Brainstem neural circuits for fixation and generation of saccadic eye movements.

We review neural connections of the superior colliculus (SC) and brainstem saccade-related neurons in relation to saccade generation mechanism. The caudal and rostral SC play a role in saccade generation and visual fixation, respectively. This functional differentiation suggests that different connections should exist between these two SC areas and their brainstem target neurons. We examined synaptic potentials evoked by stimulation of the rostral and caudal SC in inhibitory burst neurons (IBNs) and omnipause neurons (OPNs) in anesthetized cats. The caudal and rostral SC produced monosynaptic excitation and disynaptic inhibition in IBNs, respectively. Intracellular HRP staining showed that single IBNs sent their axons to abducens motoneurons, IBNs and OPNs on the opposite side. OPNs received monosynaptic excitation from the rostral SC, and disynaptic inhibition from the caudal SC via opposite IBNs. These neural connections are discussed in relation to the saccade triggering system and the model proposed by Miura and Optican.