Precerebellar paramedian tract cell groups connections: I. Oculomotor internuclear projections.

The role of internuclear neurons residing in the medial rectus subdivision of the oculomotor nucleus and projecting to the abducens nucleus is poorly understood. As a step toward remedying this, we labeled this projection in macaque monkeys by injecting the oculomotor nucleus with anterograde tracers. Labeled terminal arbors present within the main body of the abducens nucleus were diffusely distributed and mainly directed at motoneurons. However, a dense terminal field was present along the dorsomedial aspect of the nucleus, designated the infragenual region, and immediately dorsal to the facial genu, in the supragenual region. These regions contain precerebellar neurons projecting to the flocculus and are integral parts of the precerebellar system known as the paramedian tract cell groups. Consequently, we extended our analysis and found extensive internuclear axon terminal fields in each of the paramedian tract cell groups in the animals that received oculomotor nucleus injections. These data indicate that a major role of the internuclear projections is to provide a medial rectus-like efference copy signal to the flocculus. This signal likely plays an important role in adjusting eye movements, such as the vestibulo-ocular reflex, with respect to vergence angle.