Effects of contactor size and stimulation distance on the response properties of rapidly-adapting tactile fibers innervating the rat glabrous skin

Abstract

Recent population models used space-invariant attenuation functions. But this assumption is not enough to construct realistic population models, because mechanical properties of the skin vary along the skin [1]. In this study, it is hypothesized that response pro les of mechanoreceptive bers are not symmetric along proximo-distal axis due to the varying mechanical properties of the skin. In this study, sinusoidal mechanical vibrations were applied perpendicular to the skin of adult rats. Single-unit responses were recorded from sciatic nerve. Five di erent stimulus locations (2 distal, 1 RF center, 2 proximal) and three di erent contactor sizes (area: 0.39 mm2, 1.63 mm2, 2.96 mm2) were used. Averages of absolute spike thresholds (a0) and entrainment thresholds (a1) of rapidly adapting bers were plotted as a function of stimulation distance and contactor size. Also, mechanical impedance of the skin was measured at 5 di erent locations on distal phalanx of one rat. 2-way ANOVA showed that the e ect of stimulus location on a0 and a1 was signi cant (p<0.001), whereas contactor size had no signi cant e ects (p=0.642). Posthoc Tukey test showed that thresholds for proximal stimulations were higher than those for distal stimulations. A mechanical model which explains the mechanical behavior of the skin and its e ects on mechanoreceptive bers' response is presented. According to these results rate-intensity functions of RA bers shift asymmetrically along the proximo-distal axis, which suggests that the mechanical stimulus is transmitted better towards the proximal direction. This may counterbalance the effects of innervation density and cause uniform psychophysical detection thresholds for the nger.|Keywords : Somatosensation, tactile ber, vibrotactile stimulus, skin mechanics.