Starts in periaqueductal grey (PAG), PAG receives pain information via spinomescencephalic tract & processes nociceptive information → relays to rostral ventral medulla (RVM) → sends signal down spinal cord → activates endogenous opiate system → secretes enkephalins, serotonin & noradrenaline → inhibition of second-order neuron in spinothalamic pathway → reduces AP transmission up spinothalamic tract
