activity and activity of sweat glands.
c. Rubrospinal tracts
carry motor impulses to skeletal muscles to coordinate muscles and control
posture.
4. Reflex Arcs
a. A reflex arc includes a sensory receptor, a
sensory neuron, an interneuron in the spinal cord, a motor neuron, and an effector.
b.
Reflexes whose arcs pass through the spinal cord are called spinal reflexes.
5. Reflex Behavior
a. Reflexes are automatic, subconscious
responses to stimuli that help maintain homeostasis (heart rate, blood
pressure, etc.) and carry out automatic responses (vomiting, sneezing,
swallowing, etc.).
b.
The knee-jerk reflex (patellar tendon reflex) is an example of a monosynaptic
reflex (no interneuron).
c. The withdrawal reflex involves sensory
neurons, interneurons, and motor neurons.
1)
While the flexor muscles contract to
withdraw the hand, a crossed extensor reflex causes extensors in the other arm
to contract.
E. Spinal Cord Trauma and Disorders
1. Injury
to the anterior horn neurons or the ventral roots results in flaccid paralysis.
2. Injury
to the upper motor neurons in the brain results in spastic paralysis.
3. If
a dorsal root or sensory tract is damaged, paresthesia
(abnormal sensation, such as burning, prickling or tactile hallucination) occurs.
4. Poliomyelitis
results from inflammation and destruction of the anterior horn neurons by the
poliovirus.
a.
Paralysis and muscle atrophy follow.
5. Amyotrophic
lateral sclerosis results from destruction of the anterior horn neurons and the
pyramidal tract.
a.
The
victim loses the ability to swallow, speak and breathe.
b.
Death
occurs within 5 years.
(13)
F. Diagnostic Procedures for Assessing CNS Dysfunction
1.
Diagnostic procedures used to assess
neurological condition and function range from routine reflex testing to
sophisticated techniques such as pneumoencephalography,
cerebral angiography, CT scans, MRI scans, and PET scans.
G.Developmental Aspects of the Central Nervous System
1. Maternal and environmental factors may impair
embryonic brain development.
a. Oxygen
deprivation destroys brain cells.
b. Severe
congenital brain diseases include cerebral palsy, anencephaly, hydrocephalus,
and spina bifida.
2.
Premature babies have trouble regulating
body temperature because the hypothalamus is one of the last brain areas to
mature prenatally.
3.
Development of motor control indicates
progressive myelination and maturation of a child's
nervous system.
4.
Brain growth ends in young adulthood.
a. Neurons
die throughout life and are not replaced
b. Brain
weight and volume decline with age
5.
Healthy elders maintain nearly optimal
intellectual function
a.
Disease (particularly cardiovascular disease) is the major cause of declining
mental function with age.
(14)