Touch, temperature and pain

Run a fingertip over a rough surface and the skin vibrates; a receptor called a Merkel cell picks up slow vibrations of about 5 to 15 per second.

Many senses in one skin

Walk barefoot on a lawn on a hot day and you feel the warm sun, the breeze and every blade of grass under your feet. The skin does not have one sense of touch but several. OpenStax lists pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception (body position) and kinesthesia (body movement) among the general senses. Each is picked up by its own kind of receptor.

Receptors can be built in three ways. Some are bare nerve endings, called free nerve endings, with branches spread through the tissue. The pain and temperature receptors in the dermis are this kind. Some are nerve endings wrapped in a capsule of connective tissue, which makes them more sensitive to a particular kind of push. The third kind is a specialised receptor cell, such as a photoreceptor in the eye.

Hot, cold and the chilli pepper

Temperature is sensed by thermoreceptors, some tuned to warmth above normal body temperature and others to cold. How does a temperature become an electrical signal? That question was answered only recently. In the late 1990s David Julius used capsaicin, the compound that makes chilli peppers burn, to search for the sensor in nerve endings. His team found a single gene that made cells react to capsaicin. It coded for an ion channel, later named TRPV1, which also opens at temperatures we feel as painfully hot. That is why chilli feels hot even though it is not.

Julius and Ardem Patapoutian, working separately, then used menthol to find TRPM8, a channel activated by cold. Patapoutian also searched for the sensor of touch. His team poked single cells with a fine glass pipette, narrowed 72 candidate genes down one by one, and found a new pressure-activated channel they called Piezo1, after the Greek word for pressure. A related channel, Piezo2, turned out to be essential for touch and for sensing the position of the body. The two scientists shared the 2021 Nobel Prize in Physiology or Medicine.

Pain and body position

Pain is sensed by nociceptors. OpenStax describes pain as mainly a chemical sense: stressed or damaged tissue releases chemicals that activate receptor proteins on these nerve endings. Pain works as a warning that tissue is under stress or damaged.

Close your eyes and touch your nose. You can do it because of proprioception. Muscle spindles report how stretched a muscle is, and Golgi tendon organs report the tension in tendons. Stretch receptors also guard the muscle: if you stretch too far, a reflex pulls the muscle back before it tears.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.