PROPERTIES OF TOUCH RECEPTORS IN DISTAL GLABROUS SKIN OF THE MONKEY

Abstract

Each touch fiber innervates a small skin area (2.5 - 33 mm2), known as the receptive field. At any point in this field, a minute displacement of the skin may elicit a discharge of afferent impulses in the fiber. A prerequisite is that the displacement proceeds at, or above, a certain minimum velocity, and that the amplitude of displacement exceeds the receptor threshold. With linearly increased displacement, produced by a mechanical stimulator, the minimum effective velocity was found to vary from less than 0.08 mm/sec, (stimulator limit) up to 3.5 mm/sec. The threshold amplitude, measured with sharply rising mechanical pulses, was from 10 to 200 [mu]. The input-output relationship of the receptors was characterized by a modulation of the impulse frequency according to the displacement velocity in the range 0.1 - 50 mm/sec. All receptors adapted rapidly to maintained displacement with complete cessation of the discharge. Repetitive stimulation with short mechanical pulses of variable amplitude showed that the receptor threshold increases after each impulse, presumably due to summating refractoriness of the nerve ending, so that, at moderate and high frequencies, even a receptitive stimulus produces only an initial burst of impulses.