CHAPTER 7
Nervous Tissue
121
Figure 7-4.
Overview of the central and peripheral nervous systems.
The nervous system is divided into three general divisions: the
CNS
, the
PNS
, and the
ANS
. The
CNS
consists of the brain and spinal cord
(
1
). The brain is divided into F ve regions, labeled above, based on developmental considerations. A typical cross
section through the spinal
cord is illustrated in (
2)
. The
PNS
includes all of the peripheral nerves, which join or exit the spinal cord as 31 sets of
spinal roots
and 12
sets of
cranial nerves
. In general, the
posterior spinal roots
carry sensory information from the body back to the CNS; the
anterior roots
carry motor signals from the CNS to the muscles and internal organs. The spinal cord includes large bundles of axons (
white matter
in
2
)
that carry sensory signals to the brain or motor signals to motor neurons located in the gray matter of the spinal cord. The
gray matter
is
composed of concentrations of neuron cell bodies. These neurons include interneurons in sensory and motor pathways, as well as motor
neurons, which directly innervate muscle F bers. The color of the white matter derives from the shiny white lipid,
myelin
, that coats many
of the axons (±igs. 7-6 and 7-7). This coating is sparse in regions of concentrated neuron cell bodies, resulting in the gray color of those
regions.
Peripheral nerves
(
3
) are composed of several bundles (
fascicle
s
) of axons surrounded by connective tissue (±ig. 7-5A). The
ANS
is devoted to the control of internal organs, glands, blood vessels, and associated structures and is diagrammed in ±igure 7-14. It includes
sympathetic
,
parasympathetic
, and
enteric
subdivisions. (In
2
, A, anterior; P, posterior; AR, anterior ramus; PR, posterior ramus. Both the
anterior and the posterior rami contain sensory, motor, and autonomic nerve F bers.)
J. Lynch &T. Yang
Diencephalon
Midbrain
(mesencephalon)
Pons and cerebellum
(metencephalon)
Medulla
(myelencephalon)
Fascicle
Epineurium
Perineurium
Endoneurium
Spinal
nerve
Artery
Axons
Spinal cord
Preganglionic
sympathetic
fiber
Postganglionic
sympathetic
fiber
Skeletal
(striated)
muscle
Posterior root
ganglion
White matter
(myelinated
axons)
Posterior
spinal root
Posterior horn
Intermediate horn
Anterior horn
Anterior
spinal root
Gray matter
(neuron cell
bodies)
White matter
(axons)
Unipolar
sensory
neuron
Multipolar
motor
neuron
Sensory receptors
in skin, muscle,
or joints
Sympathetic
chain ganglion
Telencephalon
2
P
A
AR
PR
1
3
P
A
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