Organization of the Nervous
Nervous Tissue: Structure and Function
Central Nervous System
Peripheral Nervous System
Developmental Aspects of the Nervous System
- Structural: All nervous system structures are classified as part
of the CNS (brain and spinal cord) or PNS (nerves and ganglia).
- Functional: Motor nerves of the PNS are classified on the basis
of whether they stimulate skeletal muscle (somatic division) or smooth/cardiac
muscle and glands (autonomic division) (Figure
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- Supportive connective tissue cells
- Neuroglia support and protect neurons in the CNS. Specific
glial cells are phagocytes; others myelinate neuron processes in
the CNS or line cavities.
- Schwann cells myelinate neuron processes in the PNS (Figure
- Anatomy: All neurons have a cell body containing the nucleus
and processes (fibers) of two types; (1) axons (one per cell) typically
generate and conduct impulses away from the cell body and release
a neurotransmilter, and (2) dendrites (one to many per cell) typically
carry electrical currents toward the cell body. Most large fibers
are myelinated; myelin increases the rate of nerve impulse transmission.
- On the basis of function (direction of impulse transmission)
there are sensory (afferent) and motor (efferent) neurons and
association neurons (interneurons). Dendritic endings of sensory
neurons are bare (pain receptors), or are associated with sensory
receptors (Figure 7.3).
- On the basis of structure, there are unipolar, bipolar,
and multipolar neurons; the terminology reveals the number of
processes extending from the cell body. Motor and association
neurons are multipolar; most sensory neurons are unipolar. The
exceptions are sensory neurons in certain special sense organs
(ear, eye), which are bipolar.
- A nerve impulse is an electrochemical event (initiated
by various stimuli) that causes a change in neuron plasma membrane
permeability, allowing sodium ions (Na+) to enter the cell (depolarization).
Once begun, the action potential, or nerve impulse, continues
over the entire surface of the cell. Electrical conditions of
the resting state are restored by the diffusion of potassium
ions (K+) out of the cell (repolarization). Ion concentrations
of the resting state are restored by the sodium-potassium pump
- A neuron influences other neurons or effector cells by
releasing neurotransmitters, chemicals that diffuse across the
synaptic cleft and attach to membrane receptors on the postsynaptic
cell. The result is opening of specific ion channels and activation
or inhibition, depending on the neurotransmitter released and
the target cell (Figure
- A reflex is a rapid, predictable response to a stimulus.
There are two types: autonomic and somatic. The minimum number
of components of a reflex arc is four: receptor, effector, and
sensory and motor neurons (most, however, have one or more interneurons)
(Figure 7.6). Normal
reflexes indicate normal nervous system function.
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- The brain is located within the cranial cavity of the skull and
consists of the cerebral hemispheres (forebrain), diencephalon, brain
stem structures, and cerebellum (Figure
- The two cerebral hemispheres form the largest part of the brain.
Their surface, or cortex, is gray matter and their interior is white
matter. The cortex is convoluted and has gyri, suici, and fissures.
The cerebral hemispheres are involved in logical reasoning, moral
conduct, emotional responses, sensory interpretation, and the initiation
of voluntary muscle activity. Several functional areas of the cerebral
lobes have been identified. The basal nuclei, regions of gray matter
deep within the white matter of the cerebral hemispheres, modify
voluntary motor activity. Parkinson's disease and Huntington's chorea
are disorders of the basal nuclei.
- The diencephalon is superior to the brain stem and is enclosed
by the cerebral hemispheres. The major structures include the following:
- The thalamus encloses the third ventricle and is the relay
station for sensory impulses passing to the sensory cortex for
- The hypothalamus makes up the "floor" of the third ventricle
and is the most important regulatory center of the autonomic
nervous system (regulates water balance, metabolism, thirst,
temperature, and the like).
- The epithalamus includes the pineal body (an endocrine
gland) and the choroid plexus of the third ventricle.
- The brain stem is the short region inferior to the hypothalamus
that merges with the spinal cord.
- The midbrain is most superior and is primarily fiber tracts.
- The pons is inferior to the midbrain and has fiber tracts
and nuclei involved in respiration.
