Abstract
Field and intracellular potentials were recorded at the level of the trochlear nucleus (TN) following stimulation of the ipsi- or contralateral vestibular nerve (Vi, Vc) or the cerebellar flocculus (Floc). Vi and Vc stimulation produced clear presynaptic field potentials in the TN as the vestibulo-trochlear volleys ascending from the vestibular nucleus reached the trochlear motoneurons (TMns). Paired Vi-Vi or Vc-Vc stimulation at different intervals demonstrated a clear depression of the second of the two presynaptic potentials in the TN. A similar finding was obtained intracellularly from TMns. These results indicate a marked reduction in the excitability of the vestibular neurons following the conditioning stimulus. This period of subnormality may last for as long as 40 msec. Electrical stimulation of Vi and Vc generated an IPSP and EPSP respectively in TMns. In most instances these synaptic potentials occurred in a repetitive fashion after a single stimulation to either nerve. This reverberatory-like tendency of the vestibulo-trochlear pathways was absent in decerebellate animals, implying a cerebellar modulatory mechanism on vestibulo-trochlear transmission. Floccular stimulation produced a strong monosynaptic depression of the field and intracellular potentials evoked in the TN by Vi activation. In chronic experiments where the vestibular nerve had been transected, Floc stimulation generated a disinhibition of TMns through its suppression of the ipsilateral inhibitory vestibular neurons which project to the TN. Contralateral Floc stimulation produced a clear disfacilitation of TMns by the inhibition of contralateral excitatory vestibular neurons. Direct evidence was obtained for commissural inhibition acting on both the inhibitory and excitatory vestibular neurons projecting to the TN. The functional implications of the interaction between ipsi- and contralateral vestibular nuclei and the cerebellum and vestibular nuclei are considered in the Discussion.
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