Synaptic actions of tectofugal pathways on abducens motoneurons in the cat

Organization of pathways between the superior colliculus (CS) and abducens motoneurons (VI-MNs) was studied in cats under pentobarbital anesthesia using intracellular recordings from VI-MNs and adjacent reticular neurons. Latencies of EPSPs elicited by contralateral CS stimulation indicate that a small fraction of the excitatory pathway may be monosynaptic while its major part is disynaptic. As suggested by an analysis of synaptic responses to microstimulation of the paramedian pontine region, excitatory impulses descend in the tectobulbospinal tract after crossing at midbrain levels. An attempt was made to identify interneurons of the excitatory tectoabducens pathway in the region just ventral and rostroventral to the VI-nucleus. About one-quarter of the reticular neurons in this region received monosynaptic excitation specifically from the contralateral CS. They were acceptable as interneurons with regard to other response characteristics too. Axonal projection to, or through, the abducens nucleus was demonstrated for some of them by intranuclear microstimulation or by tracing axons after Procion yellow injections. It is suggested that "premotor" interneurons of the excitatory tectoabducens pathway are concentrated in the vicinity of the abducens nucleic. A similar investigation of inhibitory responses to ipsilateral CS-stimulation indicates that inhibitory pathways are at least disynaptic and, for the most part, contain three or more synapses. In its initial trajectory the inhibitory pathway appears to be identical with the tectobulbospinal tract,but it decussates for the second time at caudal pontine levels to reach ipsilateral VI-MNs.