Experiments on cats were performed to study the pathway and location of the secondary vestibulo-ocular neurons in response to stimulation of the ampullary nerves of the vertical, anterior or posterior, semicircular canals. Experiments on the medial longitudinal fasciculus transection disclosed that vertical canal-evoked, disynaptic excitation and inhibition were transmitted to the extraocular motoneurons through the contra- and ipsilateral medial longitudinal fasciculus respectively. Secondary vestibular neurons, which receive input from the ampullary nerve of the vertical semicircular canals and send their axons to contralateral medial longitudinal fasciculus, were intermingled in the rostral half of the descending and lateral part of the medial vestibular nuclei. A direct excitatory connection of some of these neurons to the target extraocular motoneurons was confirmed by means of a spike-triggered signal averaging technique. It was also found that neurons activated by antidromic stimulation of ipsilateral medial longitudinal fasciculus were located in the superior vestibular nucleus, some of which made direct inhibitory connections to the target extraocular motoneurons. Both excitatory and inhibitory vestibuloocular neurons made synaptic contact in about half of the impaled target motoneurons.
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