Afferent and efferent connections of the cat abducens nucleus: a study by injection of wheat germ agglutinin conjugated with horseradish peroxidase.

The ascending projection of the abducens internuclear neurons and the afferent pathway to the abducens nucleus were examined after injection of wheat germ agglutinin conjugated with horseradish peroxidase (HRP) into the abducens nucleus of the cat. Orthograde neural terminations of the internuclear neurons of the abducens nucleus were found in the contralateral medial rectus subdivision of the oculomotor complex and the medial longitudinal fasciculus (MLF). Furthermore, HRP-positive cells were also found in the ipsilateral pontine reticular formation. These findings indicate that the axons of the neurons within the pontine reticular formation project to the ipsilateral abducens nucleus, and that the internuclear neurons of the abducens nucleus project to the contralateral medial rectus subdivision of the oculomotor complex and the MLF. Clinically, the pontine reticular formation is thought to be a center for the horizontal eye movements which generates impulses to the ipsilateral abducens motor neurons and excites the internuclear neurons of the abducens nucleus projecting to the contralateral MLF as well as to the oculomotor complex. These pathways are essential for conjugate horizontal eye movements.