The autonomic nervous system (ANS) provides bidirectional communication (afferent and efferent fibers) between the brain and smooth muscle, cardiac muscle, and exocrine and endocrine glands. Its name is derived from the fact that the activities of the ANS are largely independent (ie, autonomous) of direct conscious control. The ANS is concerned primarily with necessary visceral functions like modification of blood flow to organs, digestion, and regulation of cardiac output. Afferent (sensory) fibers provide information regarding the internal and external environment and modify motor output through reflex arcs of varying size and complexity.
The ANS shares several commonalities with the endocrine system, the other major system that regulates bodily functions. These properties include high-level integration in the brain, the ability to influence processes in distant regions of the body, and extensive use of negative feedback. In the nervous system, information is transmitted between neurons and their effector cells via chemical transmission. Chemical transmission takes place through the release of small amounts of transmitter substances from the nerve terminals into the synapse. The transmitter diffuses across the synaptic cleft and binds to its specific receptor, which either activates or inhibits the postsynaptic cell.
Drugs that either mimic or block the actions of chemical transmitters can be used to selectively modify many autonomic functions such as blood pressure, heart rate, smooth muscle contraction, vasodilation, glandular secretion, and release of neurotransmitter from presynaptic nerve terminals. Drugs affecting the ANS are useful in many clinical conditions. Unfortunately, many drugs used for other purposes also have unwanted effects on autonomic function.
ANATOMY OF THE AUTONOMIC NERVOUS SYSTEM
The ANS is anatomically divided into two major portions: the sympathetic (thoracolumbar) division and the parasympathetic (craniosacral) division (Figure 4-1). Both divisions originate in nuclei within the central nervous system (CNS) and give rise to preganglionic efferent fibers that exit from the brainstem or spinal cord and terminate in motor ganglia. The sympathetic preganglionic fibers leave the CNS through the thoracic and lumbar spinal nerves. The parasympathetic preganglionic fibers leave the CNS through several of the cranial nerves and the third and fourth sacral spinal nerve roots.
Diagram comparing key anatomic and neurotransmitter features of autonomic and somatic motor nerves. Only primary neurotransmitters are shown. Parasympathetic ganglia are not shown as discrete structures because most are in or near the wall of the innervated organ. Note that some sympathetic postganglionic fibers release acetylcholine or dopamine rather than norepinephrine. The adrenal medulla, a modified sympathetic ganglion, receives sympathetic preganglionic fibers and releases mainly epinephrine and some norepinephrine into the blood. ACh, acetylcholine; D, dopamine; Epi, epinephrine; M, muscarinic receptors; N, nicotinic receptors; NE, norepinephrine.
The majority of sympathetic preganglionic fibers terminate in ganglia located in the paravertebral chains that lie on either side of the ...