RETICULOVESTIBULAR ORGANIZATION PARTICIPATING IN GENERATION OF HORIZONTAL FAST EYE MOVEMENT *

Unit activities that are closely related to eye movements have been recorded in both the vestibular nuclei and the reticular formation in the alert animal.'-'' Among these, four groups of neurons have been identified in the cat as immediate premotor neurons participating in activation or suppression of abducens motor activity during horizontal fast eye movement, i.e., excitatory and inhibitory secondary vestibular neurons and excitatory and inhibitory reticular burst neurons (EBNs and IBNS)."-'~ The locations of these neurons in the brain stem and their axonal projections to the abducens nuclei have been verified in these studies. Excitatory and inhibitory secondary vestibular neurons are located predominantly in the ventrolateral part of the medial vestibular nucleus. The former send axons to the contralateral abducens nucleus, and the latter to the ipsilateral (FIGURE EBNs are located in the dorsomedial reticular formation immediately rostra1 to the abducens nucleus and project to the ipsilateral abducens nu~ leus . ' ~ In contrast, IBNs are found mainly in the region caudal to the abducens nucleus and send their axons to the contralateral abducens nucleus (FIGURE The discharge pattern of these neurons related to horizontal nystagmus has also been specified. FIGURE 1 (right) shows their activities in one nystagmic cycle. At the beginning of the quick phase, EBNs and IBNs on the same side as the direction of fast eye movement exhibit high-frequency burst activity, and tonic discharges of secondary vestibular neurons on the same side are abruptly s ~ p p r e s s e d . ' ~ ~ ' ~ ~ ' ~ Synaptic mechanisms of this spike suppression of secondary vestibular neurons were in~estigated. '~ The results indicated that the spike suppression was caused by an abrupt generation of inhibitory postsynaptic potentials (IPSPs) in these neurons. These IPSPs are suggested to originate from activity of type I1 vestibular neurons, on the basis of the findings that type I1 neurons exhibited burst activity during the pause of secondary vestibular neurons and that type I1 neurons made monosynaptic inhibitory connection with secondary vestibular neurons in the same nucleus as revealed by postspike averaging technique (FIGURE However, the origin of input causing burst activity of type I1 neurons is so far unknown. In view of the neural connection shown in FIGURE 1, synchronous generation of a burst of EBNs, IBNs, and type I1 vestibular neurons on the same side appears