THE AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system, like the somatic nervous system, is organized on the basis of
the reflex arc. Impulses initiated in visceral receptors are relayed via afferent autonomic pathways to
the CNS, integrated within it at various levels, and transmitted via efferent pathways to visceral
effectors. The peripheral motor portions of the autonomic nervous system are made up of
preganglionic and postganglionic neurons (see figure). The cell bodies of the preganglionic neurons
are located in the visceral efferent intermediolateral gray column (IML) of the spinal cord or the
homologous motor nuclei of the cranial nerves. Their axons are mostly myelinated, relatively slowly
conducting B fibers. The axons synapse on the cell bodies of postganglionic neurons that are located
in all cases outside the CNS. Each preganglionic axon diverges to an average of eight or nine
postganglionic neurons. In this way, autonomic output is diffused. The axons of the postganglionic
neurons, mostly unmyelinated C fibers, end on the visceral effectors.
, Anatomically, the autonomic outflow is divided into two components: the sympathetic and
parasympathetic divisions of the autonomic nervous system. In the gastrointestinal tract, these both
communicate with the enteric nervous system.
Sympathetic Division
The axons of the sympathetic preganglionic neurons leave the spinal cord with the ventral
roots of the first thoracic to the third or fourth lumbar spinal nerves. They pass via the white rami
communicantes to the paravertebral sympathetic ganglion chain, where most of them end on the cell
bodies of the postganglionic neurons. The axons of some of the postganglionic neurons pass to the
viscera in the various sympathetic nerves. Others reenter the spinal nerves via the gray rami
communicantes from the chain ganglia and are distributed to autonomic effectors in the areas supplied
by these spinal nerves. The postganglionic sympathetic nerves to the head originate in the superior,
middle, and stellate ganglia in the cranial extension of the sympathetic ganglion chain and travel to the
effectors with the blood vessels. Some preganglionic neurons pass through the paravertebral ganglion
chain and end on postganglionic neurons located in collateral ganglia close to the viscera. Parts of the
uterus and the male genital tract are innervated by a special system of short noradrenergic neurons
with cell bodies in ganglia in or near these organs, and the preganglionic fibers to these postganglionic
neurons presumably go all the way to the organs.
Parasympathetic Division
The cranial outflow of the parasympathetic division supplies the visceral structures in the head
via the oculomotor, facial, and glossopharyngeal nerves, and those in the thorax and upper abdomen
via the vagus nerves. The sacral outflow supplies the pelvic viscera via the pelvic branches of the
second to fourth sacral spinal nerves. The preganglionic fibers in both outflows end on short
postganglionic neurons located on or near the visceral structures.
Chemical Transmission at Autonomic Junctions
Transmission at the synaptic junctions between pre- and postganglionic neurons and between
the postganglionic neurons and the autonomic effectors is chemically mediated. The principal
transmitter agents involved are acetylcholine and norepinephrine.
The neurons that are cholinergic are (1) all preganglionic neurons, (2) the anatomically
parasympathetic postganglionic neurons, (3) the anatomically sympathetic postganglionic neurons
that innervate sweat glands, and (4) the anatomically sympathetic neurons that end on blood vessels
in skeletal muscles and produce vasodilation when stimulated (sympathetic vasodilator nerves). The
remaining postganglionic sympathetic neurons are noradrenergic. The adrenal medulla is essentially a
sympathetic ganglion in which the postganglionic cells have lost their axons and secrete
norepinephrine, epinephrine, and some dopamine.
