Brachium conjuntivum and rubrobulbar tract: brain stem projections of red nucleus essential for the conditioned nictitating membrane response

The trajectory of the rubrobulbar tract in rabbit to the level of the accessory abducens nucleus is described: orthograde labeling of fibers of the rubrobulbar tract following horseradish peroxidase implants into red nucleus of 8 animals permitted ad hoc analysis of the effects of brain stem lesions on the rabbit's conditioned nictitating membrane response. Twenty-four rabbits, trained to give conditioned NM responses from both eyes, received unilateral lesions of the right pontine brain stem. Six of the 7 cases of post-lesion disruption of ipsilateral conditioned responding involved either ipsilateral brachium conjunctivum or the rubrobulbar tract. This finding, together with a reexamination of data from two related studies from this laboratory strongly support the conclusion that an essential premotor component of the conditioned NM response is a doubly decussating circuit from the interpositus nucleus of the cerebellum to magnocellular red nucleus, via brachium conjunctivum and its decussation, and from red nucleus caudally, via the ventral tegmental bundle and the rubrobulbar tract, to the accessory abducens nucleus, where motoneurons involved in the NM response are located. These findings are consistent with published reports on the essential role of interpositus nucleus of the cerebellum in NM conditioning. The possible role of the supratrigeminal reticular formation in this learned behavior is also discussed.