The autonomic nervous system controls involuntary
) functions and has three divisions. The sympathetic
divisions consist of two-neuron chains that connect the central nervous system
with the smooth muscle
s and gland
s of the viscera
, blood vessel
s, and skin
. The enteric
division is a largely independent system that lies in the walls of the gastrointestinal tract
and controls many digestive functions. The sympathetic system organizes the involuntary responses that anticipate maximal exertion
reaction). Conversely, the parasympathetic system organizes the involuntary responses that generally reflect visceral function in a state of relaxation
Sympathetic and parasympathetic ganglia are innervated by preganglionic neurons in the spinal cord. Sympathetic preganglionic axons arise from neurons in the thoracic and upper lumbar spinal cord. The preganglionic neurons that innervate the head and thoracic organs are in the upper and middle thoracic segments, and those that innerate the abdominal and pelvic organs are in the lower thoracic and upper lumbar segments. The parasympathetic preganglionic axons arise from neurons in the brainstem and sacral spinal cord. Many organs--including the salivary glands, heart, bladder, and sex organs--receive inputs from both the sympathetic and parasympathetic systems. Other targets receive only sympathetic innervation, including the sweat glands, the adrenal medulla, the piloerector muscles of the skin, and most blood vessels. The neurons innervated by the preganglionic sympathetic axons are mostly found in the sympathetic chain ganglia, whereas the parasympathetic motor neurons are located in ganglia within the organs they control.
The enteric nervous system, although it receives sympathetic and parasympathetic innervation, acts to some degree independently of the rest of the autonomic system. A rich intrinsic circuitry of sensory neurons, interneurons, and motor neurons interconnects different levels of the gut and coordinates activity along its length. The enteric system governs gut motility, secretion, and the transfer of substances across the gut epithelium.
Sensory inputs from the viscera modulate autonomic activity. Like other primary sensory neurons, the relevant cell bodies lie in dorsal root and cranial nerve ganglia; the visceral sensory axons that enter the spinal cord terminate mainly in the intermediate gray matter, near the preganglionic neurons of the thoracolumnar and sacral cord. Those that enter the brainstem (in cranial nerves VII, IX, and X) terminate in the nucleus of the solitary tract. Sensory fibers that travel in the sympathetic nerves convey visceral sensations, usually pain. Other fibers, including most of those that travel in the parasympathetic nerves to the nucleus of the solitary tract, convey information that does not reach consciousness, but which is important for reflex integration (for example, arterial baroreceptors and chemoreceptors).
Neuroscience, Sinaur Associates (QP355.2.N487 1997)