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

Abstract

1. A sample of 113 large, myelinated first-order afferent fibers innervating the glabrous skin of the squirrel monkey's hand proved to consist primarily of two basic types. In a sample where the only known source of sampling bias is a greater likelihood to record from larger diameter fibers, 40% of the fibers were rapidly adapting (RA) and 60% were very slowly adapting (VSA). Two units were moderately slowly adapting (MSA), and one had the properties of a Pacinian afferent (Pc). 2. The RA and VSA resemble those in the glabrous skin of other mammalian species in terms of thresholds, receptive-field areas, conduction velocities, and the coding of velocity of mechanical displacement of the skin. Mean instantaneous frequency during ramp stimulation is a power function of ramp velocity for both RA and VSA, with exponents generally less than 1.00. However, ramp discharge patterns differ for RA and VSA. 3. The VSA exhibit a wide range of coefficients of variation (CV) of their interspike-interval distributions, but form a continuous distribution with respect to this statistic. In other respects the VSA are more similar to slowly adapting type I than to slowly adapting type II. They lack spontaneous activity, have restricted receptive fields, and are relatively insensitive to skin stretch. 4. Effects of mechanical stimulus velocity and static indentation depth on static discharge rate were examined in 23 squirrel monkey and 22 raccoon SA units having receptive fields on glabrous skin of the hand. 5. Discharge rate during static indentation is a monotonic, increasing function of identation depth. However, the nature of the best-fitting function (highest r) varies from unit to unit. Using a set of standard conditions (milliseconds 100-500 of static displacements up to 960 mum, following a ramp velocity of 100 mum/ms, interstimulus interval of at least 10 s), the ratio of units for which linear, as opposed to logarithmic, functions provided the best fit was 4:3 for squirrel monkeys and 1:3 for raccoons. Few units had power functions as best fits in either species. Differences between fits for different functions within the same unit, however, were often trivial and insignificant. 6. Response rate during static skin displacement is also strongly influenced by prior stimulus ramp velocity. For at least the first 500 ms, discharge rate is positively related to onset velocity but, in many units, within the first 1 s of static displacement, this relationship reverses itself, and the inverse relationship may persist for at least 5 s.