Neural circuits for triggering saccades in the brainstem

Here we review the functional anatomy of brainstem circuits important for triggering saccades. Whereas the rostral part of the superior colliculus (SC) is considered to be involved in visual fixation, the caudal part of the SC plays an important role in generation of saccades. We determined the neural connections from the rostral and caudal parts of the SC to inhibitory burst neurons (IBNs) and omnipause neurons (OPNs) in the nucleus raphe interpositus. To reveal the neural mechanisms of triggering saccadic eye movements, we analysed the effects of stimulation of the SC on intracellular potentials recorded from IBNs and OPNs in anaesthetized cats. Our studies show that IBNs receive monosynaptic excitation from the contralateral caudal SC, and disynaptic inhibition from the ipsilateral caudal SC, via contralateral IBNs. Further, IBNs receive disynaptic inhibition from the rostral part of the SC, on either side, via OPNs. Intracellular recording revealed that OPNs receive excitation from the rostral parts of the bilateral SCs, and disynaptic inhibition from the caudal SC mainly via IBNs. The neural connections determined in this study are consistent with the notion that the "fixation zone" is localized in the rostral SC, and suggest that IBNs, which receive monosynaptic excitation from the caudal "saccade zone," may inhibit tonic activity of OPNs and thereby trigger saccades.