Touch receptors

Hairless skin carries four types of endings dedicated to touch. They are called Merkel discs, Meissner corpuscles, Pacinian corpuscles, and Ruffini endings, and each one is a distinct structure.

The difference that stands out most is the speed of adaptation, that is, how quickly they stop reporting when the stimulus does not change. Meissner and Pacini adapt quickly; Merkel and Ruffini, slowly. Some report only when something starts or stops touching; others keep reporting for as long as it lasts. That is why you feel your shirt when you put it on and stop feeling it a few minutes later.

And each one watches for something different, which is why four are needed. The Merkel ending keeps track of whether something is still pressing; the Meissner, of whether the skin moves; the Pacinian, of whether something vibrates; and the Ruffini, of whether the skin stretches.

The wire for light touch is called the A-beta fiber: it is heavily myelinated, coated in myelin, and it conducts at between 16 and 100 meters per second.