Brainstem circuits controlling lid-eye coordination in monkey.

In primate, the M-group is a cell cluster in the rostral mesencephalon which contains premotor neurons for the levator palpebrae (LP) and upward-pulling eye muscles. It is therefore thought to play a role in lid-eye coupling during vertical saccades. To further elucidate its role, the afferents to the M-group and LP motoneurons were studied in monkeys. Anterograde tracer injections were placed in one of the three eye-movement-related areas: 1. superior colliculus (SC), 2. interstitial nucleus of Cajal (INC), and 3. the omnipause neuron (OPN) region. Injections into the medial SC subtending upward saccades led to afferent labelling of the ipsilateral M-group and the adjacent rostral interstitial nucleus of the medial longitudinal fascicle (RIMLF), whereas only RIMLF was labelled after an injection into the lateral SC subtending downward saccades. Both RIMLF and M-group received bilateral projections from INC, but only RIMLF received glycineric inputs from the OPN region. This connectivity pattern supports the hypothesis that the M-group mediates lid-eye coupling during vertical upgaze, but is indirectly driven by collaterals of saccadic burst neurons in the RIMLF during lid saccades. A selective projection from the OPN area to the LP motoneurons, but not to other oculomotor neurons is reported here for the first time. The result is supported by the presence of glycinergic terminals only over LP motoneurons, and implies that a subset of OPNs may directly trigger saccade-related blinks.