Touch, Temperature and Pain

Skin receptors encode different inputs. Mechanoreceptors respond to pressure, vibration or stretch; thermoreceptors respond to warmth or cold. Nociceptors detect potentially damaging mechanical, thermal or chemical stimuli. This detection, called nociception, is distinct from felt pain. A capsule is a covering around a nerve ending. Free endings lack capsules and include temperature and nociceptive endings. Encapsulated endings include touch receptors, but unencapsulated Merkel endings detect touch too.

Rapidly adapting receptors respond strongly to changes, then quieten under steady input. Slowly adapting receptors maintain responses while pressure continues. Rapid adaptation helps steady clothing become less noticeable, but does not silence every touch receptor. Attention also affects what you notice.

Simplified textbook routes from body skin to cortex count three neurons, excluding local interneurons. For dorsal-column touch: a primary sensory neuron (cell body in a dorsal root ganglion), a medullary neuron crossing in the medulla, then a thalamic neuron projecting to somatosensory cortex. The direct spinothalamic pain/temperature route instead uses a second neuron crossing in the spinal cord. Spinal circuits can inhibit or amplify nociceptive transmission. Non-painful touch can recruit inhibitory interneurons, reducing onward transmission. Descending brain signals also alter spinal processing. Gentle rubbing near a minor bump may ease pain, but relief varies. Felt pain is not a direct measure of tissue damage.

Dorsal-column model: each cell names one neuron in the ascending chain. The third projects to cortex, the destination outside this count.

A receptive field is the skin region where stimulation affects a neuron's response. Fingertips have dense touch innervation and many small receptive fields. At the same suitable spacing, two contacts can produce more distinct activity patterns there than on the back. The nervous system reads populations of neurons, not one cortical column per contact. Like visual maps, the cortical body map gives different areas unequal space. Peripheral sampling and central connections shape these proportions; hands occupy much more cortex relative to their skin area than the torso.