Separation of the contributions of voluntary and vibratory activation of motor units in man by cross-correlograms.

The relationship between vibration and human motor unit spikes elicited by reflex and voluntary actions was studied by means of cross-correlograms. Using this method motor unit spikes could be classified into two categories, locked spikes with good correlation to vibratory frequency and unlocked ones with poor correlation to vibratory frequency. 1. Inter-spike intervals of the locked spikes were integer multiples of the cyclic time of the vibration used. This suggests that the locked spikes are elicited by the firing of ƒ¿-motoneurons that are activated by monosynaptic transmission of Ia vibratory afferents. 2. Locked spikes are only elicited immediately after vibratory application. Spike frequency soon attains its maximum and frequency plateau level is then maintained. 3. Unlocked spikes are slow in appearance and a gradual increase of their spike frequency is a characteristic feature. Increase of the total motor unit spike frequency is therefore attained by the recruitment of unlocked spikes even without apparent increase in the frequency of the locked spikes. This mechanism is held to explain the gradual increment of the tonic vibration reflex activity. 4. It is suggested on the basis of previous work (HOMMA and KANDA, 1973) that polysynaptic transmission caused by sustained Ia vibratory afferent activity elicits a slowly increasing EPSP which is directly responsible for the unlocked spikes. 5. Vibration superposed upon a voluntary effort elicits a considerable increase of locked spikes. These may be the sum of motor unit spikes activated by vibratory Ia impulses and other Ia impulses whose firing had become locked to the vibratory stimuli though originally driven by the