Projections from the superior colliculus motor map to omnipause neurons inmonkey

Descending projections from the superior colliculus (;SC) motor map to the saccadic omnipause neurons (OPNs) were examined in monkeys by using anterog rade transport of tritiated leucine. The SC was divided. into three zones: the rostral pole of the motor map, a small horizontal saccade zone in centr al SC, and a large horizontal saccade zone in caudal SC. Tracer injections into the intermediate layers of the three zones led to different patterns o f silver grain deposits in and around nucleus raphe interpositus (RIP), whi ch contains the OPNs: 1) From the rostral pole of the motor map, coarse axo n branches of the crossed predorsal bundle spread medially into the RIP, br anched, and terminated predominantly unilaterally over cells on the same si de. 2) From the small horizontal saccade zone, the axon branches were of a finer caliber and terminated diffusely in the RIP, mainly on the same side. 3) From the large horizontal saccade zone, no terminal labeling was found within the RIP. 4) From the rostral pole of the motor map and small horizon tal saccade zone, fiber branches from the ipsilateral descending pathway te rminated diffusely over RIP. 5) In addition, terminal labeling in reticulos pinal areas of the pens and medulla increased in parallel with the size of the saccade according to the SC motor map. The results suggest that there a re multiple projections directly onto OPNs from the rostral SC but not from the caudal SC associated with large gaze shifts. The efferents from the ro stral pole of the motor map may subserve the suppression of saccades during visual fixation, and those from the small horizontal saccade zone could in hibit anatagonist premotor circuits. J. Comp. Neurol. 413:55-67, 1999. (C) 1999 Wiley-Liss, Inc.