The Autonomic Nervous System
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Te autonomic nervous system (ANS) is the motor division of the
PNS that controls visceral activities, with the goal of maintaining
Comparison of the Somatic and Autonomic Nervous
Te somatic (voluntary) nervous system provides motor ﬁbers to
skeletal muscles. Te autonomic (involuntary or visceral motor)
nervous system provides motor ﬁbers to smooth and cardiac
muscles and glands.
In the somatic division, a single motor neuron forms the eﬀerent
pathway from the CNS to the eﬀectors. Te eﬀerent pathway
of the autonomic division consists of a two-neuron chain: the
preganglionic neuron in the CNS and the postganglionic neuron
in a ganglion.
Acetylcholine, the neurotransmitter of somatic motor neurons, is
stimulatory to skeletal muscle ﬁbers. Neurotransmitters released
by autonomic motor neurons (acetylcholine and norepinephrine)
may excite or inhibit target cells.
Nervous System II; Topic: Synaptic Transmission, pp. 8–11.
Te ANS consists of two divisions, parasympathetic and
sympathetic, which normally exert antagonistic eﬀects on many
of the same target organs.
Te parasympathetic division (the rest-digest system) conserves
body energy and maintains body activities at basal levels.
Parasympathetic eﬀects include constricted pupils, glandular
secretion, increased digestive tract motility, and smooth muscle
activity leading to elimination of feces and urine.
Te sympathetic division prepares the body for activity (the ﬁght-
Sympathetic responses include dilated pupils, increased heart
rate, increased blood pressure, dilated bronchioles of the
lungs, increased blood glucose levels, and sweating. During
exercise, sympathetic vasoconstriction shunts blood from the
skin and digestive viscera to the heart, brain, and skeletal muscles.
Parasympathetic (Craniosacral) Division
Parasympathetic preganglionic neurons arise from the brain stem
and from the sacral (S
) region of the spinal cord.
Preganglionic ﬁbers synapse with postganglionic neurons in
terminal ganglia located in (intramural ganglia) or close to their
eﬀector organs. Preganglionic ﬁbers are long; postganglionic
ﬁbers are short.
Cranial ﬁbers arise in the brain stem nuclei of cranial nerves
III, VII, IX, and X and synapse in ganglia of the head, thorax,
and abdomen. Te vagus nerves serve virtually all organs of the
thoracic and abdominal cavities.
Sacral ﬁbers (S
) issue from the lateral region of the cord and
form pelvic splanchnic nerves that innervate the pelvic viscera. Te
preganglionic axons do not travel within rami communicantes.
Sympathetic (Thoracolumbar) Division
Preganglionic sympathetic neurons arise from the lateral horns of
the spinal cord from the level of ±
Preganglionic axons leave the cord via white rami
communicantes and enter the sympathetic trunk ganglia in the
sympathetic trunk. An axon may synapse in a trunk ganglion
at the same or at a diﬀerent level, or it may issue from the
sympathetic trunk without synapsing. Preganglionic ﬁbers are
short; postganglionic ﬁbers are long.
When the synapse occurs in a trunk ganglion, the postganglionic
ﬁber may enter the spinal nerve ramus via the gray ramus
communicans to travel to the body periphery. Postganglionic
ﬁbers issuing from the cervical ganglia also serve visceral organs
and blood vessels of the head, neck, and thorax.
When synapses do not occur in the trunk ganglia, the
preganglionic ﬁbers form splanchnic nerves (thoracic, lumbar,
and sacral). Most splanchnic nerve ﬁbers synapse in collateral
ganglia, and the postganglionic ﬁbers serve the abdominal
viscera. Exceptions are that (1) some splanchnic nerve ﬁbers
synapse with cells of the adrenal medulla, and (2) some
lumbar and sacral splanchnic nerve ﬁbers
synapse in trunk
Visceral reﬂex arcs have the same components as somatic reﬂexes:
receptor, sensory neuron, integration center, motor neurons,
Cell bodies of visceral sensory neurons are located in dorsal root
ganglia, sensory ganglia of cranial nerves, or autonomic ganglia.
Visceral aﬀerents are located in spinal nerves and in virtually all
Neurotransmitters and Receptors
Autonomic motor neurons release two major neurotransmitters,
acetylcholine (ACh) and norepinephrine (NE). Based on the
neurotransmitter they release, ﬁbers are classiﬁed as cholinergic
(ACh) or adrenergic (NE).
ACh is released by all preganglionic ﬁbers and all
parasympathetic postganglionic ﬁbers. NE is released by all
sympathetic postganglionic ﬁbers except those serving the sweat
glands of the skin.
Neurotransmitter eﬀects depend on the receptors to which
the neurotransmitter binds. Cholinergic (ACh) receptors are
classiﬁed as nicotinic or muscarinic. Adrenergic (NE) receptors
are classiﬁed as
, or β
, or β
Nervous System II; Topic: Synaptic Transmission, pp. 8–11, 14.