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The motor (efferent) portion of the nervous system can be divided into two major subdivisions: autonomic and somatic. The autonomic nervous system (ANS) is largely autonomous (independent) in that its activities are not under direct conscious control. The ANS is concerned primarily with visceral functions that are necessary for life. The somatic system is largely concerned with consciously controlled functions such as movement and posture. Both systems have important afferent (sensory) inputs that provide sensation and modify motor output through reflex arcs of varying size and complexity.

The nervous system has several properties in common with the endocrine system, which is the other major system for control of 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. Both systems use chemicals for the transmission of information. In the nervous system, chemical transmission occurs between nerve cells and between nerve cells and their effector cells. Chemical transmission takes place through the release of small amounts of transmitter substances from the nerve terminals into the synaptic space. The transmitter crosses the space by diffusion and activates or inhibits the postsynaptic cells by binding to a specialized receptor molecule.

Drugs that mimic or block the actions of chemical transmitters can selectively modify many autonomic functions. These functions involve a variety of effector tissues, including cardiac muscle, smooth muscle, vascular endothelium, exocrine glands, and presynaptic nerve terminals. Drugs affecting the autonomic nervous system are useful in many clinical conditions. Conversely, a very large number of drugs used for other purposes have unwanted effects on autonomic function.

The autonomic nervous system lends itself to division on anatomic grounds 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 and give rise to preganglionic efferent fibers that exit from the brain stem or spinal cord and terminate in motor ganglia. The sympathetic preganglionic fibers leave the central nervous system through the thoracic and lumbar spinal nerves. The parasympathetic preganglionic fibers leave the central nervous system through several of the cranial nerves and the third and fourth sacral spinal roots.

Figure 4–1.

Schematic diagram comparing some anatomic and neurotransmitter features of autonomic and somatic motor nerves. Only the primary transmitter substances are shown. Parasympathetic ganglia are not shown as discrete structures because most are in or near the wall of the organ innervated. 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; NE, norepinephrine; N, nicotinic receptors; M, muscarinic receptors. See text.)

Sympathetic preganglionic fibers terminate in ganglia located in the paravertebral ...

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