Define referred pain. Explain its mechanism and give common examples. Add a note on the gate control theory.

Definition

Pain Pathway — Brief

Nociceptor (free nerve ending) → Aδ (fast, sharp) and C (slow, dull) fibres → Dorsal root ganglion → substantia gelatinosa → Crosses within 1–2 segments → Lateral spinothalamic tract → VPL thalamus → Sensory cortex

Mechanism of Referred Pain

• Convergence–projection theory — visceral and somatic afferents converge on the same second-order neurone in the dorsal horn
• The brain has learned from experience that such impulses usually come from the skin, and therefore misinterprets the visceral pain as somatic
• Facilitation theory — visceral impulses lower the threshold of neighbouring somatic neurones
• The site of reference follows the embryological (dermatomal) origin of the viscus, not its adult position

Common Examples

Gate Control Theory (melzack and Wall)

• Large Aβ fibres (touch) close the gate in the substantia gelatinosa; small C fibres open it
• Explains why rubbing an injured part relieves pain, and the basis of tens
• Descending inhibition from the periaqueductal grey and raphe nuclei, using endogenous opioids (endorphins, enkephalins) and serotonin, also closes the gate

Applied Aspects

• Diagnostic value — the pattern of reference often localises the diseased organ; the shifting pain of appendicitis is a classical example
• Phantom limb pain — felt in an amputated limb; due to cortical reorganisation
• Morphine acts on opioid receptors in the periaqueductal grey and dorsal horn, reinforcing descending inhibition