Lecture notes for Anatomy 350 - Functional Neuroanatomy - on a subject not adequately covered in the recommended text.
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return to the text document. J. A. Kiernan, Department of Anatomy and Cell Biology,
The control of smooth muscle, cardiac muscle or secretory tissues by the central nervous system always involves a chain of at least two neurons, preganglionic and postganglionic, with the cell body of the latter being in an autonomic ganglion.
Autonomic vs somatic innervation
The autonomic nervous system is classified into parasympathetic, sympathetic and enteric divisions. Parasympathetic ganglia are near the organs they innervate, whereas most sympathetic ganglia are paravertebral or preaortic
(See diagram).
(An important difference between a sensory ganglion and an
autonomic ganglion
is that the latter contains synapses whereas the former does not.)
The enteric ganglia are in the myenteric and submucous plexuses of the alimentary canal.
Preganglionic parasympathetic neurons are present in certain nuclei of cranial nerves III, VII, IX and X and in the intermediolateral cell columns of spinal segments S2-S4.
Parasympathetic ganglia supply the sphincter pupillae and ciliary muscles, the lacrimal and salivary glands, thoracic and abdominal viscera (including the heart), the urinary bladder and other pelvic organs, and the erectile tissue of the genitalia.
Preganglionic sympathetic neurons are in the intermediolateral cell column of spinal segments T1-L2. Their axons pass through the ventral roots and the white communicating rami into the sympathetic trunk.
Preganglionic sympathetic axons reach paravertebral ganglia
(see diagram)
by way of the sympathetic trunk, or preaortic ganglia by way of splanchnic nerves.
(See other diagram).
The adrenal medulla is a modified sympathetic ganglion, with neurons that release their transmitters directly into the blood.
Grey communicating rami carry postganglionic sympathetic axons into mixed nerves
(see diagram),
to supply blood vessels, sweat glands and piloarrector muscles. Cardiac sympathetic nerves, arising in the cervical ganglia, supply the heart. Mesenteric and similar nerve plexuses carry postganglionic sympathetic fibers from preaortic ganglia to abdominal organs.
The enteric nervous system can work independently, but its activities are normally modulated by preganglionic parasympathetic and postganglionic sympathetic axons. The
enteric plexuses
contain sensory neurons, interneurons and cells that provide excitatory and inhibitory innervation to the gut.
Acetylcholine is the principal neurotransmitter of all preganglionic neurons, and of all parasympathetic postganglionic neurons. Noradrenaline is the principal transmitter of all postganglionic sympathetic neurons except those supplying sweat glands, which are cholinergic. Several peptides also occur in autonomic neurons, and the enteric nervous system contains a particularly large variety of potential neurotransmitters.
The activities of the autonomic nervous system are subject to control by descending central pathways from the amygdala, septal area, hypothalamus and reticular formation.
The two types of visceral sensation are served by different nerves. Fibers for pain have cell bodies in dorsal root ganglia and axons that accompany the pre- and postganglionic sympathetic fibers. Most sensory neurons for visceral reflexes (and also for conscious sensation of fullness) have cell bodies in the inferior ganglion of the vagus nerve, and axons that accompany the preganglionic parasympathetic fibers.
Visceral pain is often referred to somatic structures supplied by the same spinal segments as the affected organ. The central pathway to the cerebral cortex is the spinothalamic system.
Sensory neurons for visceral reflexes project centrally to the solitary nucleus. This is connected with the hypothalamus, with various regions of the reticular formation, and with preganglionic autonomic neurons.
--------------------------The autonomic nervous system and visceral innervation
Copyright (c) 2002 J. A. Kiernan and The University of Western Ontario
The University of Western Ontario, London, Canada
Last updated: January 2003
