Projections from the trigeminal nucleus caudalis in the squirrel monkey

The role of nucleus caudalis as a relay station in the trigeminal brain-stem nuclear complex was studied in the squirrel monkey ( Saimiri sciureus). Evoked potentials incident to peripheral trigeminal stimulation were recorded from sites in the upper brain stem by means of stereotaxically placed macroelectrodes. A dorsolateral medullotomy at the level of the obex isolating nucleus caudalis from peripheral afferent input was then performed. Those potentials that persisted following the medullotomy were assumed to relay in the more rostral trigeminal relay nuclei while those that were abolished were thought to relay primarily through nucleus caudalis. Trigeminal projections dependent on a relay at nucleus caudalis were identified in the CM complex (centrum medianum and nucleus parafascicularis), nucleus paracentralis of intralaminar nuclei, posterior nuclear group of the thalamus, and ventromedial tegmental fasciculus of the midbrain. In contrast to similar findings in the cat, there was no significant projection from nucleus caudalis to nucleus ventralis posteromedialis in the primate. These findings further confirmed nucleus caudalis as a major relay station to regions of the midbrain and thalamus which have been considered as major contributive elements in the integration and perception of pain.