FIGURE 3. Sensory transduction by epithelial cells.

a, Hair cell of the inner ear. Mechanical deflection of stereocilia opens non-selective cation channels, which depolarize the cell to increase the rate of glutamate release onto synaptically connected auditory afferents. b, Taste bud epithelium. Chemical tastants activate a G-protein-coupled receptor–ion channel pathway in a 'receptor' epithelial cell, which communicates through paracrine signalling (wavy line) to an adjacent 'output' epithelial cell. The output cell releases neurotransmitter onto a synaptically connected taste afferent⁵⁸. c, Proposed models for keratinocyte and Merkel cell involvement in cutaneous thermotransduction and mechanotransduction. Touch activates an unknown transduction mechanism in Merkel cells (blue, left), which are synaptically connected to cutaneous A afferents. The myelin sheath of the A afferent is lost as it approaches a Merkel cell. Projections from the Merkel-cell surface represent microvilli. Heat activates TRPV3 and TRPV4 ion channels (green, right) expressed in keratinocytes. Paracrine signalling from keratinocytes to adjacent afferents (wavy line) results in neuronal activation. For simplicity, direct thermotransduction and mechanotransduction by cutaneous afferents are not shown.