Kyoya Takagi

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

Modulation by head and trunk positions of the vestibulo-spinal reflexes was studied in the soleus muscle activities induced by galvanic stimulation of the labyrinth. The stimulation was applied using a bipolar-biaural method with the cathode on the right ear and the anode on the left ear. The intensity was 1 mA and duration 3 s. 1) When the labyrinth was stimulated with the head facing forward, soleus muscle activities increased on the right side and decreased on the left, with a latency of about 100 ms. In spite of the same stimulation, activities of the soleus muscle on both sides decreased with the head rotated to the right and increased with the head rotated to the left. 2) The responses in upper-body rotation were the same as in the case of head rotation. 3) In trunk rotation, some cases showed the same responses as with the trunk facing forward in rotation to the right and left, while others showed changes in muscular activities with the head rotation caused by rotating the trunk. 4) Changes in labyrinthine evoked EMG by head and trunk positions were considered to be due to interaction of vestibular and proprioceptive inputs on the interneurons of the spinal cord.

Studies on vestibulo-spinal reflexes by examination of labyrinthine-evoked EMGs of lower limbs.

Evoked electromyograms (EMGs) induced by galvanic stimulation of the labyrinth were examined in the muscles of the lower limbs. The labyrinthine-evoked EMGs appeared in the muscles of the femoral and gluteal regions and of the legs, and these muscle activities were changed by the head position in spite of the labyrinthine stimulation being the same. The vestibulo-spinal reflexes fulfil these functions in coordination with the neck proprioceptor.

Analysis of the Vestibulo-spinal system with a Five-dimensional feedback model

The role of the vestibulo-spinal system involved in the maintenance of upright standing posture was studied by a time series analysis with a 5-dimensional feedback model. The system was composed of sways of the head, shoulders, hips and activities of the nuchal and soleus muscles. The results were displayed as determinants of noise correlation matrix, power spectrum, correlogram, transfer function and relative power contribution. The five movements constitute a feedback system in normal subjects. Bilateral loss of labyrinthine function indicates a break in the feedback system. The power spectrum and relative noise contribution indicate that the vestibulo-spinal system regulates low-frequency body sway in upright standing. However, the contribution of the system was low in the overall control of standing posture.

Analysis of standing posture in patients with bilateral loss of labyrinthine excitability

The forward-backward and right-left sways in the upright standing posture were examined using stabilometry and analysis with a 5-dimensional feedback model. The forward-backward sway was found to have a fast, hidden periodicity, composed of sways due to a loss of the labyrinthine righting reflex upon the head and interaction between the head and knee sways. Furthermore, on the basis of our previous investigation, the forward-backward sway was found to depend on the increased spinal reflexes caused by a loss of labyrinthine inhibitory control upon the antigravity muscles. The right-left sway is slow and irregular, being mainly produced by disturbance of the head righting reflex.

TRANSFER FUNCTION OF RIGHT- AND LEFTWARD EYE MOVEMENTS IN PATIENTS WITH UNILATERAL LABYRINTH

In patients with loss of unilateral labyrinthine function, the dynamic characteristics of rightward eye movement and leftward eye movement induced by the healthy labyrinth were displayed respectively. In patients who were examined within a few months after loss of unilateral labyrinthine function, the gain of eye movement toward the healthy labyrinth was low compared with that of eye movement toward the opposite direction. These differences in the gain were remarkable in the frequency range from 0.01 to 0.1 Hz. But those differences in the gain were not detected in patients who were examined more than 3 months after loss of unilateral labyrinthine function.