Chapter 16: The Autonomic Nervous System and Visceral Reflexes
The autonomic nervous system controls many of the unconscious functions
necessary for life. Its two divisions are the sympathetic and parasympathetic systems, the
focus of this chapter.
General Properties of the Autonomic Nervous System
The autonomic nervous system (ANS) controls glands, cardiac muscle, and
smooth muscle. It is also called the visceral motor system. The primary target organs
are the viscera of the thoracic and abdominopelvic cavities, and, as such, it regulates
processes that are vital to maintaining homeostasis.
The word “autonomic” means “self-governed.” The ANS usually functions
unconsciously, without conscious intent, in contrast to the somatic motor system which
responds to voluntary commands. The distinction is not clear-cut, however, because
there are somatic reflexes and unconscious control of postural muscles. In addition,
biofeedback studies have shown that there can be some conscious control of autonomic
functions such as blood pressure and heart rate.
The ANS adjusts the function of visceral effectors rather than providing initial
stimulation. This is different than skeletal muscles, where if a nerve is severed, the
muscle cannot function. If an autonomic nerve is severed, the organ or gland will still
function, but will not be able to efficiently respond to changing conditions.
The ANS is responsible for the body’s visceral reflexes—unconscious, automatic,
stereotyped responses to stimulation of visceral receptors. Autonomic activity involves a
reflex arc that requires several components: receptors, afferent neurons leading to the
CNS, efferent neurons, and effectors.
Divisions of the Autonomic Nervous System
The two subdivisions of the ANS are the sympathetic and parasympathetic
divisions. The sympathetic division adapts the body for extraordinary circumstances that
call for action. When it is stimulated, there is increased alertness, heart rate, blood
pressure, blood glucose, and blood flow to the skeletal muscles. The catch phrase for the
system is “fight or flight”, but it is also involved in less dramatic day-to-day stress, anger,
competition, and sexual arousal. The parasympathetic division has a calming effect on
body functions, and results in decreased energy expenditure, and normal maintenance
activities such as digestion. It is often called the “rest and digest” part of the nervous