Modulation by Head and Trunk Positions of the Vestibulo-spinal Reflexes Evoked by Galvanic Stimulation of the Labyrinth

Abstract

Modulation by head and trunk positions of the vestibulo-spinal reflexes was studied with respect to soleus muscle activities induced by galvanic stimulation of the labyrinth (labyrinthine evoked EMG). A subject was asked to stand with eyes closed and was stimulated with galvanic current by a bipolar- biaural method with the cathode on the right ear and the anode on the left ear. The intensity was 1mA and duration was 3s. The vestibulo-spinal responses induced by the stimulation were examined in terms of the activities of both soleus muscles. When the labyrinth was stimulated with the head facing forward, soleus muscle activities increased on the right side and decreased on the lift side with a latency of about 100ms. When the head was rotated to the right, both soleus muscle activities decreased. When the head was rotated to the left, both soleus muscle activities increased. Despite the same labyrinthine stimulation, soleus muscle activities provoked by galvanic stimulation with the head turned to the right were opposite to those with the head turned to the right. Changes by upper-body rotation of the labyrinthine evoked EMGs was similar to those by head rotation. When the trunk was rotated to the right and left with the head and lower body kept stationary, galvanic induced soleus activities were similar to those with the trunk facing forward in 3 of 6 subjects. However, 3 other subjects showed modulated response induced by head rotation, indicating that the influence of head position was stronger than that of waist position. We speculate that the properties of the vestibulo-spinal responses are due to interaction of vestibular and proprioceptive inputs on the interneuron of the spinal cord. © 1989, Japan Society for Equilibrium Research. All rights reserved.