- The medulla oblongata is the most inferior part of the
brain stem. In addition to fiber tracts, it contains autonomic
nuclei involved in the regulation of vital life activities (breathing,
heart rate, blood pressure, etc.).
- The cerebellum is a large, cauliflower-like part of the
brain posterior to the fourth ventricle. It coordinates muscle
activity and body balance.
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- Protection of the CNS
- Bones of the skull and vertebral column are the most external
- Meninges are three connective tissue membranes: dura mater
(tough outermost), arachnoid mater (middle weblike), and pia mater
(innermost delicate) (Figure
7.8). The meninges extend beyond the end of the spinal cord.
- Cerebrospinal fluid (CSF) provides a watery cushion around
the brain and cord. CSF is formed by the choroid plexuses of the
brain. It is found in the subarachnoid space, ventricles, and central
canal (Figure 7.9). CSF is
continually formed and drained.
- The blood-brain barrier is composed of relatively impermeable
- Brain dysfunctions
- Head trauma may cause concussions (reversible damage) or contusions
(nonreversible damage). When the brain stem is affected, unconsciousness
(temporary or permanent) occurs. Trauma-induced brain injuries may
be aggravated by intracranial hemorrhage or cerebral edema, both
of which compress brain tissue-
- Cerebrovascular accidents (CVAs, or strokes) result when blood
circulation to brain neurons is blocked and brain tissue dies. The
result may be visual impairment, paralysis, and aphasias.
- Alzheimer's disease is a degenerative brain disease in which
abnormal protein deposits and other structural changes appear. It
results in slow, progressive loss of memory and motor control plus
- Techniques used to diagnose brain dysfunctions include the
EEG, simple reflex tests, pneumo-encephalography, angiography, and
CT, PET, and MRI scans.
- The spinal cord is a reflex center and conduction pathway. Found
within the vertebral canal, the cord extends from the foramen magnum
to L1 or L2. The cord has a central bat-shaped area of gray matter surrounded
by columns of white matter, which carry motor and sensory tracts from
and to the brain.
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- A nerve is a bundle of neuron processes wrapped in connective tissue
coverings (endoneurium, perineurium, epineurium) (Figure
- Cranial nerves: Twelve pairs of nerves that extend from the brain
to serve the head and neck region. The exception is the vagus nerves,
which extend into the thorax and abdomen.
- Spinal nerves: Thirty-one pairs of nerves are formed by the union
of the dorsal and ventral roots of the spinal cord on each side. The
spinal nerve proper is very short and splits into dorsal and ventral
rami. Dorsal rami serve the posterior body trunk; ventral rami (except
T1 through T12) form plexuses (cervical, brachial, lumbar, sacral) that
serve the limbs.
- Autonomic nervous system: Part of the PNS, composed of neurons
that regulate the activity of smooth and cardiac muscle and glands.
This system differs from the somatic nervous system in that there is
a chain of two motor neurons from the CNS to the effector. Two subdivisions
serve the same organs with different effects (Figure
- The sympathetic division is the "fight-or-flight" subdivision,
which prepares the body to cope with some threat. Its activation
results in increased heart rate and blood pressure. The pre-ganglionic
neurons are in the gray matter of the cord. The postganglionic neurons
are in sympathetic chains or in collateral ganglia. Postganglionic
axons secrete norepinephrine.
- The parasympathetic division is the "housekeeping" system and
is in control most of the time. This division maintains homeostasis
by seeing that normal digestion and elimination occur, and that
energy is conserved. The first motor neurons are in the brain or
the sacral region of the cord. The second motor neurons are in the
terminal ganglia close to the organ served. Postganglionic axons
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- Maternal and environmental factors may impair embryonic brain development.
Oxygen deprivation destroys brain cells. Severe congenital brain diseases
include cerebral palsy, anencephaly, hydrocephalus, and spina bifida.
- Premature babies have trouble regulating body temperature because
the hypothalamus is one of the last brain areas to mature prenatally.
- Development of motor control indicates the progressive myelination
and maturation of a child's nervous system. Brain growth ends in young
adulthood. Neurons die throughout life and are not replaced; thus, brain
mass declines with age.
- Healthy aged people maintain nearly optimal intellectual function.
Diseaseóparticularly cardiovascular diseases the major cause of declining
mental function with age.