Gate control theory (Melzack & Wall, Science 1965):
Pain signals travel via thin Aδ and C fibers to the dorsal horn. Substantia gelatinosa (SG) interneurons act as a gate:
C fiber (small): INHIBITS SG → opens gate → pain ↑
Aβ fiber (large): EXCITES SG → closes gate → pain ↓
This explains why rubbing an injury reduces pain — large mechanical fibers are activated, inhibiting the nociceptive signal.
TENS (transcutaneous electrical nerve stimulation) works by selectively activating large Aβ fibers.
Descending modulation:
The brain can modulate pain via descending pathways from:
· Periaqueductal gray (PAG)
· Rostral ventromedial medulla (RVM)
· Locus coeruleus (LC)
These release endorphins, enkephalins (opioid peptides) and serotonin, norepinephrine at dorsal horn interneurons.
Placebo effect is real descending modulation — endogenous opioid release confirmed by naloxone reversal.
Stress analgesia: acute stress activates PAG → opioid suppression of pain. Chronic stress → hyperalgesia (sensitization).