The Merkel cell as a possible mechanoreceptor cell.

Merkel cells (MCs) are abundant at the basal layer of various skin in vertebrates, and make synaptic contacts with nerve endings to form the Merkel cell-neurite complex (MCN-complex). It has been established that the MCN-complex is involved in slowly adapting mechanoreception, cutaneous afferents of which are called SAI units in mammals or Ft-I units in frogs. However, the MC function has been the focus of attention, and some hypotheses propose that the site of mechanoreception is at the nerve terminals but not at the MC. In the present review, the possibility that MCs are the mechanoreceptors was focused on recent findings. Irradiation of quinacrine-loaded MCs in the rat hairy skin using excitation light degenerates the MCs selectively with the nerve terminals left intact. Correspondingly, SAI units decrease tonic discharges rapidly, but phasic responses remain intact. Blocking synaptic transmission in the MCN-complexes by divalent or alkyl Ca antagonists in mammals or frogs heavily decreases the tonic mechanical responses of the afferent units, but the phasic responses are rather resistant. Application of anodal current on the Ft-I receptive spots produces tonic discharges as in hair cells or taste cells, while the threshold to elicit the first spike is lower with cathodal than anodal stimulation, in contrast with other secondary sensory cells. These findings indicate that MCs are mechanoreceptors to yield tonic responses, while the nerve terminals may transduce the transient phase. Further studies, particularly on mechanically-gated ionic channels in the MC membrane and on transmitters between the MCs and nerve terminal, are necessary to establish the MC as mechanoreceptors.