Spatial organization of neck and vestibular reflexes acting on the forelimbs of the decerebrate cat

Abstract

EMG recording was used to study the spatial organization of vestibular and tonic neck reflexes acting on forelimb and shoulder muscles of the decerebrate cat. Neck reflexes were studied in preparations with intact labyrinths as well as those with acute or chronic labyrinthectomies. Reflexes were described by response vectors whose orientation component is aligned with the optimal excitatory direction of tilt or head rotation. A muscle's vector orientation remained reasonably stable over a period of hours, although there was sometimes drift at the beginning or end of an experiment. Orientation of muscle response vectors did not change systematically with stimulus frequency of 0.05-2.0 Hz. For vestibular reflexes this is so, although their dynamics are consistent with convergent input from semicircular canals and otolith organs. Regardless of the preparation, a consistent reflex pattern emerged. Vestibular reflexes are characterized by response vector orientation near ear-down roll. Neck vector orientation lies in the opposite direction from the vestibular vector but typically lies further from the roll plane: Nose-up pitch is excitatory for the shoulder muscles supra- and infraspinatus, and for the medial and lateral heads of triceps, whereas nose-down pitch excites the long head of triceps. Our results generally agree with the pattern proposed by Roberts (28) for neck reflexes but disagree in part with his proposed pattern of vestibular reflexes; we did not see the expected consistent excitation by nose-down pitch.