Sources of direct excitatory and inhibitory inputs from the medial rhombencephalic tegmentum to lateral and medial rectus motoneurons in the cat

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Abstract
The paramedian pontine and bulbar tegmentum was explored by microstimulation to outline the sites of origin of direct excitatory and inhibitory inputs to lateral rectus (LR) and medial rectus (MR) motoneurons (MNs). In order to avoid activation of fibers of passage and axon reflexes originating outside the stimulation sites, experiments were carried out 4–22 days after brain stem transections causing degeneration of vestibulo-ocular pathways. Additionally, in some experiments the paramedian tegmentum was isolated from the contralateral side by midline transections. It is concluded that the paramedian rhombencephalic reticular formation contains four pools of premotor neurons related to coordination of conjugate horizontal eye movements. Two of them are excitatory for LR- and MR-MNs with ipsilateral ON-directions, the other two mediate reciprocal inhibition of the antagonistic motor nuclei.