Chapter 18 - Review of the autonomic nervous system

Abstract

The autonomic nervous system (ANS) is part of the peripheral nervous system, and controls the body's internal environment. It regulates involuntary physiologic processes such as heart rate, blood pressure, respiration, digestion, and sexual arousal. There are three distinct divisions of the ANS: the sympathetic, parasympathetic, and enteric nervous systems. The ANS can also be described as the visceral portion of the nervous system. It has neurons in both the central nervous system and the peripheral nervous system. The ANS uses efferent and afferent pathways to regulate body functions. The sympathetic and parasympathetic divisions have largely opposite functions. For example in the heart, the sympathetic division increases the rate and force of heartbeat, while the parasympathetic division decreases it. Preganglionic sympathetic fibers are myelinated, while postganglionic sympathetic fibers are unmyelinated. The major transmitters of the ANS include acetylcholine (utilizing cholinergic receptors) and norepinephrine (utilizing adrenergic receptors). Their effects can be excitatory, inhibitory, or neither. Cholinergic receptors are either nicotinic or muscarinic. Autonomic plexuses are large nerve networks that help distribute sympathetic, parasympathetic, and afferent fibers. Activity of the ANS is regulated by the cerebral cortex, hypothalamus, brainstem, and spinal cord.