In one series of experiments, vestibular neurons that could be activated antidromically by stimulation of the contralateral medial reticular formation were studied with extracellular recording in cats under pentobarbital anesthesia. These neurons were found in all of the four main vestibular nuclei, but were less prevalent in dorsal Deiter's nucleus and in the central region of the superior vestibular nucleus than elsewhere. Regions of the pontine and medullary reticular formation from which neurons in different vestibular nuclei were activated corresponded to the pattern of vestibuloreticular projections described by neuroanatomists. Latencies of antidromic responses to stimulation of the contralateral reticular formation ranged from 0.6 to over 3 ms, indicating a relatively slow transfer of activity from vestibular nuclei to reticular formation. Forty four percent of the neurons activated antidromically by stimulating the contralateral reticular formation were also activated transsynaptically by stimulating the ipsilateral vestibular nerve. Response latencies were usually in excess of 1.3 ms, indicating the firing was evoked di or polysynaptically. The remaining neurons did not respond to stimulation of the vestibular nerve with trains of strong shocks. Vestibular neurons responding to stimulation of the vestibular nerve were more prevalent among neurons activated antidromically from the medullary reticular formation than among neurons activated from the pontine reticular formation. In a second series of experiments excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) evoked by stimulation of the four main vestibular nuclei were recorded intracellularly from neurons in the medial pontomedullary reticular formation in cats under pentobarbital anesthesia. Some cats had chronic lesions of the medial cerebellum to eliminate fastigioreticular fibers. Stimulation of the ipsilateral or contralateral vestibular nuclei evoked EPSPs and/or IPSPs in most of the medial pontomedullary reticular neurons studied with intracellular recording. Latency measurements suggested that at least half, and possibly more, of the vestibular evoked PSPs were produced by monosynaptic pathways. Short latency EPSPs were most readily evoked in both pontine and medullary reticular neurons by stimulation of the superior vestibular nucleus. No one nucleus could be established as the preferred site for evoking IPSPs. If a reticular neuron responded to stimulation of one of the vestibular nerves, stimuli applied to the vestibular nuclei on the same side usually evoked a similar response at shorter latency. In addition, stimulation of the vestibular nuclei often, evoked responses that were not evoked by strong stimulation of the vestibular nerve. Interpretation of the data suggests that the vestibuloreticular system has a widespread, direct excitatory and inhibitory action on medial reticular neurons. This system does not appear to function solely as a simple relay for vestibular afferent activity since many of the neurons sending axons to the reticular formation could not be fired by vestibular nerve stimulation. Such neurons may preferentially relay activity reaching the vestibular nuclei from other sources, such as the cerebellum or spinal cord.
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