Coding of mechanical stimulus velocity and indentation depth by squirrel monkey and raccoon glabrous skin mechanoreceptors

Abstract

Median nerve fibers supplying mechanoreceptors of glabrous skin of squirrel monkey (Saimiri sciureus) hand were isolated by microdissection and examined for their response to controlled mechanical deformations of their receptive fields. Most fibers were either rapidly adapting (RA), responding only during mechanical ramp stimulation or very slowly adapting (VSA), continuing to respond for many seconds of static skin displacement. Mean instantaneous frequency during ramp stimulation was a power function of ramp velocity for both RA and VSA with exponents generally < 1.00. In both squirrel monkey and raccoon (Procyon lotor) slowly adapting 1st-order afferents, the nature of the best-fitting function relating static discharge rate to indentation depth varied from unit to unit, linear functions predominating for squirrel monkey units, logarithmic functions for raccoon units. An initially strong positive relation between static discharge and prior ramp velocity may change within the 1st s to an inverse relationship, lasting at least 5 s.