Synaptic potentials were recorded in identified extraocular motoneurons in anesthetized cats, following stimulation of ampullary nerves of the anterior and posterior semicircular canals. Superior rectus motoneurons received disynaptic EPSPs and IPSPs following stimulation of the two ampullary nerves of the anterior and posterior semicircular canals, respectively. In the inferior rectus motoneurons, the effects of anterior and posterior semicircular canal stimulation were a mirror image of those on superior rectus motoneurons. Inferior oblique motoneurons developed disynaptic EPSPs and IPSPs following stimulation of the ampullary nerves of the contralateral anterior and ipsilateral posterior semicircular canals, respectively. In addition, some inferior oblique motoneurons displayed disynaptic IPSPs following stimulation of the contralateral ampullary nerve of the posterior semicircular canal. In the superior oblique (trochlear) motoneurons, disynaptic EPSPs and IPSPs were recorded after stimulation of the contralateral posterior and ipsilateral anterior semicircular canals, respectively. There was no significant connection between the ampullary nerves of the vertical semicircular canals and motoneurons innervating lateral and medial rectus muscles.
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