The vestibular nuclei and the interstitial nucleus of Cajal (INC) have been regarded as key elements of the velocity-to-position integrator for vertical eye movements. This paper reports a class of type II vestibular neurons that receives input from the INC and carries vertical eye movement signals that appear to represent an intermediate stage of the integration process. Extracellular recordings were made from neurons in and near the vestibular nuclei in alert cats. We encountered 39 neurons that exhibited an intense burst of spikes for downward saccades and a position-related tonic activity during intersaccadic intervals (d-type II neurons). They had a very high saccadic sensitivity (4.3+/-2.7 spikes/deg, mean +/- SD) as well as a high position sensitivity (3.2+/-1.6 (spikes/sec)/deg). Unlike the bursts of motoneurons, the bursts of these neurons declined gradually with an exponential-like time course and lasted well beyond the end of saccades. The mean time constant of the burst decay was 139+/-43 ms. The d-type II neurons were excited with disynaptic or trisynaptic latencies following stimulation of the contralateral vestibular nerve. The responses to vertical head rotations suggested inputs from the contralateral posterior canal. The d-type II neurons were excited with short latencies following stimulation of the ipsilateral INC, suggesting that they receive a direct excitatory input from vertical eye movement-related INC neurons with downward on-directions. The d-type II neurons were located in the rostral portion of the vestibular nuclei and the underlying reticular formation. These results suggest that d-type II neurons may be interposed between the burst-tonic neurons in the INC and pure tonic neurons in the vestibular nuclei and contribute to the oculomotor velocity-to-position integration.
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