Anatomy & Physiology

Anatomy & Physiology Study Guide The autonomic nervous system, composed of the sympathetic and parasympathetic divisions, is involved in the unconscious regulation of visceral functions. Conscious thoughts, plans, and actions represent a tiny fraction of the activities of the nervous system. The autonomic nervous system coordinates respiratory, cardiovascular, urinary, digestive, endocrine, and reproductive functions¬– all activities that humans execute without conscious control. In doing so, the ANS adjusts internal water, electrolyte, nutrient, and dissolved gas concentrations in body fluids. The ANS is responsible for the operation of all processes that humans need to survive. 15.3 The Autonomic Nervous System Both the SNS and ANS are efferent divisions of the central nervous system. There are some primary structural differences between the two. In the SNS, motor neurons of the central nervous system exert direct control over skeletal muscles. In the ANS, by contrast, motor neurons of the central nervous system synapse on visceral motor neurons. Divisions of the ANS There are two primary subdivisions of the ANS: the sympathetic division and the parasympathetic division. Most often, these two divisions have opposing effects; if the sympathetic division causes excitation, the parasympathetic inhibits. Though, this occurrence is not always the case. The sympathetic division activates only during periods of emergency, exertion, or stress; it is known as the “fight or flight” response. The parasympathetic division predominates under resting condition and responds as “rest and digest.” Sympathetic The sympathetic division (thoracolumbar) prepares the body for heightened levels of somatic activity. With full activation, the “fight or flight” response readies the body for a crisis that may require intense physical activity. Sympathetic activity increases alertness and stimulates tissue metabolism. The general pattern of the sympathetic response also includes reduced digestive and urinary functions, activation of energy reserves, increased respiratory rate and dilation of respiratory passageways, increased heart rate and blood pressure, and activation of sweat glands. The sympathetic division consists of preganglionic neurons and ganglionic neurons involved in using energy and increasing metabolic rate. This division contains preganglionic neurons that are located between segments T1 and L2 of the spinal cord, and ganglionic neurons that are located in ganglia near the vertebral column. The cell bodies of the preganglionic neurons are situated in the lateral gray horns. After passing through the intervertebral foramen, each ventral root gives rise to a myelinated white ramus, which carries myelinated preganglionic fibers into a nearby sympathetic chain ganglion. These fibers may synapse within the sympathetic chain ganglia, at one of the collateral ganglia, or in the suprarenal medullae.

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