Direct projection of type II vestibular neurons to eye movement-related pause neurons in the cat pontine reticular formation.

Brain stem pause neurons play an important role in the regulation of rapid eye movements. However, the input sources that drive pause neurons are uncertain. In the present study, horizontal canal type II neurons in the medial vestibular nucleus were antidromically activated by electrical stimulation of the pause neuron region. Systematic microstimulation tracks within that region showed an antidromic activation pattern of low-threshold sites separated by high-threshold sites consistent with axonal branching of type II neurons in the pause neuron region. Spike-triggered averaging of single spontaneously firing type II vestibular neuronal discharges in the pause neuron region resulted in short-latency, positive field responses. These results supported the conclusion that horizontal canal type II neurons of the medial vestibular nucleus project to and inhibit pause neurons monosynaptically.