Principles of Manual Medicine
Autonomic Nervous System - Overview
The portion of the nervous system that controls the visceral functions of the body is called the autonomic nervous system. The autonomic nervous system is controlled mainly by centers located in the spinal cord, brain stem, and hypothalamus, and functions to control arterial pressure, gastrointestinal motility and secretion, urinary output, sweating, body temperature, and many other bodily activities.
In general, the sympathetic system often mediates the response of the body to stress by speeding up heart rate, increasing blood pressure, and mobilizing the body's energy stores for an emergency response. In contrast, the parasympathetic system acts to conserve the body's resorces and maintain homeostasis by slowing heart rate, decreasing blood pressure, and preparing the body to rest. While the sympathetic and parasympathetic divisions normally act in synergy, they will at other times oppose one another.
The autonomic nervous system, like the somatic nervous system, is organized on the basis of the reflex arc. Sensory signals from appropriate parts of the body send impulses into the centers of the cord, brain stem, or hypothalamus, and these in turn transmit appropriate reflex responses back to the visceral organs to control their activities. 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. The cell bodies of all preganglionic neurons are located within the central nervous system (CNS). The cell bodies of symapathetic preganglionic neurons are located in the visceral efferent (lateral gray) column of the spinal cord. The cell bodies of parasymapathetic preganglionic neurons are located in the homologous motor nuclei of the cranial nerves. The axons of preganglionic neurons are mostly myelinated, relatively slow-conducting fibers. These axons synapse on the cell bodies of postganglionic neurons which are located in all cases outside the CNS.
It will be recalled that somatic motor fibers secrete acetylcholine. This is also true of the preganglionic neurons of both the sympathetic and parasympathetic system, and it is true of the postganglionic neurons of the parasympathetic system (Click here to review neurotransmitters of the autonomic nervous system). These fibers are said to be cholinergic because they secrete acetylcholine at their nerve endings. While a few postganglionic endings of the sympathetic nervous system secrete acetylcholine, the majority of the sympathetic endings secrete norepinephrine. These fibers are said to be adrenergic. Since each preganglionic axon diverges to an average of 8-9 postganglionic neurons, autonomic output is widely diffused. Consequently, there is overlap of control at the level of the spinal cord, and variation among individuals.
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