Neuronal mechanism generating vertical eye position signals.

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The vestibular nucleus (VN) and the interstitial nucleus of Cajal (INC) contain neurons with activity related to vertical eye movement. To examine a possible interaction between the two nuclei, projection of INC neurons to VN and vestibular input to these neurons were studied in the alert cat.Sixty-two INC neurons with activity related to vertical eye position were tested for the response to stimulation of the ipsilateral VN. Forty-one had a downward and 21 an upward on-direction (to be called downward and upward neurons, respectively). Thirteen of 41 downward neurons were antidromically activated following stimulation of the ipsilateral VN with thresholds 36 to 220 muA. None of 21 upward neurons showed antidromic responses. Histological study showed that the stimulation sites had been in the ventral part of VN at the level of the Vlth nucleus. All the downward INC neurons antidromically activated from VN responded orthodromically to stimulation of the contralateral vestibular nerve at disynaptic latencies (1.3 to 2.0 msec). We suggest that secondary VN neurons carrying vertical eye position signals make excitatory connections with contralateral downward INC neurons which in turn send the signal ipsilaterally to VN. This neural pathway may form an important part of the neural circuitry responsible for the generation of vertical eye position signals.