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 med ial rectus motoneurons, which also increase their activity during conv ergence. Most neurons in the abducens nucleus decrease their firing ra te during convergence, and the source of this vergence signal is unkno wn. Oculomotor internuclear neurons (OINs) in monkeys project primaril y from the medial rectus subdivisions of the oculomotor nucleus to the contralateral abducens nucleus, although there is a smaller ipsilater al projection as well. Because of these anatomic connections, it has b een suggested that the OIN input may be responsible for the vergence s ignal seen on abducens neurons. The behavior of the OINs during eye mo vements and their synaptic drive are not known. Thus the goal of this study is to determine the behavior of these neurons during conjugate a nd disjunctive eye movements and to determine if these neurons have an excitatory or inhibitory drive on the abducens neurons. 2. Single-uni t recording studies in alert rhesus monkeys were used to characterize the behavior of OINs. Eighteen OINs were identified by antidromic acti vation and collision testing. The recorded OINs displayed a burst-toni c pattern of activity during adducting saccades, and the majority of t hese cells displayed an increase in tonic activity with convergent eye movements. 3. Identified OINs were compared with a large sample of no nactivated and untested horizontal burst-tonic cells in the medial rec tus 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 wh ere both the OINs and medial rectus motoneurons were located resulted in a large adducting twitch of the ipsilateral eye and a smaller abduc ting twitch of the contralateral eye. The latter effect was presumed t o be the result of OIN innervation of the contralateral abducens nucle us. 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 rever sible inactivation of the medial rectus motoneurons and OINs. As expec ted, the inactivation of medial rectus motoneurons resulted in an exop horia and weakness of adduction for the eye ipsilateral to the lidocai ne injection. In addition, the lidocaine injection resulted in hypomet ric and slowed abducting saccades in the eye contralateral to the inje ction site. This result also suggests that the crossed OIN pathway is excitatory. 6. Taken together, these findings indicate that the OINs a re sending an appropriate signal to the contralateral abducens nucleus for conjugate eye movements but an inappropriate signal for convergen ce eye movements. The OINs then, like the abducens internuclear neuron s, may have evolved for the coordination of horizontal conjugate movem ents. The OINs are not the source of the vergence signal seen on later al rectus motoneurons and abducens internuclear neurons. On the contra ry, an increased excitatory drive to the abducens nucleus during conve rgence might explain why most abducens neurons do not decrease their f iring rate as much for convergence movements as for conjugate movement s of similar magnitude.