Modulation of cutaneous mechanoreceptors by sympathetic stimulation

It has long been known that stimulation of the sympathetic nerve supply to skeletal muscle causes relief of fatigue (Orbeli, 1923). In the frog, this has recently been shown to be due to a facilitation of neuromuscular transmission brought about by an increased sensitivity of the motor end-plate to acetylcholine (Hutter & Loewenstein, 1955). The effect is small, the improvement of transmission seldom exceeding 10%. Teleological considerations led to the present work. The smallness of the effect induced speculation on a more general sympathetic facilitation fractioned at various stages of the reflex system. The possibility of a sympathetic influence on cutaneous mechanoreceptors is here tested. It was found that stimulation of the sympathetic nerve supply to an isolated frog's skin or application of adrenaline or noradrenaline alter the response of a tactile receptor to a mechanical stimulus, decreasing adaptation rate and threshold, and, eventually, causing spontaneous activity in the receptor. A preliminary report has appeared elsewhere (Loewenstein, 1955). Physiological interest in a possible action of the sympathetic system on sense organs dates from Claude Bernard's (1851) observations of changes in cutaneous sensibility following the extirpation of the cervical ganglion in the cat. Since then, a great number of contradictory claims of a sympathetic influence on many kinds of sense modalities have been presented (see v. Briicke, 1932, for a review). Within the technical limitations then existing the conclusions were based either on subjective tests, on clinical observations or on reflex excitability measurements, without eliminating the possibility of the sympathetic system influencing parts of the reflex arc other than the receptor and, generally, of circulatory changes masking the observed effects.