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U N I T 1 0
Nervous System
33 of which form the spinal cord and spinal nerves, and
10 of which form the brain and its cranial nerves.
The basic pattern of the CNS is that seen in the spi-
nal cord—a central cavity surrounded by an inner core
of gray matter and a superficial layer of white matter
(Fig. 34-7). The brain retains this organization, but it
also contains additional regions of gray matter that are
not evident in the spinal cord. The gray matter is func-
tionally divided into longitudinal columns of nerve cell
bodies called the
cell columns
. The superficial white
matter region contains the longitudinal tract systems of
the CNS. The dorsal half or dorsal horn of the gray mat-
ter contains afferent neurons. The ventral portion, or
ventral horn
, contains efferent neurons that communi-
cate by way of the ventral roots with effector cells of the
body segment. Many CNS neurons develop axons that
grow longitudinally as tract systems that communicate
between neighboring and distal segments of the neural
tube.
Each segment of the CNS is accompanied by two
pairs of bundled nerve roots—a dorsal and ventral pair.
The paired dorsal roots connect a pair of dorsal root
ganglia and their corresponding CNS segment. The dor-
sal root ganglia contain many afferent nerve cell bodies,
each having two axonlike processes—one that ends in
a peripheral receptor and another that enters the dorsal
horn of its respective CNS segment. The paired ventral
roots of each segment contain fibers supplying skeletal
muscles and visceral structures of the body segment.
All brain segments, except segment 2, retain some
portion of the basic segmental organization of the ner-
vous system in which each segment has multiple paired
branches containing a grouping of component axons.
The classic pattern of spinal nerve organization, which
consists of a pair of dorsal and a pair of ventral roots,
is a later development that has not occurred in the
cranial nerves. Consequently, the cranial nerves, which
are arbitrarily numbered 1 through 12, retain the early
pattern of development, with more than one cranial
nerve branching from a single segment. The truly seg-
mental nerve pattern of the cranial nerves is altered
because all branches from segment 2 and most of the
branches from segment 1 are missing. Cranial nerve 2,
also called the
optic nerve
, is not a segmental nerve, but
a brain tract connecting the retina (modified brain) with
the first forebrain segment from which it developed.
Cell Columns
Anatomically, the body is organized into soma (e.g.,
skin, muscles, and skeletal structures of the body wall)
and viscera (internal organs). The organizational struc-
ture of the nervous system can be best explained and
simplified as a pattern in which functionally specific
PNS and CNS afferent and efferent somatic and visceral
neurons are repeated as parallel cell columns running
lengthwise along the nervous system. In this organiza-
tional pattern, afferent neurons, dorsal horn cells, and
ventral horn cells are organized as a bilateral series of
11 cell columns.
The cell columns on each side can be further grouped
according to their location in the PNS: four in the dorsal
root ganglia that contain sensory neurons, four in the
dorsal horn that contain sensory
input association
(IA)
interneurons, and three in the ventral horn that contain
motor neurons (Fig. 34-8). The processes of sensory
neurons that enter the dorsal horn communicate with IA
neurons, which then distributes the afferent information
to local reflex circuitry and to more rostral segments of
the CNS. The ventral horns contain
output association
(OA) interneurons and lower motor neurons, which
project to the effector muscles. The afferent and efferent
cell columns of the PNS and CNS, their projections, and
the type of information they transmit are summarized in
Table 34-1.
Between the IA neurons and the OA neurons are
networks of small interconnecting neurons arranged in
complex circuits that provide the discreteness, appropri-
ateness, and intelligence of responses to stimuli. Most
of the billions of CNS cells in the spinal cord and brain
gray matter are interconnecting neurons.
Dorsal Horn Cell Columns.
Four columns of affer-
ent (sensory) neurons in the dorsal root ganglia directly
innervate four corresponding columns of IA neurons
in the dorsal horn (see Fig. 34-8). These are the special
somatic and special visceral afferents and the general
somatic and general visceral afferents.
The special somatic afferent
(SSA) neurons are con-
cerned with internal sensory information such as joint
and tendon sensation. They relay information to local
reflexes concerned with posture and movement, as well
as to the cerebellum, contributing to coordination of
movement, and to the forebrain, contributing to expe-
rience. Afferents innervating the vestibular system of
the inner ear also belong to the special somatic afferent
category.
Dorsal
root
Ventral
root
IA neuron
Dorsal root
ganglion
Spinal
nerve
Dorsal root
ganglion
neuron
Segments
FIGURE 34-7.
Three segments of the spinal cord showing
dorsal roots entering the dorsal lateral surface of the cord and
ventral roots exiting the ventral surface of the cord.The dorsal
root ganglion contains dorsal root ganglion cells, whose axons
bifurcate: one process enters the spinal cord in the dorsal
root, and the other extends peripherally to supply the skin and
muscle of the body.The ventral root is formed by axons from
motor neurons in the spinal cord. IA, input association.
