Periaqueductal grey stimulation: an association between selective inhibition of dorsal horn neurones and changes in peripheral circulation.

In barbiturate-anaesthetized and paralysed cats, dorsal horn neurones were studied during electrical stimulation of the periaqueductal grey matter (PAG) and the midbrain ventral tegmentum (VT). Responses to impulses in unmyelinated primary afferents were selectively inhibited by stimulation in the PAG, whereas stimulation in the VT non-selectively reduced both these responses and those to innocuous cutaneous stimuli. Stimulation in the PAG but not the VT produced changes in peripheral circulation. This was observed as a rise in the levels of carbon dioxide in expired air, a rise in muscle temperature in the hind limb and a fall in skin temperature of the pinna or glabrous skin. The combination of suppression of spinal transmission of impulses related to pain and an increase in perfusion of muscles may be a mechanism appropriate to coping with a potentially injurious environment.