C h a p t e r 3 4
Organization and Control of Neural Function
835
The
general visceral efferent
(GVE) neurons are
structurally and functionally divided between either the
sympathetic or the parasympathetic nervous systems of
the ANS (discussed later in this chapter). Their axons
project through the segmental ventral roots to innervate
smooth and cardiac muscle and glandular cells of the
body, most of which are in the viscera.
The
general somatic efferent
(GSE) neurons supply
skeletal muscles of the body and head, including those
of the trunk, limbs, and tongue and the extrinsic eye
muscles. Because they transmit the commands of the
CNS to peripheral effectors, the skeletal muscles, they
are considered the “final common pathway neurons” in
the sequence leading to motor activity. They are often
called
lower motor neurons
(LMNs) because they are
under the control of
upper motor neurons
(UMNs) that
have their origin in the CNS.
The
pharyngeal efferent
(PE) neurons innervate the
muscles of mastication and facial expression, as well
as the muscles of the pharynx and larynx. Pharyngeal
efferent neurons also innervate the muscles responsible
for moving the head.
Longitudinal Tracts
The gray matter of the cell columns in the CNS is sur-
rounded by bundles of myelinated axons (i.e., white
matter) and unmyelinated axons that travel longitu-
dinally along the length of the neural axis. This white
matter can be divided into three layers: inner, middle,
and outer (Fig. 34-9). The inner layer contains short
fibers that project for a maximum of approximately
five segments before reentering the gray matter. The
fibers of the middle layer project to six or more seg-
ments. The outer layer contains large-diameter axons
that can travel the entire length of the nervous system
(Table 34-2).
Suprasegmental
is a term that refers to
higher levels of the CNS, such as the brain stem and
cerebrum and structures above a given CNS segment.
The middle and outer layer fibers have suprasegmental
projections.
The longitudinal layers are arranged in bundles, or
fiber tracts, that contain axons that have the same desti-
nation, origin, or function (Fig. 34-10). These longitudi-
nal tracts are named systematically to reflect their origin
and destination; the origin is named first, and the desti-
nation is named second. For example, the spinothalamic
tract
originates in the spinal cord and terminates in the
thalamus and the
corticospinal tract
originates in the
cerebral cortex and ends in the spinal cord.
The Inner Layer.
The inner or archi layer of white mat-
ter contains the axons of neurons that connect neighbor-
ing segments of the nervous system. Axons of this layer
permit the pool of motor neurons of several segments
to work together as a functional unit. They also allow
the afferent neurons of one segment to trigger reflexes
that activate motor units in the same or a neighboring
segment. The inner layer is the first of the longitudinal
layers to become functional, and is the most primitive.
Its circuitry may be limited to reflex movements, such as
those of the fetus (i.e., quickening) that begin during the
fifth month of intrauterine life.
The inner layer of the white matter differs from the
other two layers in one important aspect. Many neurons
in the embryonic gray matter migrate out into this layer,
resulting in a rich mixture of neurons and local fibers
called the
reticular formation
. The circuitry of most
reflexes is contained in the reticular formation. In the
brain stem, the reticular formation becomes quite large
and acts in vital reflexes controlling respiration, cardio-
vascular function, swallowing, and vomiting. A func-
tional system called the
reticular activating system
(RAS)
TABLE 34-2
Characteristics of the Concentric Subdivisions of the Longitudinal Tracts in theWhite
Matter of the Central Nervous System
Characteristics
ArchilayerTracts
PaleolayerTracts
NeolayerTracts
Segmental span
Intersegmental (
<
5 segments)
Suprasegmental (
≥
5 segments)
Suprasegmental
Number of synapses
Multisynaptic
Multisynaptic but fewer than
archilayer tracts
Monosynaptic with target
structures
Conduction velocity
Very slow
Fast
Fastest
Examples of functional
systems
Flexor withdrawal reflex
circuitry
Spinothalamic tracts
Corticospinal tracts
Gray matter
(dorsal horn)
Archilayer
tracts
Paleolayer
tracts
Neolayer
tracts
Tract fibers
synapsing
on lower
motor
neurons
FIGURE 34-9.
The three concentric subdivisions of the tract
systems of the white matter of the spinal cord. Migration
of neurons into the archilayer converts it into the reticular
formation of the white matter.
