Lability of the postauricular and inion microreflexes, studied in the normal human subject.

Experiments have been carried out to investigate the lability of 2 brain-stem microreflexes, viz., the postauricular and inion responses to auditory stimulation. The size of the postauricular response was reduced by conditioning auditory or median nerve stimulation. Both forms of conditioning stimulus also induced a reduction in ongoing EMG activity in postauricular muscles, which suggests that there may be a direct inhibitory effect on facial nerve motoneurones. The effectiveness of conditioning auditory stimulation in blocking the response to a subsequent auditory testing stimulus was related to the size of the response evoked by the conditioning stimulus, which suggests that part of the inhibitory effect may be due to afterhyperpolarization or a feedback type of inhibition which is dependent on motoneurone activation. By contrast, auditory and median nerve stimulation were not found to have an inhibitory effect on the inion response.