FRCA Notes

Pain Gate Theory

The Primary syllabus asks for knowledge of pain "neuromodulatory systems" and the "influence of therapy on nociceptive mechanisms".

In the Final syllabus the theory underpins "the role of interventional procedures for pain relief, TENS, and other alternative treatments".

Resources

  • The gate control theory was described by Melzack and Wall in 1965
  • It describes modulation of nociceptive transmission at spinal cord level, which is itself influenced by descending pathways
  • Although since refined, it explains why non-painful stimuli can reduce pain, and led to the development of TENS

  • Transmission of a painful stimulus from first to second order neurones occurs through a 'gate' in the dorsal horn
  • The gate is formed by inhibitory interneurones in the substantia gelatinosa (SG)
  • These interneurones are themselves stimulated or inhibited by other afferent and descending inputs, which determines whether the gate is open or closed

The gate is opened by

  • C fibres: pre-synaptically via substance P
  • Aδ fibres: post-synaptically, by inhibiting the inhibitory enkephalinergic interneurones

The gate is closed by

  • Aβ fibres: pre-synaptically, via GABA-mediated inhibition
  • Descending inhibitory pathways from the PAG/NRM (5-HT) and locus coeruleus (noradrenaline)
    • These activate enkephalinergic interneurones, inhibiting onward transmission
  • Aδ fibres, indirectly: collaterals stimulate the PAG and so enhance descending inhibition
Diagram of the pain gate in the substantia gelatinosa showing inputs which open and close the gate

Inputs which open and close the pain gate in the substantia gelatinosa (SG)


Aβ stimulation

  • 'Rubbing it better': touch and pressure activate Aβ fibres, closing the gate
  • Conventional TENS: high-frequency, low-intensity stimulation preferentially activates Aβ fibres
  • Spinal cord stimulation was originally developed on the same principle, by stimulating the dorsal columns

Aδ stimulation

  • Acupuncture and acupuncture-like TENS: low-frequency, high-intensity stimulation activates Aδ fibres
  • Analgesia is mediated by enhanced descending inhibition and release of endogenous opioids

Psychological factors

  • The theory gave a physiological basis for the effect of mood, attention and distraction on pain, via descending control of the gate