Statistical analysis and modeling of variance in the SA-I mechanoreceptor response to sustained indentation

The slowly-adapting type I mechanoreceptor (SA-I) exhibits variability in its steady-state firing rate both within an afferent upon repeated stimulation and between afferents. Additionally, inter-spike intervals of the SA-I are extremely variable during this steady-state firing. While variability of the SA-I response has been noted previously, the work presented herein provides a finer analysis of the impact of force and fiber on the SA-I response. Specifically, we test two hypotheses, that 1) fiber-to-fiber variation will significantly impact firing rate over the range of applied forces, and that 2) fiber-to-fiber variation will significantly impact the coefficient of variation (CV) of inter-spike intervals over the range of applied forces. Utilizing an ex vivo skin nerve preparation in the mouse, experiments were conducted with six SA-I fibers from five mice, and with compressive stimuli with force magnitudes up to 9.59 mN. We found fiber to significantly impact both firing rate and CV. These findings motivated the construction of a generalized input (force)-output (firing rate) model composed of a baseline response profile and a multiplicative fiber sensitivity factor. This work will inform future efforts to attribute variability to differences in skin, neuron, and receptor properties, and will contribute to the understanding of how much variability is acceptable in systems designed to provide tactile feedback to the nervous system.

I. Introduction

The slowly adapting type I (SA-I) mechanoreceptor is characterized by high spatial acuity to edges and curvature, responses over a range of frequencies, and slow adaptation in spike firing elicited by static indentation [1]–[3]. SA-Is are found in both hairy and glabrous skin of vertebrates, including primates [4], [5] and mice [6]. The end organ associated with the SA-I afferent is the Merkel cell-neurite complex (MCNC). The MCNC features a tree-like cluster of a few to dozens of Merkel cells (dependent on species and body site) [6], [7] and is located in the basal layer of the epidermis, near the dermis [8].