What is Somatosensation?

The skin holds several kinds of mechanoreceptor, each an axon ending in its own capsule: Merkel cells for fine pressure and texture (slowly adapting, small fields), Meissner corpuscles for light touch and slip (rapidly adapting), Pacinian corpuscles deep in the skin for vibration (very rapidly adapting, huge fields), Ruffini endings for stretch. The transducer in most of them is Piezo2, a giant ion channel with a propeller of blades that opens when the membrane is stretched; without it touch and the sense of body position are lost, and the same channel in the lungs and bladder senses their filling. Temperature is sensed by channels of the TRP family tuned to different ranges — TRPV1, opened by heat above $43\,{}^{\circ}\mathrm{C}$ and by capsaicin, which is why chilli is “hot”; TRPM8, opened by cold and by menthol — and pain by free nerve endings, the nociceptors, that respond to damaging heat, pressure or chemicals (protons, ATP, bradykinin) and whose thresholds are lowered by inflammation (Chapter 15). Proprioception, the sense of where the limbs are, comes from the muscle spindles and tendon organs of Chapter 17 and from Piezo2 in joint capsules. The density of receptors sets acuity: two points $2\,\mathrm{mm}$ apart are told apart on a fingertip, $40\,\mathrm{mm}$ apart on the back.