The Merkel cells and the rapidly adapting mechanoreceptors of the salamander skin

We have made correlated physiological and morphological studies of salamander skin to look for a possible relationship between Merkel cells and the mechanoreceptors of the skin. Using electrophysiological techniques, a systematic survey was made of the apparent thresholds to mechanical stimulation over areas of approximately 1 mm2 of skin surface; the same skin was then serially sectioned for electromicroscopic examination. From an analysis of the physiological findings we could define highly mechanosensitive discrete areas on the skin which indicated the locations of the rapidly adapting mechanoreceptors. The same skin was then examined morphologically to construct a map showing the locations of the Merkel cells. The morphological map was corrected for distortion due to shrinkage, by using as ‘internal markers’ the gland openings which can be identified both in the morphological study and during the physiological survey. The final superimposition of the physiological map with the corrected morphological map revealed a highly significant correlation between the touch-sensitive areas and the Merkel cells. We conclude that each rapidly adapting touch receptor, which we know is only rarely supplied by more than one axon, is comprised of mechanosensory endings in intimate association with a single Merkel cell.