Postsynaptic inhibition of oculomotor neurons involved in vestibulo-ocular reflexes arising from semicircular canals of rabbits.

In anesthetized albino rabbits, electric stimulation of vestibular nerve branches innervating semicircular canals produced not only reflex contraction in certain extraocular muscles, but also a transient relaxation in others. From relaxing muscles was recorded a slow muscle potential that reflected depression of spontaneous spike discharges in muscle fibers. When recorded monophasically, spontaneous spikes of muscle fibers were superposed to form a direct current potential, and depression of the spikes resulted in a transient reduction of this direct current potential, i.e., the slow muscle potential. The slow muscle potential was correlated to the postsynaptic inhibition induced in oculomotor neurons through the vestibulo-ocular reflex are for the following reasons; its latency was compatible with that of the IPSP's recorded from oculomotor neurons; it was removed by severing axons of the inhibitory second-order vestibular neurons; it was blocked by intravenous injection of picrotoxin as were the IPSP's in oculomotor neurons. By recording slow muscle potentials, a specific canal-muscle relationship for the vestibulo-ocular reflex inhibition of oculomotor neurons was shown to be complementary to that obtained for the vestibulo-ocular reflex excitation.