468
UNIT 3
Regulation and Integration of the Body
12
Neuronal Pathways
All major spinal tracts are part of
multineuron pathways
that
connect the brain to the body periphery. Tese great ascending
and descending pathways contain not only spinal cord neurons
but also parts of peripheral neurons and neurons in the brain.
Before we get specific about the individual tracts, we will note
four key generalizations about them and the pathways to which
they contribute:
Decussation.
Most pathways cross from one side of the CNS
to the other (decussate) at some point along their journey.
Relay.
Most pathways consist of a chain of two or three neurons
(a relay) that contribute to successive tracts of the pathway.
Somatotopy.
Most pathways exhibit
somatotopy
, a precise
spatial relationship among the tract fibers that reflects the
orderly mapping of the body. For example, in an ascending
sensory tract, fibers transmitting inputs from sensory recep-
tors in superior body regions lie lateral to those conveying
sensory information from inferior body regions.
Symmetry.
All pathways and tracts are paired symmetrically
(right and le±), with a member of the pair present on each
side of the spinal cord or brain.
Ascending Pathways to the Brain
Te ascending pathways conduct sensory impulses upward, typ-
ically through chains of three successive neurons (first-, second-,
and third-order neurons) to various areas of the brain. Note that
both second- and third-order neurons are interneurons.
First-order neurons
, whose cell bodies reside in a ganglion
(dorsal root or cranial), conduct impulses from the cutaneous
receptors of the skin and from proprioceptors to the spinal cord
or brain stem, where they synapse with second-order neurons.
Impulses from the facial area are transmitted by cranial nerves,
dorsal and ventral roots are very short and fuse laterally to form
the
spinal nerves
(see Chapter 13).
Te spinal gray matter can be divided further according to
its neurons’ relative involvement in innervating the somatic
and visceral regions of the body. Spinal gray matter has the
following four zones (Figure 12.29):
somatic sensory (SS)
,
visceral sensory (VS)
,
visceral (autonomic) motor (VM)
,
so-
matic motor (SM)
.
White Matter
Te white matter of the spinal cord is composed of myelinated
and nonmyelinated nerve fibers that allow communication be-
tween different parts of the spinal cord and between the cord
and brain. Tese fibers run in three directions:
Ascending
—up to higher centers (sensory inputs)
Descending
—down to the cord from the brain or within the
cord to lower levels (motor outputs)
Transverse
—across from one side of the cord to the other
(commissural fibers)
Ascending and descending tracts make up most of the white
matter.
Te white matter on each side of the cord is divided into
three
white columns
, or
funiculi
(fu-nik
9
u-li; “long ropes”),
named according to their position as
dorsal (posterior)
,
lat-
eral
, and
ventral (anterior) funiculi
(Figure 12.28b). Each fu-
niculus contains several fiber tracts, and each tract is made up
of axons with similar destinations and functions. With a few
exceptions, the names of the spinal tracts reveal both their ori-
gin and destination.
Figure 12.30
schematically illustrates the
principal ascending and descending tracts of the spinal cord in
cross-sectional view.
Somatic sensory neuron
Dorsal root
(sensory)
Dorsal root
ganglion
Visceral sensory
neuron
Somatic motor neuron
Spinal nerve
Ventral root (motor)
Ventral horn
(motor neurons)
Dorsal horn (interneurons)
Visceral motor
neuron
SS
VS
VM
SM
Interneurons receiving input from somatic sensory neurons
Interneurons receiving input from visceral sensory neurons
Visceral motor (autonomic) neurons
Somatic motor neurons
SS
VS
VM
SM
Figure 12.29
Organization of the gray matter
of the spinal cord.
The gray matter of the spinal
cord is divided into a sensory half dorsally and a
motor half ventrally. Note that the dorsal and ventral
roots are part of the PNS, not of the spinal cord.
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