Mechanoreceptors in rat glabrous skin: redevelopment of function after nerve crush

In the glabrous skin of the rat's hindfoot the same triple set of low-threshold mechanoreceptors is present as has been found in other mammals: slowly adapting (SA), rapidly adapting (RA), and very rapidly adapting Pacinian corpuscle-like (PC) receptors. Their functional characteristics were examined in normal rats and compared with those of sensitive mechanoreceptors found in the glabrous skin of the foot 2-24 wk after crush of the plantar nerves, resulting in regeneration of the transected nerve fibers. After 2 wk of nerve regeneration, low-threshold RA and SA cutaneous mechanoreceptors reappeared in the foot skin. Responses of PC receptors were recorded again after 3 wk, at which time the proportion of fibers that could be identified as low-threshold cutaneous mechanoreceptors had regained control level. Discharge patterns of regenerated cutaneous mechanosensitive receptors were very similar to those of normal skin mechanoreceptors. Their sensitivity to controlled mechanical stimulation was, however, still reduced 4 wk after the lesion. After 8 wk RA and SA receptors had regained their normal dynamic sensitivity, i.e., the responsiveness to the velocity of skin indentation. The static sensitivity of SA receptors, i.e., responsiveness to maintained skin indentation, was not consistently reestablished within 24 wk. No shift in sensitivity could be deduced from tuning curves of PC receptors examined 3-24 wk after nerve crush. In addition to the low-threshold mechanoreceptors, high-threshold (HT) mechanoreceptive fibers were found in controls and in animals with regenerating nerves. This type of fiber was most frequently found 1 wk after the nerve crush, when reinnervation of the foot started. They probably represent fibers not connected to specific mechanoreceptor end organs. Thus, functional restitution of the highly specific cutaneous mechanoreceptors occurs fairly soon after invasion of the original territory by the regenerating nerve. It is assumed that the underlying mechanism is the rapid reconnection of fibers with the end organs that have either survived during the period of denervation or regenerated subsequent to reinnervation of the skin.