Role of Skin Material Properties in Determining the Sensitivity of Stretch-Sensitive Cutaneous Mechanoreceptors

Rotating a joint stretches the soft tissues on one side of the joint and relaxes them on the other side. All the soft tissues around joints contain mechanoreceptor neurons that are activated by stretch. Thus stretch responses of mechanoreceptors potentially contain information about joint displacements or movements, and are therefore of interest in studies of motor control. In this presentation I shall describe properties of mechanoreceptor (in particular cutaneous) afferent neurons from skin overlying the knee, that are activated by stretch. The results are all derived from in vitro studies of isolated, innervated, hairy skin from rats and mice. Skin is a biaxial material, and stretching it creates biaxial states of tensile, compressive, and shear stresses and strains. A key question is, which of these states causes activation of neuronal mechanoreceptors? This is a point of some interest, since it is widely accepted that at a molecular level, displacements (i.e., strains) must be the key variable. After all, molecules have to change shape in order for their function to be altered, implying a dependence on strain. Yet many macroscopic experiments have shown that mechanoreceptor responses are more closely associated with stress variables than strain variables [1]. I shall present data that shows that many macroscopic variables are associated with spike responses, but that spike responses are most strongly associated with stress variables. In order to determine if stress or strain variables are more strongly associated with spikes, the experimental design has to decouple stress and strain. Stress and strain have been decoupled by using dynamic stimuli, which causes a phase shift between stress and strain in viscoelastic materials. Using dynamic stimuli also allows for studying rapidly adapting (RA) mechanoreceptors, which are the most abundant type found in hairy skin. The strength of associations between stochastic spikes and continuous stimulus variables has been determined using logistic regression analysis [2]. As in previous studies, neuronal responses have been found to be most strongly associated with stress rather than strain variables. Spike responses were strongly associated with the rate of change of tensile stress, and more weakly associated with tensile stress. Spikes were weakly associated with the rate of change of tensile strain, and there was no association between spikes and tensile strain. There was only a weak association at all between spikes and incremental strain energy. There were significant memory effects associated with these variables [2]. It is widely believed that the …