Touch, pain, and temperature share one body map
The somatosensory system, part of sensory nerves, senses the outside world, the inside body, and helps regulate movement. One subdivision handles touch, vibration, pressure, and skin tension; another handles nociceptive pain and temperature. As of 2024, debates continue about mechanisms and links to emotion.
This branch of the nervous system does three jobs: registering stimuli from outside, registering signals from inside the body, and tracking posture and balance, known as proprioception. It reaches every major part of the vertebrate body, from receptors in skin, muscle and organs to neurons deep in the brain. Touch counts as a general sense, as opposed to special senses such as sight or hearing. People can even convey emotions like anger, fear, love, gratitude and sympathy through touch alone far more accurately than chance, and deafblind people often communicate through tactile signing.
Hairless skin such as the fingertips holds four kinds of low-threshold receptor that respond to harmless contact. Merkel endings pick up slow vibration around 5 to 15 Hz and steady pressure, reporting fine detail about edges and shapes. Tactile corpuscles, concentrated in fingertips and lips, respond to light touch and 10 to 50 Hz vibration and make Braille reading possible. Pacinian corpuscles are the most vibration-sensitive, peaking near 250 Hz, but quickly ignore constant pressure, which is why we stop noticing clothes. Bulbous corpuscles register sustained stretch and help us sense slippage and finger position. High-threshold receptors, by contrast, react to harmful stimuli.
Signals travel in relays of three neurons. The first has its cell body in a dorsal root ganglion beside the spinal cord; the second sits in the cord or brainstem and crosses to the opposite side; the third lies in the thalamus and projects to the cortex. Touch and vibration climb the dorsal columns and switch sides in the medulla, while face and head input enters through cranial nerves such as the trigeminal. The wiring preserves location, so neighbouring cortical neurons map neighbouring patches of body, forming the sensory homunculus.
The primary somatosensory cortex sits in the postcentral gyrus of the parietal lobe as Brodmann areas 3, 1 and 2. Area 3 takes the heaviest input from the thalamus, area 1 handles texture and area 2 size and shape. A secondary area links touch with memory structures such as the hippocampus, and the insula contributes to feelings of body ownership, itch and pain. Researchers were still disputing parts of this model in 2024.
Source: Somatosensory system
