EFFECT OF MLF TRANSECTION ON THE VERTICAL VESTIBULOCOLLIC REFLEX IN DECEREBRATE CATS

1. In the cat, motoneurons supplying biventer cervicis, complexus and rectus capitis posterior receive disynaptic input from the posterior s emicircular canals and from the contralateral anterior canal via the m edial vestibulospinal tract (MVST). Disynaptic excitation from the ips ilateral anterior canal reaches these motoneurons via the lateral vest ibulospinal tract. 2. We hypothesized that if the MVST has a unique ro le in the production of the vertical vestibulocollic reflex (VCR) in t hese muscles then interruption of this tract by transection of the med ial longitudinal fasciculus (MLF) would change the VCR responses. Spec ifically, response vector orientations would shift toward the plane of the ipsilateral anterior canal, and response gains would drop at high frequencies. 3. We lesioned the MLF bilaterally and observed no subst antial effect on neck-muscle responses. Response vector orientations d id not shift systematically toward a single plane, nor was there a con sistent decrease in response gains at high frequencies. 4. As in the h orizontal VCR, there is no unique contribution from MVST neurons; para llel pathways must play an important role in the vertical VCR.