VARIATION IN THE DEGREE OF SYNCHRONIZATION EXHIBITED BY MOTOR UNITS LYING IN DIFFERENT FINGER MUSCLES IN MAN

1. Cross-correlation analysis was performed on the discharges of motor units recorded from the same or from different finger muscles during weak, voluntary isometric contractions in man. 2. In 88% of cases such cross-correlograms contained a narrow central peak indicating synchronization between the firing of the contributary motor units. Expressed in terms of the synchronization index, b (peak area/total number of reference plus response spikes) the amount of synchronization found in these different recordings ranged from b = 0.0057 to b = 0.1436 (n = 1230). The duration of the synchronization (measured across the base of the peak) ranged from 5 to 31 ms (mode = 13 ms). 3. For each pair of motor units examined the amount of synchronization between their firing varied from minute to minute of a long recording. In around half the cases studied this variation appeared to correspond to variation in the firing rates of the two motor units whereas in the other half of cases tested no relationship was found between the firing rates and the amount of synchronization. 4. Some motor unit pairings consistently showed more synchronization than other motor unit pairings within the same muscle. A frequency histogram of the synchrony measurements from all of the motor unit pairings tested in that muscle showed a unimodal and continuous distribution. 5. Some subjects consistently showed two or three times more motor unit synchronization than others in equivalent recordings. This rank order of motor unit synchronization in different subjects was found to be the same in all muscle pairings tested. 6. A similar distribution in the amount of motor unit synchronization found in different muscle pairings was found in all subjects tested. In the first place the firing of motor units which act on widely separated fingers was less synchronized than the firing of motor units acting on adjacent fingers. Secondly, motor units acting on the lateral fingers (thumb, index) showed less synchronization in their discharges than motor units acting on medial fingers (ring, little). Finally the firing of motor units in the finger flexor muscles were less synchronized than the firing of motor units in either the finger abductor or the finger extensor muscles. 7. The synchronization of motor unit activity in different muscles indicates the presence of a widespread projection pattern for the branches of some last-order input fibres to finger muscle motoneurones. We have discussed the possibility that some of these presynaptic fibres may act as 'movement generators' whose widespread projections determine the spatial distribution of activity in different muscles during co-ordinated finger movements. Other last-order inputs may have a more restricted branching pattern, projecting mostly to the motoneurones of a single pool and with only weak projections to adjacent motor nuclei. The degree of divergence of the last-order inputs from any particular source of drive during a movement may reflect the level of necessity for separate control over the different fingers during that movement.