Axonal projection of the prepositus hypoglossi and reticular neurons in the brainstem of the cat.

Abstract

1. Extracellular unit spikes responsive to horizontal head rotation were recorded in and around the cat nucleus prepositus hypoglossi. Type II neurons (activated by contralateral angular acceleration) were much more common than type I neurons. The axonal projections of single prepositus and reticular neurons to the rostral brainstem were investigated by a systematic survey of antidromic responses using microstimulation. 2. Most type II prepositus and reticular neurons sent axons to the rostral brainstem either ipsilaterally or contralaterally (i-type II or c-type II neurons). The stem axons ran through the reticular formation without forming a discrete fiber tract, usually reaching the midbrain tegmentum. 3. Axons of type II prepositus and reticular neurons were presumed to terminate in some of the following areas: the contralateral prepositus and reticular region, pontine reticular formation, raphe nucleus, pontine tegmental reticular nucleus, midbrain reticular formation, and lateral midbrain tegmentum near the parabigeminal nucleus. 4. The possibility of efferent connections to the abducens and oculomotor nuclei was closely examined. Some of type II prepositus and reticular neurons (24%) sent axonal branches into the abducens nucleus on the ipsi- or contralateral side. The ramification of such axonal branches seemed to be poor. Axonal projection to the oculomotor nuclei was also observed but less commonly (5%).