An electrophysiological study of the sacculo-ocular pathways in cats

Microelectrode recordings of neuronal activities in the cat were made in the vestibular main nuclei and the y group cells in response to stimulation of the saccule and IIIrd (oculomotor) nuclei. A relatively new anaesthetic (Ketalar, Parke Davis Co.) that apparently did not suppress transmissions through the sacculo-oculomotor pathways was used. A social accessory arm to replace one side of the standard stereotaxic frame was designed to permit surgical access to the inner ear. Orthodromic focal potentials indicating monosynaptic activation were recorded in the region of the lateral and inferior vestibular nuclei in response to ipsilateral stimulation of saccule. Antidromic field potentials in response to IIIrd nuclear stimulation were also recorded. Similar results were obtained from the y group cells. Unit recordings demonstrated that neurones in the rostral part of the lateral vestibular nucleus received monosynaptic excitation from the saccule before projecting to the IIIrd nuclei. However, studies on the y group cells indicated the existence of a separate and distinct pathway in the sacculo-oculomotor reflex. This pathway involves at least three synapses before reaching the motoneurones.