Transduction in Merkel Cell Mechanoreceptors — What Is the Job of the Merkel Cell?

Abstract

Most cutaneous mechanoreceptors are known to be primary mechanoreceptors. Mechano-electric transduction occurs in the terminal of the afferent nerve fibre, while the surrounding structures, e.g. lamellae in Meissner and Vater-Pacini corpuscles assist in directing the mechanical stimulus towards cytoplasmic protrusions extending from the nerve terminal. The latter are believed to be the site of mechanically gated ion channels. Merkel cell mechanoreceptors are different in the sense that special cells-Merkel cells-inake close contact with nerve terminals showing dense core granules in the adjacent part of the Merkel cell cytoplasm and synapse-like membrane specializations in the contact area. Cytosplasmic protrusions are regularly seen extending from the Merkel cells rather than the nerve terminals. This has led to the hypothesis that the Merkel cell may be the mechanotransducer proper employing synaptic transmission towards the nerve terminal. Various substances as possible candidates for the postulated synaptic transmission were found in the dense core granules of Merkel cells. However, until recently, there has been no conclusive evidence for neurotransmitter function of any of these substances between Merkel cell and nerve terminal and no direct electrophysiological measurements of receptor potentials have so far been successful, due to the location of these receptors between epidermis and dermis making them virtually inaccessible for microelectrode recordings. Recently, mechanically induced increases in intracellular calcium of Merkel cells have provided evidence for mechanically gated calcium channels in Merkel cells and calcium-induced calcium release from intracellular stores within the Merkel cell. Glutamate receptor antagonists have been shown to reduce static responses of Merkel cell mechanoreceptors. Thus, it appears that slowly maintained adapting responses of Merkel cell mechanoreceptors involve a mechano-electric transduction process in Merkel cells and glutamatergic neurotransmission to the associated nerve terminals. However, the specific glutamate receptor still remains to be identified.