Horizontal canal input to cat extraocular motoneurons

Synaptic potential were recorded in identified extraocular motoneurons in the anesthetized cat, following stimulation of the horizontal canal nerve (HCN). Weak stimulation of the HCN evoked disynaptic EPSPs in ipsilateral medial rectus (i-MR), contralateral lateral rectus (c-LR) and disynaptic IPSPs in i-LR motoneurons. Weak stimulation of the HCN produced longer latency IPSPs (probably trisynaptic) in c-MR motoneurons. It is suggested that the HCN projects to an excitatory interneuron in the vestibular nucleus whose axon in turn projects to a third order inhibitory interneuron in the IIIrd nucleus which finally projects to c-MR motoneurons. Essentially there is no influence of the HCN stimulation on bilateral superior rectus (SR), inferior rectus (IR), superior oblique (SO) and inferior oblique (IO) motoneurons.