Chapter 14
The Autonomic Nervous System
525
14
Comparison of the Somatic
and Autonomic Nervous Systems
In our previous discussions of motor nerves, we have focused
largely on the somatic nervous system. So, before describing au-
tonomic nervous system anatomy, let’s point out key differences
between the somatic and autonomic systems as well as areas of
functional overlap.
Both systems have motor fibers, but the somatic and auto-
nomic nervous systems differ in: (1) their effectors, (2) their
efferent pathways and ganglia, and (3) target organ responses to
their neurotransmitters. Consult
Figure 14.2
for a summary of
the differences as we discuss them next.
Effectors
Te somatic nervous system stimulates skeletal muscles,
whereas the ANS innervates cardiac and smooth muscle and
glands. Differences in the physiology of the effector organs ac-
count for most of the remaining differences between somatic
and autonomic effects on their target organs.
Efferent Pathways and Ganglia
In the somatic nervous system, the motor neuron cell bodies are
in the CNS, and their axons extend in spinal or cranial nerves
all the way to the skeletal muscles they activate. Somatic motor
fibers are typically thick, heavily myelinated group A fibers that
conduct nerve impulses rapidly.
In contrast, the ANS uses a
two-neuron chain
to reach its
effectors:
1.
Te cell body of the first neuron, the
preganglionic neu-
ron
, resides in the brain or spinal cord. Its axon, the
pre-
ganglionic axon
, synapses with the second motor neuron.
2.
Te
postganglionic neuron
(sometimes called the
gangli-
onic neuron
), is the second motor neuron. Its cell body is
in an
autonomic ganglion
outside the CNS. Its axon, the
postganglionic axon,
extends to the effector organ.
If you think about the meanings of all these terms while refer-
ring to Figure 14.2, understanding the rest of the chapter will be
much easier.
Preganglionic axons are thin, lightly myelinated fibers, and
postganglionic axons are even thinner and nonmyelinated.
Consequently, conduction through the autonomic efferent
chain is slower than conduction in the somatic motor system.
For most of their course, many pre- and postganglionic fibers
are incorporated into spinal or cranial nerves.
Keep in mind that autonomic ganglia are
motor
ganglia,
containing the cell bodies of motor neurons. ±echnically, they
are sites of synapse and information transmission from pre-
ganglionic to postganglionic neurons. Also, remember that
the somatic motor division
lacks
ganglia entirely. Te dorsal
root ganglia are part of the sensory, not the motor, division of
the PNS.
Neurotransmitter Effects
All
somatic motor neurons release
acetylcholine (ACh)
at their
synapses with skeletal muscle fibers. Te effect is always
excit-
atory
, and if stimulation reaches threshold, the muscle fibers
contract.
Autonomic postganglionic fibers release two neurotransmit-
ters:
norepinephrine (NE)
secreted by most sympathetic fibers,
and ACh secreted by parasympathetic fibers. Depending on the
type of receptors on the target organ, the effect may be excita-
tory or inhibitory (Figure 14.2; see ±able 14.2 on p. 534).
Overlap of Somatic and Autonomic Function
Higher brain centers regulate and coordinate both somatic and
autonomic motor activities, and most spinal nerves (and many
cranial nerves) contain both somatic and autonomic fibers.
Moreover, most of the body’s adaptations to changing inter-
nal and external conditions involve both skeletal muscles and
visceral organs. For example, when skeletal muscles are work-
ing hard, they need more oxygen and glucose, so autonomic
Central nervous system (CNS)
Peripheral nervous system (PNS)
Motor (efferent) division
Sensory (afferent)
division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Figure 14.1
Place of the ANS in the structural organization of the nervous system.
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