Chapter 13: 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 and postganglionic sympathetic and parasympathetic neurons.

• Name the neurotransmitters that are released by preganglionic autonomic neurons, postganglionic sympathetic neurons, postganglionic parasympathetic neurons, and adrenal medullary cells.

• Name the types of receptors on autonomic ganglia and on various target organs and list the ways that drugs can act to alter the function of the processes involved in transmission within the autonomic nervous system.

• Describe functions of the sympathetic and parasympathetic nervous systems.

• Describe the location of some forebrain and brainstem neurons that are components of central autonomic pathways.

• Describe the composition and functions of the enteric nervous system.

INTRODUCTION

The autonomic nervous system (ANS) is the part of the nervous system that is responsible for homeostasis. Except for skeletal muscle, which gets its innervation from the somatomotor nervous system, innervation to all other organs is supplied by the ANS. Nerve terminals are located in smooth muscle (eg, blood vessels, the wall of the gastrointestinal tract, and urinary bladder), cardiac muscle, and glands (eg, sweat glands and salivary glands). Although survival is possible without an ANS, the ability to adapt to environmental stressors and other challenges is severely compromised. The importance of understanding the functions of the ANS is underscored by the fact that so many drugs used to treat a vast array of diseases exert their actions on elements of the ANS. Also, many neurologic diseases or disorders result directly from a loss of preganglionic sympathetic neurons (eg, multiple system atrophy and Shy–Drager syndrome) and other common diseases (eg, Parkinson disease and diabetes) are associated with autonomic dysfunction (Clinical Box 13–1).

The ANS has two major and anatomically distinct 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 system 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 system as the cranial and spinal motor neurons. A modern definition of the ANS takes into account the descending pathways from several forebrain and brainstem regions as well as visceral afferent pathways that set the level of activity in sympathetic 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.