The effect of different head positions on vestibulo-spinal reflexes was investigated by measuring the activities of the soleus and tibialis anterior muscles induced by galvanic stimulation of the labyrinth (labyrinthine evoked EMG).
A subject was asked to stand with eyes closed and was stimulated with a galvanic current by a bipolarbiaural method with the cathode on the right ear and the anode on the left ear. The intensity was 1 mA and the duration was 3 s. The evoked EMG was observed with the head facing foward and the head turned to the left and right at angles of 30°, 45° and 90°.
1. With the head facing forward, soleus muscle activities increased on the right side and decreased on the left side. The potential changes of the tibialis anterior muscle varied with the individual subject.
2. When the head was turned to the right at an angle of 30°, soleus muscle activities increased on the right side and decreased on the left side, but the potential changes of both soleus muscles were smaller than when the head was facing foward. Tibialis anterior muscle activities increased on the right side and decreased on the left side.
3. When the head was turned to the right at an angle of 45°, both soleus muscle activities decreased and tibialis anterior muscle activities increased on the right side and decreased on the left side. The potential changes of the soleus muscle on the right side were opposite to those when the angle was 30°.
4. When the head was turned to the right at an angle of 90°, both soleus muscle activities decreased and both tibialis anterior muscle activities increased.
5. The evoked potentials with the head turned to the left at angles of 30°, 45°, and 90° were usually opposite to those when the head was turned to the right.
6. The vestibulospinal responses induced by galvanic stimulation indicated sequential changes corresponding to the angle of rotation of the head.
The vestibular system produces spinal reflexes after processing information transmitted from the neck proprioceptors.