Vestibular projections to medial rectus subdivision of oculomotor nucleus

Abstract

Extra- and intracellular records were obtained from the medial rectus subdivision of the oculomotor complex in anesthetized cats following electrical stimulation of the vestibular nerve, nuclei, pontine tegmentum and medial longitudinal fasciculi (MLF). Stimulation of the ipsilateral vestibular nerve produced large-amplitude disynaptic EPSP [excitatory post-synaptic potentials] in medial rectus motoneurons with a latency of 1.3 ms. Electrical stimulation of the contralateral vestibular nerve produced small-amplitude disynaptic EPSP in a majority of medial rectus motoneurons with a latency of 1.4 ms. Disynaptic IPSP [inhibitory post-synaptic potential] were not observed. Suprathreshold electrical stimulation of the contralateral vestibular nerve produced the aforementioned short-latency EPSP followed either by an EPSP and/or IPSP with a minimum latency of 2.8 ms in the latter case. Simultaneous reversal of EPSP elicited from the vestibular nerve (ipsi- and contralateral) by intracellular injections of depolarizing current demonstrated that the excitatory synaptic terminations on the somadendritic membrane from contralateral vestibular neurons were more distally placed on the dendritic tree. The complete EPSP reversal confirms the chemical as opposed to electrotonic mode of synaptic transmission for all vestibular excitatory pathways terminating on the medial rectus subdivision in the mammal. The absence of a disynaptic, reciprocal, semicircular canal-evoked excitation-inhibition in the medial rectus subgroup is strikingly different than the synaptic organization found for any other vertical or horizontal extraocular motoneuron. The supranuclear afferents ending on the 2 conjugate eye movers for horizontal gaze (medial rectus and abducens) are substantially different. Such findings imply that important integrative and scaling operations must be occurring at both abducens and medial rectus motoneuronal levels.