Systematic mapping of descending inhibitory control by the medulla of nociceptive spinal neurones in cats.

Inhibition descending the spinal cord from the medullary brainstem was systematically mapped in two dimensions in anaesthetized cats. Fifty-four ipsi- and contralateral sites and 27 ipsilateral sites were electrically stimulated with concentric bipolar or with monopolar electrodes, respectively, whilst extracellularly recording firing of dorsal horn neurones to noxious heat. Effective bipolar stimulation, but not monopolar stimulation caused gross lesions at the stimulated sites. There were 3 ipsilateral sites at which descending inhibition was evoked with the smallest stimuli. These areas corresponded to nucleus raphe magnus, nucleus reticularis magnocellularis and the lateral tegmental field. Contralateral stimulation was as effective or more effective than ipsilateral stimulation. The inhibition of dorsal horn neurones was not due to the cardiovascular changes which sometimes accompanied stimulation of the brainstem. The data are discussed in relation to the analgesia which results from stimulating these areas of the brainstem.