STUDIES OF MECHANORECEPTORS IN SKIN OF THE SNOUT OF THE ECHIDNA TACHYGLOSSUS-ACULEATUS

The echidna Tachyglossus aculeatus, together with the platypus, belong s to the monotremes, a group of mammals with a number of reptilian cha racteristics. A structure unique to the skin of monotremes is the push rod-a compacted column of epidermal cells that is 20 mu m wide and 10 0 mu m long with its tip at the skin surface, and that is able to move relatively independently of adjacent tissue. At the base of each push rod is a cluster of encapsulated nerve endings. Push rods are common in skin of the snout and have been postulated to have a mechanosensory function. Experiments were carried out on four anesthetized echidnas with the aim of determining the function of push rods. Recordings made from the infraorbital nerve, which supplies the skin of the upper jaw , yielded responses from a total of 46 afferents. Two were electrorece ptors; the others were mechanoreceptors. Within the group of mechanore ceptors with rapidly adapting responses, three responded to high-frequ ency vibration and resembled pacinian corpuscles. There were 26 slowly adapting (SA) mechanoreceptors, which. based on the regularity of the ir discharge, could be divided into two groups: SA I or Merkel type, a nd SA II or Ruffini type. SA I receptors had very discrete receptive f ields with diameters of 100 mu m. The receptive fields of two SA I rec eptors were marked, and after histological processing, one was seen to lie near two push rods. It is concluded that mechanoreceptor response s in the echidna's snout skin resemble those in other mammals in many aspects. We could not unequivocally associate responses to mechanical stimulation with the push rods.