Merkel cell

The skin has an outer layer called the epidermis, home to four cell types of its own: keratinocytes, melanocytes, Langerhans cells and Merkel cells. The last of these is neuroendocrine, that is, it communicates by releasing chemical signals like those of the nervous system; it arises from the epidermis itself, and its job has to do with sensation.

Touch is its business. It works as a slowly adapting receptor, and that means something specific: when the stimulus is repeated, the cell lowers the rate at which it fires until it returns to its baseline firing rate. It does not work alone; it is paired with a sensory nerve ending, the tip of a nerve that picks up information.

How the message gets across is still described in cautious terms. It is proposed that the cell releases transmitter molecules into the space that separates it from the nerve fiber, like someone slipping a note under a closed door. That junction resembles a synapse, the point where one cell hands a signal to another. That is where the nerve impulse is thought to arise.