Relations between stimulus force, skin displacement, and discharge characteristics of slowly adapting type I cutaneous mechanoreceptors in glabrous skin of squirrel monkey hand.

The contributions of viscoelastic properties of squirrel monkey glabrous skin to slowly adapting Type I (SAI) mechanoreceptive afferent fiber discharge were examined in the present study. Individual fibers of the median and ulnar nerves were isolated by microdissection in six monkeys anesthetized with pentobarbital sodium. Utilizing mechanical stimulation and data analysis techniques identical to those of a previous study of raccoon glabrous skin and its mechanoreceptors (Pubols, 1982a; Pubols and Maliniak, 1984), we studied and compared responses to punctate mechanical stimuli controlled with respect to force or displacement. Squirrel monkey glabrous skin was found to be more compliant than raccoon glabrous skin, in that a given force applied to either a digital or a palmar skin pad produced a greater displacement of squirrel monkey skin. Skin displacement increased approximately linearly with increasing forces at the beginning of static stimulation, but over time (at least up to 20 sec), the relationship became negatively accelerated. Absolute-force thresholds of individual SAI units were significantly lower in squirrel monkey (mean = 122 mg, range = 48-340 mg) than in raccoon (mean = 484 mg, range = 70-1,290 mg). However, absolute-displacement thresholds were insignificantly lower (squirrel monkey: mean = 17.24 microns, range = 5-30 microns; raccoon: mean = 30 microns, range = 5-185 microns). Application of suprathreshold forces (range = 1-20 g) and displacements (range = 500-1,000 microns) revealed greater interunit variability in response to maintained stimulation than previously found in raccoon. In 8 out of 15 fibers, the rate of adaptation was significantly greater during constant-displacement than during constant-force stimulation; in 4 cases there was no significant difference; and in 3 cases the rate of adaptation was significantly greater during constant-force stimulation. Potential sources of interunit variability include surface topography of the hand, properties of cutaneous and subcutaneous tissues in the vicinity of the receptor, and experimental variables such as stimulus amplitude and rate of stimulus onset. It is suggested that both regional and species differences in functional properties of cutaneous mechanoreceptors are more likely attributable to differences in mechanical properties of skin and subjacent tissues than to any inherent differences in receptor properties.