Large fibre input inhibits transmission of small fibre pain signals
Expansion
In the substantia gelatinosa of the dorsal horn, inhibitory interneurones act as a gate on transmission to the ascending pathways.
- Large A beta fibres (touch, vibration) excite the inhibitory interneurone, closing the gate and reducing pain transmission
- Small A delta and C fibres (pain) inhibit the interneurone, opening the gate
This explains rubbing an injury, and it is the rationale for transcutaneous electrical nerve stimulation and for dorsal column stimulators.
Descending inhibition adds a second layer, from the periaqueductal grey and rostral ventromedial medulla, using endogenous opioids, serotonin and noradrenaline. This is why opioids act both spinally and supraspinally, and why tricyclics and serotonin-noradrenaline reuptake inhibitors relieve neuropathic pain by enhancing descending inhibition.
The theory also accommodates psychological modulation: attention, anxiety and expectation alter descending control, which is the physiological basis of placebo analgesia and of the observation that pain is not simply proportional to tissue damage.