The influence of vibration on the excitability of alpha motoneurones.

Transcranial magnetic brain stimuli were delivered to 6 healthy subjects at different time intervals after the beginning of muscle vibration. Vibration of 6 sec duration in 4 subjects and of 100 msec duration in 6 subjects was applied to the right abductor digiti minimi muscle using an electromagnetic mechanical stimulator. The responses to brain stimuli were enhanced in this muscle when vibration began 9 msec before the transcranial stimulus, i.e., when the descending volley and the monosynaptic afferent Ia volley arrived simultaneously at the anterior horn cell. With long lasting vibration an enhancement of responses to brain stimuli was seen, which began after 120 msec and continued for up to 5 sec after the onset of vibration. This is consistent with a tonic, probably polysynaptic, excitatory Ia influence on homonymous alpha motoneurones, as well as the well known monosynaptic effect.