THE EFFECTS OF CALORIC STIMULATION OF THE LABYRINTH ON THE SOLEUS MOTOR POOL IN MAN

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Abstract
The effects of caloric stimulation of the labyrinth on the soleus motor pool were investigated using the Achilles tendon reflex, the H [Hoffman] reflex and the vibratory inhibition of the H reflex. The excitability of the myotatic reflex arc was augmented from the beginning of irrigation and remains so until the end of nystagmus. The effects were bilateral and roughly symmetrical. The effects were due to simultaneous activation of non-specific, probably reticular, and specific, doubtless vestibular, mechanisms. The Achilles tendon reflex was more facilitated than the H reflex, and the vibratory inhibition of the H reflex was not modified following irrigation of the ear canal. This indicated a simultaneous facilitation of .alpha. and .gamma. extensor motoneurons, but .gamma. effects clearly predominated. This interpretation was in good agreement with the findings of animal neurophysiology. The vibratory inhibition argued against a modification of presynaptic inhibition. Before interpreting this fact as different from animal mechanisms, the particular experimental conditions in man must be taken into account: the stimulus was complex and interactions between various nervous structures were possible.