The Autonomic Nervous System
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
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
for a summary of
the diﬀerences as we discuss them next.
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
to reach its
Te cell body of the ﬁrst neuron, the
, resides in the brain or spinal cord. Its axon, the
, synapses with the second motor neuron.
(sometimes called the
), is the second motor neuron. Its cell body is
outside the CNS. Its axon, the
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
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
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
ganglia entirely. Te dorsal
root ganglia are part of the sensory, not the motor, division of
somatic motor neurons release
synapses with skeletal muscle ﬁbers. Te eﬀect is always
, and if stimulation reaches threshold, the muscle ﬁbers
Autonomic postganglionic ﬁbers release two neurotransmit-
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
Place of the ANS in the structural organization of the nervous system.