Neural mechanisms of saccadic eye movements: common ground with the vestibular system

The superior colliculus (SC) plays an important role in controlling eye and head movements. The neural organization of the pathways from the SC to motoneurons in the horizontal oculomotor system has been well analyzed, but the neural mechanisms in the vertical saccade have not yet been analyzed in detail. This article reviews the current state of knowledge of the neural mechanisms of the horizontal and vertical saccadic eye movement system, and shows that they have common features: both systems contain excitatory and inhibitory burst neurons which receive inputs from the SC and directly project to motoneurons. Our recent study showed the presence of commissural inhibition between the upper and lower fields of motor map of the bilateral SCs. This reciprocal inhibition between the medial upward saccade area in one SC and the lateral downward saccade area in the other SC is very similar to that in the vestibular system. This similarity of the patterns of reciprocal inhibition in the SC system and the semicircular canal system implies that the SC output system may use the coordinate system based on the semicircular canals.