Responses of Effector Organs to Autonomic Nerve Impulses
The effects of stimulation of the noradrenergic and cholinergic postganglionic nerve fibers to
the viscera are listed in the table. The smooth muscle in the walls of the hollow viscera is generally
innervated by both noradrenergic and cholinergic fibers, and activity in one of these systems increases
the intrinsic activity of the smooth muscle, whereas activity in the other decreases it. However, there
is no uniform rule about which system stimulates and which inhibits. In the case of sphincter muscles,
The autonomic nervous system, like the somatic nervous system, is organized on the basis of
the reflex arc. Impulses initiated in visceral receptors are relayed via afferent autonomic pathways to
the CNS, integrated within it at various levels, and transmitted via efferent pathways to visceral
effectors. The peripheral motor portions of the autonomic nervous system are made up of
preganglionic and postganglionic neurons (see figure). The cell bodies of the preganglionic neurons
are located in the visceral efferent intermediolateral gray column (IML) of the spinal cord or the
homologous motor nuclei of the cranial nerves. Their axons are mostly myelinated, relatively slowly
conducting B fibers. The axons synapse on the cell bodies of postganglionic neurons that are located
in all cases outside the CNS. Each preganglionic axon diverges to an average of eight or nine
postganglionic neurons. In this way, autonomic output is diffused. The axons of the postganglionic
neurons, mostly unmyelinated C fibers, end on the visceral effectors.
, Anatomically, the autonomic outflow is divided into two components: the sympathetic and
parasympathetic divisions of the autonomic nervous system. In the gastrointestinal tract, these both
communicate with the enteric nervous system.
Sympathetic Division
The axons of the sympathetic preganglionic neurons leave the spinal cord with the ventral
roots of the first thoracic to the third or fourth lumbar spinal nerves. They pass via the white rami
communicantes to the paravertebral sympathetic ganglion chain, where most of them end on the cell
bodies of the postganglionic neurons. The axons of some of the postganglionic neurons pass to the
viscera in the various sympathetic nerves. Others reenter the spinal nerves via the gray rami
communicantes from the chain ganglia and are distributed to autonomic effectors in the areas supplied
by these spinal nerves. The postganglionic sympathetic nerves to the head originate in the superior,
middle, and stellate ganglia in the cranial extension of the sympathetic ganglion chain and travel to the
effectors with the blood vessels. Some preganglionic neurons pass through the paravertebral ganglion
chain and end on postganglionic neurons located in collateral ganglia close to the viscera. Parts of the
uterus and the male genital tract are innervated by a special system of short noradrenergic neurons
with cell bodies in ganglia in or near these organs, and the preganglionic fibers to these postganglionic
neurons presumably go all the way to the organs.
Parasympathetic Division
The cranial outflow of the parasympathetic division supplies the visceral structures in the head
via the oculomotor, facial, and glossopharyngeal nerves, and those in the thorax and upper abdomen
via the vagus nerves. The sacral outflow supplies the pelvic viscera via the pelvic branches of the
second to fourth sacral spinal nerves. The preganglionic fibers in both outflows end on short
postganglionic neurons located on or near the visceral structures.
Chemical Transmission at Autonomic Junctions
Transmission at the synaptic junctions between pre- and postganglionic neurons and between
the postganglionic neurons and the autonomic effectors is chemically mediated. The principal
transmitter agents involved are acetylcholine and norepinephrine.
The neurons that are cholinergic are (1) all preganglionic neurons, (2) the anatomically
parasympathetic postganglionic neurons, (3) the anatomically sympathetic postganglionic neurons
that innervate sweat glands, and (4) the anatomically sympathetic neurons that end on blood vessels
in skeletal muscles and produce vasodilation when stimulated (sympathetic vasodilator nerves). The
remaining postganglionic sympathetic neurons are noradrenergic. The adrenal medulla is essentially a
sympathetic ganglion in which the postganglionic cells have lost their axons and secrete
norepinephrine, epinephrine, and some dopamine.
Responses of Effector Organs to Autonomic Nerve Impulses
The effects of stimulation of the noradrenergic and cholinergic postganglionic nerve fibers to
the viscera are listed in the table. The smooth muscle in the walls of the hollow viscera is generally
innervated by both noradrenergic and cholinergic fibers, and activity in one of these systems increases
the intrinsic activity of the smooth muscle, whereas activity in the other decreases it. However, there
is no uniform rule about which system stimulates and which inhibits. In the case of sphincter muscles,
