DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

Neurotransmission: The Autonomic
and Somatic Motor Nervous Systems
Thomas C. Westfall and
David P. Westfall
ANATOMY AND GENERAL FUNCTIONS
The autonomic nervous system, also called the visceral,
vegetative, or involuntary nervous system, is distributed
widely throughout the body and regulates autonomic
functions that occur without conscious control. In the
periphery, it consists of nerves, ganglia, and plexuses
that innervate the heart, blood vessels, glands, other
visceral organs, and smooth muscle in various tissues.
Differences Between Autonomic and Somatic Nerves.
The efferent nerves of the involuntary system supply
all innervated structures of the body except skeletal
muscle, which is served by somatic nerves. The most
distal synaptic junctions in the autonomic reflex arc
occur in ganglia that are entirely outside the cerebrospinal
axis. These ganglia are small but complex
structures that contain axodendritic synapses between
preganglionic and postganglionic neurons. Somatic
nerves contain no peripheral ganglia, and the synapses
are located entirely within the cerebrospinal axis. Many
autonomic nerves form extensive peripheral plexuses,
but such networks are absent from the somatic system.
Whereas motor nerves to skeletal muscles are myelinated,
postganglionic autonomic nerves generally are
nonmyelinated. When the spinal efferent nerves are
interrupted, the denervated skeletal muscles lack myogenic
tone, are paralyzed, and atrophy, whereas smooth
muscles and glands generally retain some level of spontaneous
activity independent of intact innervation.
Visceral Afferent Fibers. The afferent fibers from visceral
structures are the first link in the reflex arcs of the autonomic
system. With certain exceptions, such as local
axon reflexes, most visceral reflexes are mediated
through the central nervous system (CNS).
Information on the status of the visceral organs is
transmitted to the CNS through two main sensory
systems: the cranial nerve (parasympathetic) visceral
sensory system and the spinal (sympathetic) visceral
afferent system (Saper, 2002). The cranial visceral sensory
system carries mainly mechanoreceptor and
chemosensory information, whereas the afferents of the
spinal visceral system principally convey sensations
related to temperature and tissue injury of mechanical,
chemical, or thermal origin. Cranial visceral sensory
information enters the CNS by four cranial nerves: the
trigeminal (V), facial (VII), glossopharyngeal (IX), and
vagus (X) nerves. These four cranial nerves transmit
visceral sensory information from the internal face and
head (V); tongue (taste, VII); hard palate and upper part
of the oropharynx (IX); and carotid body, lower part of
the oropharynx, larynx, trachea, esophagus, and thoracic
and abdominal organs (X), with the exception of
the pelvic viscera. The pelvic viscera are innervated by
nerves from the second through fourth sacral spinal
segments.
The visceral afferents from these four cranial
nerves terminate topographically in the solitary tract
nucleus (STN) (Altschuler et al., 1989). The most massive
site for termination of the fibers from the STN is
the parabrachial nucleus, which is thus the major relay
station. The parabrachial nucleus consists of at least
13 separate subnuclei, which in turn project extensively
to a wide range of sites in the brainstem, hypothalamus,
basal forebrain, thalamus, and cerebral cortex. Other
direct projections from the STN also innervate these
brain structures.
Sensory afferents from visceral organs also enter the CNS
from the spinal nerves. Those concerned with muscle chemosensation
may arise at all spinal levels, whereas sympathetic visceral
sensory afferents generally arise at the thoracic levels where sympathetic
preganglionic neurons are found. Pelvic sensory afferents from
spinal segments S2–S4 enter at that level and are important for the