Stretch sensitivity of cutaneous afferent neurons

Experiments were done to characterize responses to stretch and to determine what stretch-related mechanical variables are most closely associated with stretch. Cutaneous afferent neurons were recorded using an in vitro preparation of rat hairy skin. SA2 afferents and mechanically sensitive C afferents were studied using static stretch stimuli in which the skin was stretched along different directions. RA afferents were studied using uniaxial, dynamic stimuli. Both applied loads and skin deformations were recorded; they were used to calculate tensile stresses and strains, respectively. SA2 afferents were highly directionally sensitive and had low thresholds for activation. Their responses to static stimuli were closely related to tissue stress and poorly related to strain. C mechanoreceptors lacked directional selectivity, and had thresholds for activation that were similar to SA2s. Their responses to static stretch were more strongly related to stress than to strain variables. The main focus of this chapter is RA afferents. It was possible to apply powerful methods for determining the relationship between RA responses and cutaneous stresses and strains. RA responses were strongly driven by rate of change of stress, with a very small contribution from the static component of stress. There were strong memory effects. The strongest influence on RA afferents was by the rate of change of stress approximately 30 msec prior to the occurrence of a spike.