Human Physiology, 14th edition (2016)

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9.1 NEURAL CONTROL OF

INVOLUNTARY EFFECTORS

The autonomic nervous system helps regulate the activi-

ties of cardiac muscle, smooth muscles, and glands. In this

regulation, impulses are conducted from the CNS by an

axon that synapses with a second autonomic neuron. It is

the axon of this second neuron in the pathway that inner-

vates the involuntary effectors.

Autonomic motor nerves innervate organs whose func-

tions are not usually under voluntary control. The effectors that

respond to autonomic regulation include cardiac muscle (the

heart), smooth muscles, and glands. These effectors are part

of the visceral organs (organs within the body cavities) and of

blood vessels. The involuntary effects of autonomic innerva-

tion contrast with the voluntary control of skeletal muscles by

way of somatic motor neurons.

Autonomic Neurons

Neurons of the peripheral nervous system (PNS) that conduct

impulses away from the central nervous system (CNS) are

known as motor, or efferent, neurons (chapter 7, section 7.1).

There are two major categories of motor neurons: somatic

and autonomic. Somatic motor neurons have their cell bodies

within the CNS and send axons to skeletal muscles, which are

usually under voluntary control. This was briefly described in

chapter 8 (see fig. 8.28), in the section on the reflex arc. The

control of skeletal muscles by somatic motor neurons is dis-

cussed in depth in chapter 12, section 12.5.

Unlike somatic motor neurons, which conduct impulses along

a single axon from the spinal cord to the neuromuscular junction,

autonomic motor control involves two neurons in the efferent path-

way ( fig. 9.1 and table 9.1 ). The first of these neurons has its cell

body in the gray matter of the brain or spinal cord. The axon of this

neuron does not directly innervate the effector organ but instead

synapses with a second neuron within an autonomic ganglion (a

ganglion is a collection of cell bodies outside the CNS). The first

neuron is thus called a preganglionic neuron. The second neuron

in this pathway, called a postganglionic neuron, has an axon that

extends from the autonomic ganglion to an effector organ, where it

synapses with its target tissue ( fig. 9.1 ).

Preganglionic autonomic fibers originate in the midbrain

and hindbrain and in the upper thoracic to the fourth sacral levels

of the spinal cord. Autonomic ganglia are located in the head,

neck, and abdomen; chains of autonomic ganglia also parallel

the right and left sides of the spinal cord. The origin of the pre-

ganglionic fibers and the location of the autonomic ganglia help

to distinguish the sympathetic and parasympathetic divisions of

the autonomic system, discussed in later sections of this chapter.

Sofia was nervous about her upcoming exam, and to

quiet her anxiety she placed a finger on her radial artery

and concentrated as she attempted to lower her pulse

rate. It felt good to close her eyes! Her pupils had been

dilated for an eye exam that morning. She sniffed and

took a cold pill from her purse. It contained pseudo-

ephedrine, with a warning that people with hypertension

should be careful. Well, her blood pressure was under

control with a combination of drugs that included aten-

olol. When the exam arrived, she took a puff from her

asthma inhaler and began.

Some of the new terms and concepts you will

encounter include:

• Alpha and beta-adrenergic receptor types and their
  effects


• Atropine, atenolol, pseudoephedrine, albuterol, and
  sympathomimetics

Clinical Investigation

Figure 9.1 The autonomic system
has preganglionic and post-ganglionic
neurons. The preganglionic neurons of the
autonomic system have cell bodies in the
CNS, whereas the postganglionic neurons
have cell bodies within autonomic ganglia.
The sympathetic and parasympathetic
divisions differ in the particular locations of
their preganglionic neuron cell bodies within
the CNS, and in the location of their ganglia.

CNS

Autonomic

ganglion Involuntary

effector

Smooth

muscle

Postganglionic

neuron

Preganglionic

neuron

LEARNING OUTCOMES

After studying this section, you should be able to:

1. Describe the organization of autonomic motor
   neurons.


2. Describe how neural regulation of smooth and
   cardiac muscles differs from neural regulation of
   skeletal muscles.