Sympathetic Nervous System
In the course of phylogeny an efficient control system evolved that enabled the functions of individual organs to be orchestrated in increasingly complex life forms and permitted rapid adaptation to changing environmental conditions. This regulatory system consists of the CNS (brain plus spinal cord) and two separate pathways for two-way communication with peripheral organs, viz., the somatic and the autonomic nervous systems. The somatic nervous system comprising extero- and interoceptive afferents, special sense organs, and motor efferents, serves to perceive external states and to target appropriate body movement (sensory perception: threat _ response: flight or attack). The autonomic (vegetative) nervous system (ANS), together with the endocrine system, controls the milieu interieur. It adjusts internal organ functions to the changing needs of the organism. Neural control permits very quick adaptation, whereas the endocrine system provides for a long-term regulation of functional states. The ANS operates largely beyond voluntary control; it functions autonomously. Its central components reside in the hypothalamus, brain stem, and spinal cord. The ANS also participates in the regulation of endocrine functions.
The ANS has sympathetic and parasympathetic branches. Both are made up of centrifugal (efferent) and centripetal (afferent) nerves. In many organs innervated by both branches, respective activation of the sympathetic and parasympathetic input evokes opposing responses.
In various disease states (organ malfunctions), drugs are employed with the intention of normalizing susceptible organ functions. To understand the biological effects of substances capable of inhibiting or exciting sympathetic or parasympathetic nerves, one must first envisage the functions subserved by the sympathetic and parasympathetic divisions (A, Responses to sympathetic activation). In simplistic terms, activation of the sympathetic division can be considered a means by which the body achieves a state of maximal work capacity as required in fight or flight situations.
In both cases, there is a need for vigorous activity of skeletal musculature. To ensure adequate supply of oxygen and nutrients, blood flow in skeletal muscle is increased; cardiac rate and contractility are enhanced, resulting in a larger blood volume being pumped into the circulation. Narrowing of splanchnic blood vessels diverts blood into vascular beds in muscle.
Because digestion of food in the intestinal tract is dispensable and only counterproductive, the propulsion of intestinal contents is slowed to the extent that peristalsis diminishes and sphincteric tonus increases. However, in order to increase nutrient supply to heart and musculature, glucose from the liver and free fatty acid from adipose tissue must be released into the blood. The bronchi are dilated, enabling tidal volume and alveolar oxygen uptake to be increased.
Sweat glands are also innervated by sympathetic fibers (wet palms due to excitement); however, these are exceptional as regards their neurotransmitter (ACh).
Although the life styles of modern humans are different from those of hominid ancestors, biological functions have remained the same.
Thanks & Best Regards,
Mukesh Patel
