Activity of medial vestibulospinal tract cells during rotation and ocular movement in the alert squirrel monkey.

1. Chronic unit and eye movement recording and microstimulation techniques were used to study the discharge properties of identified medial vestibulospinal tract (MVST) cells in the alert squirrel monkey. MVST cells were antidromically activated from the ventromedial funiculus at C1 and responded to orthodromic stimulation of the ipsilateral VIIIth nerve (Vi) at mono- or disynaptic latencies. Cell discharges were examined during imposed sinusoidal yaw rotation to activate horizontal semicircular canal afferents and during voluntary ocular pursuit and fixation of visual targets with the head held stationary. 2. MVST cells represented 15% (22 of 147 cells) of the population of horizontal canal-related cells recorded in the vestibular nuclei. Twelve MVST cells were monosynaptically related to Vi; of these cells, 7 (58%) were characterized as ipsilateral eye and head velocity, having a discharge modulation related to the velocity of both ipsilaterally directed eye movement and yaw rotation, and 3 also had an ipsilateral eye position sensitivity. Most (8 of 10, 80%) MVST cells disynaptically related to Vi responded only to contralateral head velocity; the other 2 cells carried a combined contralateral head and ipsilateral eye movement signal. A pause or burst of discharge associated with fast or saccadic eye movements made in any direction was not present on the 22 MVST cells. 3. The MVST is an output pathway of the vestibular nuclei through which the labyrinth controls reflex head movements. The results show that MVST cells transmit the movement and position of eyes in orbit, with vestibular signals, to the cervical spinal cord and suggest that the MVST may play a dynamic role in voluntary gaze stabilization and eye/head tracking.