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CHAPTER

17

The Autonomic

Nervous System

OBJECTIVES

After studying this chapter, you should be able to:

■

Describe the location of the cell bodies and axonal trajectories of preganglionic

sympathetic and parasympathetic neurons.

■

Describe the location and trajectories of postganglionic sympathetic and para-

sympathetic neurons.

■

Name the neurotransmitters that are released by preganglionic autonomic neu-

rons,

postganglionic sympathetic neurons, postganglionic parasympathetic neu-

rons, and adrenal medullary cells.

■

Outline the functions of the autonomic nervous system.

■

List the ways that drugs act to increase or decrease the activity of the components

of the autonomic nervous system.

■

Describe the location of neurons that provide input to sympathetic preganglionic

neurons.

■

Describe the composition and functions of the enteric nervous system.

INTRODUCTION

The autonomic nervous system (ANS) is the part of the ner-

vous system that is responsible for homeostasis. Except for

skeletal muscle, which gets its innervation from the somato-

motor nervous system, innervation to all other organs is sup-

plied by the ANS. Nerve terminals are located in smooth

muscle (eg, blood vessels, gut wall, urinary bladder), cardiac

muscle, and glands (eg, sweat glands, salivary glands).

Although survival is possible without an ANS, the ability to

adapt to environmental stressors and other challenges is

severely compromised (see Clinical Box 17–1). The ANS has

two major divisions: the
sympathetic
and
parasympathetic

nervous systems. As will be described, some target organs are

innervated by both divisions and others are controlled by only

one. In addition, the ANS includes the

enteric nervous sys-

tem

within the gastrointestinal tract. The classic definition of

the ANS is the preganglionic and postganglionic neurons

within the sympathetic and parasympathetic divisions. This

would be equivalent to defining the somatomotor nervous sys-

tem as the cranial and spinal motor neurons. A modern defini-

tion of the ANS takes into account the descending pathways

from several forebrain and brain stem regions as well as vis-

ceral afferent pathways that set the level of activity in sympa-

thetic and parasympathetic nerves. This is analogous to

including the many descending and ascending pathways that

influence the activity of somatic motor neurons as elements of

the somatomotor nervous system.