Synaptic organization of the vestibulo-trochlear pathway.

Abstract

Field and intracellular potentials evoked in the trochlear nucleus (TN) of the cat following stimulation of the ipsi and contralateral vestibular nerves (Vi, Vc) and the vestibular nuclei (VN) were recorded with microelectrodes. Single shock stimulation of either Vc or Vi evokes in the TN the presynaptic potentials, n1 and n2, which are generated by the action currents of repetitively firing axons of vestibular neurons reaching the TN via the medial longitudinal fascicle (MLF). In the case of Vc stimulation a slow negative potential (n3) follows the presynaptic components of the field complex while a slow positive potential (p-wave) is evoked by Vi stimuli. The n3 wave is composed of the excitatory synaptic and action currents generated in trochlear motoneurons (TMns) while the p-wave is produced by the inhibitory synaptic current. Disynaptic EPSPs and IPSPs are recorded intracellularly in TMns following Vc and Vi stimulation, respectively. Each synaptic potential shows a biphasic rising phase due to the synchronous n1 and n2 presynaptic barrage. On stimulation of the ipsilateral superior and contralateral medial vestibular nuclei, the latencies of the IPSPs and EPSPs, respectively, are reduced to the monosynaptic range. Thus, it has been directly demonstrated that the VN are the mediating links for both the short latency excitatory and inhibitory vestibuloocular reflexes. The above data suggest that IPSPs are for the most part generated at or near the soma of the motoneurons. As for the site of generation of the EPSPs, a predominantly dendritic origin is suggested. The organization of the neuronal circuitry is discussed in relation to the vestibular induced eye movements.