Stimulation sites in periaqueductal gray, nucleus raphe magnus and adjacent regions effective in suppressing oral-facial reflexes.

Electrode penetrations were made in the mesencephalon and caudal brainstem at the levels of the periaqueductal gray matter (PAG) and nucleus raphe magnus (NRM) in chloralose-anaesthetized or decerebrate cats. In a systematic fashion, mesencephalic and brainstem loci at every 1 mm of vertical depth were electrically stimulated in a series of mediolateral or anteroposterior electrode penetrations to determine the lowest stimulation threshold at each locus for suppressing the digastric jaw-opening reflex evoked by tooth pulp or infraorbital nerve stimulation; at some loci, the threshold current required for suppressing infraorbital nerve-evoked neck reflexes was also determined. Stimulation at sites within large regions of the mesencephalon and caudal brainstem was effective in suppressing these reflexes at less than 4 X the lowest threshold for reflex suppression in each animal. However, in these regions the areas of lowest threshold in the mesencephalon generally corresponded to the ventrolateral PAG and adjacent nucleus cuneiformis and part of the lateral reticular formation (LRF) and in the caudal brainstem they corresponded to NRM and the adjacent nuclei reticularis magnocellularis (RMC) and gigantocellularis (RGC). These findings suggest that there may be mesencephalic and caudal brainstem areas in addition to PAG and NRM that are equally effective in modulating reflex activity.