Antidromic identification of midbrain near response cells projecting to the oculomotor nucleus

Medial rectus motoneurons carry both conjugate and vergence eye position signals. Abducens internuclear neurons, whose axons travel in the medial longitudinal fasciculus, provide these motoneurons with the major signal for conjugate eye movements but not for vergence eye movements. A vergence signal appropriate for these motoneurons is seen on the near response cells that are found in the mesencephalic reticular formation within 2 mm of the oculomotor nucleus. The goal of the present study was to determine if midbrain near response cells project to the medial rectus subdivision of the oculomotor nucleus. Near response cells were recorded in two trained rhesus monkeys with ocular search coils. A stimulating electrode was positioned within the medial rectus subdivision of the oculomotor nucleus. Twenty-eight near response cells were found that could be driven by single pulse microstimulation of the ipsilateral medial rectus subdivision. In all cases, antidromic activation was confirmed by collision testing. Attempts to antidromically activate midbrain near response cells from the contralateral medial rectus subdivision were unsuccessful. Most antidromically activated cells had a steady state firing rate proportional to vergence angle. One cell also showed burst activity during the vergence eye movements. Divergence cells were not antidromically activated.