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 semicircular canals and from the contralateral anterior canal via the medial vestibulospinal tract (MVST). Disynaptic excitation from the ipsilateral anterior canal reaches these motoneurons via the lateral vestibulospinal tract. 2. We hypothesized that if the MVST has a unique role in the production of the vertical vestibulocollic reflex (VCR) in these muscles then interruption of this tract by transection of the medial longitudinal fasciculus (MLF) would change the VCR responses. Specifically, 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 substantial effect on neck-muscle responses. Response vector orientations did not shift systematically toward a single plane, nor was there a consistent decrease in response gains at high frequencies. 4. As in the horizontal VCR, there is no unique contribution from MVST neurons; parallel pathways must play an important role in the vertical VCR.