Human Physiology, 14th edition (2016)

The Autonomic Nervous System 245

Feature Somatic Motor Autonomic Motor

Effector organs Skeletal muscles Cardiac muscle, smooth muscle, and glands

Presence of ganglia No ganglia Cell bodies of postganglionic autonomic fibers

located in paravertebral, prevertebral (collateral),

and terminal ganglia

Number of neurons from CNS to effector One Two

Type of neuromuscular junction Specialized motor end plate No specialization of postsynaptic membrane; all

areas of smooth muscle cells contain receptor

proteins for neurotransmitters

Effect of nerve impulse on muscle Excitatory only Either excitatory or inhibitory

Type of nerve fibers Fast-conducting, thick (9–13 m m), and

myelinated

Slow-conducting; preganglionic fibers lightly

myelinated but thin (3 m m); postganglionic fibers

unmyelinated and very thin (about 1.0 m m)

Effect of denervation Flaccid paralysis and atrophy Muscle tone and function persist; target cells show

denervation hypersensitivity

Table 9.1 | Comparison of the Somatic Motor System and the Autonomic Motor System

The sensory neurons that conduct information from the viscera
for autonomic nerve reflexes can have the same anatomy as those
sensory neurons involved in somatic motor reflexes (chapter 8,
fig. 8.28). That is, the sensory information enters the spinal cord
on the dorsal roots of the spinal nerves. However, some important
visceral sensory information can instead enter the brain in cranial
nerves. For example, information about blood pressure, plasma
pH, and oxygen concentration is carried into the brain by sensory
axons in cranial nerves IX and X. These are mixed nerves, con-
taining both sensory and parasympathetic motor axons.

Visceral Effector Organs

Because the autonomic nervous system helps regulate the
activities of glands, smooth muscles, and cardiac muscle, auto-
nomic control is an integral aspect of the physiology of most
of the body systems. Autonomic regulation, then, plays roles
in endocrine regulation (chapter 11), smooth muscle function
(chapter 12), the functions of the heart and circulation (chap-
ters 13 and 14), and, in fact, all the remaining systems to be
discussed. Although the functions of the target organs of auto-
nomic innervation are described in subsequent chapters, at this
point we will consider some of the common features of auto-
nomic regulation.
Unlike skeletal muscles, which enter a state of flaccid paral-
ysis and atrophy when their motor nerves are severed, the invol-
untary effectors are somewhat independent of their innervation.
Smooth muscles maintain a resting tone (tension) in the absence
of nerve stimulation, for example. In fact, damage to an auto-
nomic nerve makes its target tissue more sensitive than normal
to stimulating agents. This phenomenon is called denervation
hypersensitivity. Such compensatory changes can explain why,
for example, the ability of the stomach mucosa to secrete acid

may be restored after its neural supply from the vagus nerve has

been severed. (This procedure is called vagotomy, and is some-

times performed as a treatment for ulcers.)

In addition to their intrinsic (“built-in”) muscle tone, car-

diac muscle and many smooth muscles take their autonomy

a step further. These muscles can contract rhythmically, even

in the absence of nerve stimulation, in response to electrical

waves of depolarization initiated by the muscles themselves.

Autonomic innervation simply increases or decreases this

intrinsic activity. Autonomic nerves also maintain a resting

tone, in the sense that they maintain a baseline firing rate

that can be either increased or decreased. A decrease in the

excitatory input to the heart, for example, will slow its rate

of beat.

The release of acetylcholine (ACh) from somatic motor

neurons always stimulates the effector organ (skeletal mus-

cles). By contrast, some autonomic nerves release transmit-

ters that inhibit the activity of their effectors. An increase

in the activity of the vagus, a nerve that supplies inhibitory

fibers to the heart, for example, will slow the heart rate,

whereas a decrease in this inhibitory input will increase the

heart rate.

| CHECKPOINT

1. Describe the preganglionic and postganglionic
   neurons in the autonomic system. Use a diagram to
   illustrate the difference in efferent outflow between
   somatic and autonomic nerves.


2. Compare the control of cardiac muscle and smooth
   muscles with that of skeletal muscles. How is each type
   of muscle tissue affected by cutting its innervation?