Neck Influences on the Spatial Properties of Vestibulospinal Reflexes in Decerebrate Cats: Role of the Cerebellar Anterior Vermis

Abstract

The vestibulospinal (VS) reflexes elicited by animal rotation modify the activity of limb musculature, thus preserving balance and postural stability. We investigated whether the orientation of these postural responses is strictly dependent upon the direction of head displacement or else can be modified by extralabyrinthine inputs to the goal of stabilizing body position. The experiments were performed in decerebrate cats, in which the effects of static body-to-head displacements were tested on the multiunit EMG responses of the medial head of the triceps brachii to wobble of the whole animal at 0.15 Hz, 10°, both in the clockwise (CW) and counterclockwise (CCW) direction. These stimuli allowed us to determine the muscle response vector, whose orientation component corresponds to the direction of head displacement giving rise to the maximal EMG response. When the animal body was kept straight with respect to the head, the triceps response vector was always oriented close to the transverse axis, pointing to the side-down direction. Following 30° of body-to-head displacement around a vertical axis passing through the first-second cervical joints, the response vectors of both the left and the right muscles shifted in the same direction of body rotation, thus remaining approximately perpendicular to the body axis. The change in muscle vector orientation corresponded on the average to the angle of body-to-head displacement. Only slight changes in amplitude of the muscle responses were observed. These findings imply that the maximal activation of the triceps brachii always occurred for the same direction of body displacement, irrespective of the pattern of discharge of vestibular afferents, which is determined by the direction of head displacement. The rotation of the triceps response vector induced by body-to-head displacement was reduced or suppressed by inactivation of the ipsilateral cerebellar anterior vermis, following local microinjection of the GABA_A agonist muscimol. These findings indicate that 1) the sensory input which results from changing the body position with respect to the head, probably originating from neck receptors, is able to modify the pattern of the VS reflexes, which appear to be organized in a body-centered reference frame, and 2) the cerebellar vermis is required for the proper execution of this sensorimotor transformation.