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