Characteristics of antidromically identified oculomotor internuclear neurons during vergence and versional eye movements.

1. Previous studies have shown that midbrain near response cells that increase their activity during convergent eye movements project to medial rectus motoneurons, which also increase their activity during convergence. Most neurons in the abducens nucleus decrease their firing rate during convergence, and the source of this vergence signal is unknown. Oculomotor internuclear neurons (OINs) in monkeys project primarily from the medial rectus subdivisions of the oculomotor nucleus to the contralateral abducens nucleus, although there is a smaller ipsilateral projection as well. Because of these anatomic connections, it has been suggested that the OIN input may be responsible for the vergence signal seen on abducens neurons. The behavior of the OINs during eye movements and their synaptic drive are not known. Thus the goal of this study is to determine the behavior of these neurons during conjugate and disjunctive eye movements and to determine if these neurons have an excitatory or inhibitory drive on the abducens neurons. 2. Single-unit recording studies in alert rhesus monkeys were used to characterize the behavior of OINs. Eighteen OINs were identified by antidromic activation and collision testing. The recorded OINs displayed a burst-tonic pattern of activity during adducting saccades, and the majority of these cells displayed an increase in tonic activity with convergent eye movements. 3. Identified OINs were compared with a large sample of non-activated and untested horizontal burst-tonic cells in the medial rectus subdivisions of the oculomotor nucleus. The results indicate that the OINs behave similarly to medial rectus motoneurons during vergence and versional eye movements. None of the OINs displayed vertical eye position sensitivity. 4. Microstimulation of the oculomotor nucleus where both the OINs and medial rectus motoneurons were located resulted in a large adducting twitch of the ipsilateral eye and a smaller abducting twitch of the contralateral eye. The latter effect was presumed to be the result of OIN innervation of the contralateral abducens nucleus. This result suggests that the crossed OIN pathway is predominately, if not entirely, excitatory. 5. Injection of 10% lidocaine HCl into the medial rectus subdivision of the oculomotor nucleus caused a reversible inactivation of the medial rectus motoneurons and OINs. As expected, the inactivation of medial rectus motoneurons resulted in an exophoria and weakness of adduction for the eye ipsilateral to the lidocaine injection. In addition, the lidocaine injection resulted in hypometric and slowed abducting saccades in the eye contralateral to the injection site. This result also suggest that the crossed OIN pathway is excitatory.(ABSTRACT TRUNCATED AT 400 WORDS)