Reflex control of acute postural asymmetry and compensatory symmetry after a unilateral vestibular lesion

Abstract

The roles of certain multisensory tonic postural reflexes in determining acute postural asymmetry and compensatory symmetry were studied in guinea-pigs after a unilateral vestibular neurectomy. The frequency of uncompensated eye nystagmus is modulated by neck proprioception, limb afferent blockade and orientation to gravity. The effect of orientation to gravity upon the frequency of nystagmus is seen for 3–4 days and the neck proprioceptive effect for 2–3 days after the unilateral neurectomy. Neck and trunk asymmetries induced by vestibular input are reversed in direction when the animal is turned upside down and is forced to press its feet against a surface. This reaction is called a Head Deviation Reversal. Restraint of the position of the head during compensation does not alter the compensation of the frequency of eye nystagmus but the rate of compensation of lateral head deviation is altered depending on the position of head restraint. Compensatory symmetry is disrupted by limb blockades and by lesions to the spinal lateral and dorso- and ventrolateral funiculi but not to the dorsal columns. Lesions just above C-1 produce severe body asymmetries in ‘compensated’ animals that last for several days, and the direction of body and eye asymmetries depends on the side of the C-1 lesion. In contrast, lumbar lesions produce milder asymmetries that last for a few hours and that are in the original uncompensated direction regardless of the side of the spinal lesion. It is concluded that the integrative operations that occur during vestibular eye nystagmus and lateral head deviation are under significant reflexive tonic influences from the spinal cord and the otolithic end organs. Spinal influences are, however, a very minor factor in the plasticity mechanisms that underlie eye nystagmus compensation. In contrast the compensation mechanisms for lateral head deviation appear to involve influences from the lower and upper spinal cord. The achievement of compensatory eye and body postural symmetries apparently involves an alteration of the functional relationships between the spinal circuits and the suprasegmental postural circuitry. This compensatory relationship is more pronounced for upper cervical than for lower spinal influences, and does not involve the dorsal column system.