412
UNIT THREE
especially an axon. The terminology for the connective tissue
holding them together, however, is similar. In both cases, for
example, F bers are bundled into fascicles, whereas epineu-
rium in nerves corresponds to epimysium in muscles, and so
forth (see F
gs. 11.23, 11.24, 9.2, and 9.3).
Nerve and Nerve Fiber Classif
cation
Recall that nerves are bundles of nerve fibers, or axons.
Nerves that have only F
bers of sensory neurons, conduct-
ing impulses into the brain or spinal cord, are called
sensory
nerves.
Nerves that have only F bers involved in motor con-
trol are
motor nerves.
Most nerves include both sensory and
motor F
bers and are called
mixed nerves.
Nerves originating from the brain that communicate
with other body parts are called
cranial nerves,
whereas
those originating from the spinal cord that communicate
with other body parts are called
spinal nerves.
The nerve
F bers in these structures can be subdivided further into four
groups as follows:
1.
General somatic e±±erent f
bers
carry motor impulses
outward from the brain or spinal cord to skeletal
muscles and stimulate them to contract.
2.
General visceral e±±erent f
bers
carry motor impulses
outward from the brain or spinal cord to various smooth
muscles and glands associated with internal organs,
causing certain muscles to contract or glands to secrete.
3.
General somatic a±±erent f
bers
carry sensory impulses
inward to the brain or spinal cord from receptors in the
skin and skeletal muscles.
4.
General visceral a±±erent f
bers
carry sensory impulses
to the CNS from blood vessels and internal organs.
Structure o± Peripheral Nerves
A peripheral nerve consists of connective tissue surround-
ing bundles of nerve F bers. The outermost layer of the con-
nective tissue, called the
epineurium,
is dense and includes
many collagenous F
bers. Each bundle of nerve F
bers (fas-
cicle) is, in turn, enclosed in a sleeve of looser connective
tissue called the
perineurium.
A small amount of loose con-
nective tissue called
endoneurium
surrounds individual
nerve fibers
(figs. 11.23
and
11.24)
. Blood vessels in the
epineurium and perineurium give rise to a network of capil-
laries in the endoneurium that provides oxygen and nutri-
ents to the neurons.
The term “muscle F
ber” refers to an entire muscle cell,
whereas the term “nerve F ber” refers to a cellular process,
TABLE
11.7
|
Major Parts of the Brain
Part
Characteristics
Functions
1. Cerebrum
Largest part of the brain; two hemispheres
connected by the corpus callosum
Controls higher brain functions, including interpreting sensory impulses, initiating
muscular movements, storing memory, reasoning, and determining intelligence
2. Basal nuclei
(ganglia)
Masses of gray matter deep within the cerebral
hemispheres
Relay stations for motor impulses originating in the cerebral cortex and passing
into the brainstem and spinal cord
3. Diencephalon
Includes masses of gray matter (thalamus and
hypothalamus)
The thalamus is a relay station for sensory impulses ascending from other parts
of the nervous system to the cerebral cortex; the hypothalamus helps maintain
homeostasis by regulating visceral activities and by linking the nervous and
endocrine systems
4. Brainstem
Connects the cerebrum to the spinal cord
a. Midbrain
Contains masses of gray matter and bundles of nerve
F
bers that join the spinal cord to higher regions of the
brain
Contains re±
ex centers that move the eyes and head, and maintains posture
b. Pons
A bulge on the underside of the brainstem that
contains masses of gray matter and nerve F
bers
Relays nerve impulses to and from the medulla oblongata and cerebrum; helps
regulate rate and depth of breathing
c. Medulla
oblongata
An enlarged continuation of the spinal cord that
extends from the foramen magnum to the pons and
contains masses of gray matter and nerve F
bers
Conducts ascending and descending impulses between the brain and spinal
cord; contains cardiac, vasomotor, and respiratory control centers and various
nonvital re±
ex control centers
5. Cerebellum
A large mass of tissue inferior to the cerebrum and
posterior to the brainstem; includes two lateral
hemispheres connected by the vermis
Communicates with other parts of the CNS by nerve tracts; integrates sensory
information concerning the position of body parts; and coordinates muscle
activities and maintains posture
TABLE
11.8
|
Subdivisions of the
Nervous System
1. Central nervous system (CNS)
a. Brain
b. Spinal cord
2. Peripheral nervous system (PNS)
a. Cranial nerves arising from the brain
(1)
Somatic F
bers connecting to the skin and skeletal muscles
(2)
Autonomic F
bers connecting to viscera
b. Spinal nerves arising from the spinal cord
(1)
Somatic F
bers connecting to the skin and skeletal muscles
(2)
Autonomic F
bers connecting to viscera
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