Roll tilt reflexes after vestibulospinal tract lesions.

The effects of lesions of the vestibulospinal tracts on vestibular reflexes evoked by roll tilt in forelimb and neck extensors were examined in decerebrate cats. Sectioning the medial longitudinal fasciculus, which contains the medial vestibulospinal tract, had no major effect on the phase of the reflex, although some gain was usually lost at high stimulus frequencies. Spinal lesions at C2-C3, both cord hemisections and more restrictive lesions which cut the lateral vestibulospinal tract, produced two major effects on the forelimb. Background EMG activity was usually abolished in the triceps ipsilateral to the lesion, with partial loss of activity in the opposite limb. The tilt reflex response in the ipsilateral limb appeared normal, although it was usually necessary to raise the background excitability of the preparation by administering L-Dopa in order to observe the reflex. In contrast, the response in the contralateral limb showed a phase reversal of 180 deg at low stimulus frequencies, implying that the reflex in intact cats receives a crossed otolith-spinal input. Responses in the neck extensors splenius and biventer, recorded from compartments caudal to the spinal lesion, were relatively unaffected.