Nociception is not pain: the nerve alarm that fires before feeling
Charles Scott Sherrington coined the word nociception to separate two things we usually blur together: the nerve activity triggered by harm, and the subjective feeling of pain. The first can make you sweat, pale or faint before you are aware of anything, and it runs in leeches, sea slugs and fruit flies too.
The term comes from the Latin nocere, to harm. It describes how the sensory system detects a damaging stimulus, turns it into a nerve signal and passes it on so the body can defend itself. The detectors are nerve endings called nociceptors, found in the skin, on the membrane covering bone, on joint surfaces and in some internal organs. They respond to three broad threats: chemical ones such as capsaicin, the heat in chilli, mechanical ones such as cutting or crushing, and extremes of heat or cold.
Each nociceptor has a threshold, a minimum intensity it needs before it fires, which makes the system an adaptive alarm rather than a constant hum. Researchers exploit this in threshold testing: animals are given a painkiller and the rise in their threshold is tracked until it drifts back to baseline as the drug wears off. The alarm can also misbehave. With some conditions the pain fibres grow more excitable the longer a stimulus lasts, known as hyperalgesia, and after nerve injury ordinary touch fibres may start to register as painful.
Once in the spinal cord, signals fan out through a surprising number of brain regions. One relay area combines taste with pain and, if pain coincides with normal temperatures and an active sense of taste, treats it as a sign of poison. The thalamus is thought to be where pain enters perception, acting like a bouncer that lets some intensities through and turns others away. The somatosensory cortex pins down location, the insula gauges intensity and even lets us imagine pain, and the amygdala and hippocampus store its memory and emotion.
Warmth itself is only comfortable within a narrow band. Moderate temperatures of about 24 to 28 degrees Celsius are handled by ordinary temperature sensing; beyond that range nociceptors take over. Across the animal kingdom, from fish to nematode worms, nociceptive neurons tend to share the same triggers: heat of 40 degrees Celsius or more, acidity, capsaicin and tissue damage.
Source: Nociception
