180
Chapter 6
The simplest distinction between the somatic and auto-
nomic systems is that the neurons of the somatic division inner-
vate skeletal muscle, whereas the autonomic neurons innervate
smooth and cardiac muscle, glands, and neurons in the gastro-
intestinal tract. Other differences are listed in
Table 6–9
.
The somatic portion of the efferent division of the
peripheral nervous system is made up of all the nerve fi bers
going from the central nervous system to skeletal muscle
cells. The cell bodies of these neurons are located in groups
in the brainstem or the ventral horn of the spinal cord. Their
large-diameter, myelinated axons leave the central nervous
system and pass without any synapses to skeletal muscle cells.
The neurotransmitter these neurons release is acetylcholine.
Because activity in the somatic neurons leads to contraction of
the innervated skeletal muscle cells, these neurons are called
motor neurons.
Excitation of motor neurons leads only to the
contraction
of skeletal muscle cells; there are no somatic neu-
rons that inhibit skeletal muscles. Muscle relaxation involves
the inhibition of the motor neurons in the spinal cord.
Autonomic Nervous System
The efferent innervation of tissues other than skeletal mus-
cle is by way of the autonomic nervous system. A special case
occurs in the gastrointestinal tract, where autonomic neurons
innervate a nerve network in the wall of the intestinal tract.
Chapter 15 will describe this network, termed the
enteric
nervous system,
in more detail.
In contrast to the somatic nervous system, the auto-
nomic nervous system is made up of two neurons in series
that connect the central nervous system and the effector cells
(
Figure 6–43
). The fi rst neuron has its cell body in the cen-
tral nervous system. The synapse between the two neurons is
outside the central nervous system in a cell cluster called an
autonomic ganglion.
The neurons passing between the cen-
tral nervous system and the ganglia are called
preganglionic
neurons;
those passing between the ganglia and the effector
cells are
postganglionic neurons.
Anatomical and physiological differences within the
autonomic nervous system are the basis for its further subdivi-
sion into
sympathetic
and
parasympathetic
divisions (review
Figure 6–37). The neurons of the sympathetic and parasym-
pathetic divisions leave the central nervous system at different
levels—the sympathetic fi bers from the thoracic (chest) and
lumbar regions of the spinal cord, and the parasympathetic
fi bers from the brainstem and the sacral portion of the spinal
cord (
Figure 6–44
). Therefore, the sympathetic division is
also called the thoracolumbar division, and the parasympa-
thetic is called the craniosacral division.
The two divisions also differ in the location of ganglia.
Most of the sympathetic ganglia lie close to the spinal cord
and form the two chains of ganglia—one on each side of the
cord—known as the
sympathetic trunks
(see Figure 6–44).
Other sympathetic ganglia, called collateral ganglia—the
celiac, superior mesenteric, and inferior mesenteric ganglia—
are in the abdominal cavity, closer to the innervated organ
(see Figure 6–44). In contrast, the parasympathetic ganglia
lie within, or very close to, the organs that the postganglionic
neurons innervate.
Preganglionic sympathetic neurons leave the spinal
cord only between the fi rst thoracic and second lumbar
segments, whereas sympathetic
trunks
extend the entire
length of the cord, from the cervical levels high in the neck
down to the sacral levels. The ganglia in the extra lengths
of sympathetic trunks receive preganglionic neurons from
the thoracolumbar regions because some of the pregangli-
onic neurons, once in the sympathetic trunks, turn to travel
upward or downward for several segments before forming
Table 6–9
Peripheral Nervous System: Somatic
and Autonomic Divisions
Somatic
1. Consists of a single neuron between central nervous system
and skeletal muscle cells
2. Innervates skeletal muscle
3. Can lead only to muscle excitation
Autonomic
1. Has two-neuron chain (connected by a synapse) between
central nervous system and effector organ
2. Innervates smooth and cardiac muscle, glands, and GI
neurons
3. Can be either excitatory or inhibitory
Ganglion
Preganglionic
fiber
Postganglionic
fiber
Autonomic nervous system
Somatic nervous system
Skeletal
muscle
Smooth or
cardiac
muscles,
glands, or
GI neurons
Effector organ
CNS
CNS
Figure 6–43
Efferent division of the peripheral nervous system, including
an overall plan of the somatic and autonomic nervous systems.
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