Sympathetic and Parasympathetic “Tone”

Sympathetic and Parasympathetic “Tone”

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 nervoussystem both to increase and to decrease the activity of a stimulated organ. For instance, sympathetic tone nor-mally keeps almost all the systemic arterioles con-stricted 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 sym-pathetic system could cause only vasoconstriction, never vasodilation.

Another interesting example of tone is the back-ground “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 gastrointesti-nal activity.

Tone Caused by Basal Secretion of Epinephrine and Norepi-nephrine by the Adrenal Medullae. The normal restingrate 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 sympa-thetic nervous system results from basal secretion of epinephrine and norepinephrine in addition to the tone resulting from direct sympathetic stimulation.

Effect of Loss of Sympathetic or Parasympathetic Tone After Denervation. Immediately after a sympathetic orparasympathetic nerve is cut, the innervated organ loses its sympathetic or parasympathetic tone. In the case of the blood vessels, for instance, cutting the sym-pathetic nerves results within 5 to 30 seconds in almost maximal vasodilation. However, over minutes, hours, days, or weeks, intrinsic tone in the smooth muscle of the vessels increases—that is, increased tone caused by increased smooth muscle contractile force that is not the result of sympathetic stimulation but of chemical adaptations in the smooth muscle fibers themselves. This intrinsic tone eventually restores almost normal vasoconstriction.

Essentially the same effects occur in most other effector organs whenever sympathetic or parasympa-thetic tone is lost. That is, intrinsic compensation soon develops to return the function of the organ almost to its normal basal level. However, in the parasympa-thetic system, the compensation sometimes requires many months. For instance, loss of parasympathetic tone to the heart after cardiac vagotomy increases the heart rate to 160 beats per minute in a dog, and this will still be partially elevated 6 months later.