Functional connectivity of the superior colliculus with saccade-related brain stem neurons in the cat

Effects of stimulation of the superior colliculus on saccade-related brain stem neurons were studied in the alert cat. Extracellular recordings were made from medium-lead burst neurons (MLBNs), omnipause neurons (OPNs) and burster-driving neurons (BDNs) in the paramedian pontomedullary region rostral and caudal to the abducens nucleus. MLBNs were activated from the contralateral superior colliculus with monosynaptic latencies when single-pulse stimulation was given during saccades or ipsilateral head rotation, although this activation was not observed during fixation periods. The caudal SC was more effective than the rostral SC in monosynaptic activation of MLBNs. Most OPNs were also activated monosynaptically from the SC. In contrast to MLBNs, the activation of OPNs was more frequently induced from the rostral SC than from the caudal SC. Stimulation of the caudal SC often induced suppression of spikes in OPNs. BDNs received excitation from the ipsilateral SC through a di- or trisynaptic pathway. Like MLBNs, BDNs tended to receive stronger input from the caudal SC than the rostral SC. Results indicate the existence of tectofugal excitatory pathways to MLBNs and BDNs and an inhibitory pathway to OPNs. It seems likely that these pathways originate from saccade-related burst cells in the SC. Since excitation of BDNs and inhibition of OPNs increase the excitability of MLBNs, all of these pathways may contribute to burst activity in MLBNs and thereby saccade generation. Results also support the current idea that cells in the rostral SC may participate in fixation by activating OPNs.