Normally, the sympathetic and parasympathetic systems are continually active, and the basal rates of activity are known, respectively, as sympathetic tone and parasympathetic tone.
The value of tone is that it allows a single nervous system both to increase and to decrease the activity of a stimulated organ. For instance, sympathetic tone normally keeps almost all the systemic arterioles constricted to about one half their maximum diameter. By increasing the degree of sympathetic stimulation above normal, these vessels can be constricted even more; conversely, by decreasing the stimulation below normal, the arterioles can be dilated. If it were not for the continual background sympathetic tone, the sympathetic system could cause only vasoconstriction, never vasodilation.
Another interesting example of tone is the background "tone" of the parasympathetics in the gastrointestinal tract. Surgical removal of the parasym-pathetic supply to most of the gut by cutting the vagus nerves can cause serious and prolonged gastric and intestinal "atony" with resulting blockage of much of the normal gastrointestinal propulsion and consequent serious constipation, thus demonstrating that parasympathetic tone to the gut is normally very much required. This tone can be decreased by the brain, thereby inhibiting gastrointestinal motility, or it can be increased, thereby promoting increased gastrointestinal activity.
Tone Caused by Basal Secretion of Epinephrine and Norepi-nephrine by the Adrenal Medullae. The normal resting rate of secretion by the adrenal medullae is about 0.2 |mg/kg/min of epinephrine and about 0.05 mg/kg/min of norepinephrine. These quantities are considerable— indeed, enough to maintain the blood pressure almost up to normal even if all direct sympathetic pathways to the cardiovascular system are removed. Therefore, it is obvious that much of the overall tone of the sympathetic nervous system results from basal secretion of epinephrine and norepinephrine in addition to the tone resulting from direct sympathetic stimulation.
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