Nociception

Nociception is the encoding and transmission of noxious stimuli by the nervous system. Pain is the conscious experience that may or may not follow. The distinction is not pedantic: nociception occurs under general anaesthesia with no pain, and pain occurs without nociception in phantom limb and many chronic syndromes. Treating the two as one term makes both harder to reason about.

Free nerve endings carry specialised receptors — mechanical, thermal, chemical, polymodal. Fast myelinated A-delta fibres carry sharp, well-localised first pain; slow unmyelinated C fibres carry the dull second pain that follows. Injured tissue releases mediators that lower firing thresholds, so an area becomes tender to stimuli it would normally ignore.

The signal is modulated, not merely relayed. Gate control, proposed by Melzack and Wall in 1965, holds that large-fibre input in the dorsal horn can inhibit transmission of nociceptive traffic — the reason rubbing a knock helps. Details of that model were later revised, but spinal gating survived. Descending modulation from brainstem structures suppresses or amplifies the same traffic from above, which is how attention, expectation and mood change what reaches awareness without changing the injury.

One practical consequence: pain cannot be measured from outside. Only self-report gives access to it, so it is a case where the patient is the Ground Truth and everything else is inference — an Observability problem in the strict sense. Clinical instructions phrased "as tolerated" acknowledge this by delegating a threshold the clinician cannot read. A Regional Nerve Block interrupts the traffic pharmacologically, and analgesic drugs act on the same pathways, which is where Physical Dependence enters as a separate question.