A single sympathetic preganglionic fiber branches a number of times after entering a ganglion and makes synaptic connection with a number of postganglionic neurons. Furthermore, some branches of this pregan-glionic fiber may ascend or descend to adjacent vertebral ganglia and terminate on an additional number of postganglionic neurons in these ganglia as well. Therefore, activity in a single sympathetic preganglionic neuron may result in the activation of a number of effector cells in widely separated regions of the body. Anatomically, the sympathetic nervous system is designed to produce widespread physiological activity. The sympathetic nervous system prepares the body for strenuous muscular activity, stress, and emergencies.
By contrast, parasympathetic preganglionic neurons are extremely limited in their distribution. In general, a single parasympathetic preganglionic fiber makes a synaptic connection with only one or two postgan-glionic neurons. For this reason, along with the fact that the ganglia are near or are embedded in the organs innervated, individual parasympathetic preganglionic neurons influence only a small region of the body or affect only specific organs. The parasympathetic nervous system is involved with the accumulation, storage, and preservation of body resources.
When the sympathetic integrative centers in the brain are activated (by anger, stress, or emergency), the body's resources are mobilized for combat or for flight. Stimulation of the sympathetic nervous system results in acceleration of the heart rate and an increase in the contractile force of the heart muscle. There is increased blood flow (vasodilation) through skeletal muscle and decreased blood flow (vasoconstriction) through the skin and visceral organs. Activity of the gastrointestinal tract, such as peristaltic and secretory activity, is decreased, and intestinal sphincters are contracted. The pupils are dilated. The increased breakdown of glyco-gen (glycogenolysis) in the liver produces an increase in blood sugar, while the breakdown of lipids (lipolysis) in adipose tissue produces an increase in blood fatty acids; these biochemical reactions make energy available for active tissues. In addition to generalized activation of the sympathetic system in response to stress, there can be more discrete homeostatic activation of the sympathetic system. For example, a selective reflex-associated alteration in the sympathetic outflow to the cardiovascular system can occur.
The parasympathetic system is designed to function more or less on an organ system basis, usually under conditions of minimal stress. For example, the activation of the gastrointestinal tract takes place during digestion of a meal; constriction of the pupil and accommodation for near vision are essential for reading.
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