Synchronization of motor units in human masseter during a prolonged isometric contraction.

1. The action potentials of concurrently active motor units in human masseter were recorded on several fine-wire intramuscular electrodes and synchronization was assessed by cross-correlation of their firing times. 2. Long recording sessions and a test of significance for peaks in histograms were used to enhance detection of synchrony. 3. Synchrony in masseter units was found to be very weak, and was observed consistently only in histograms with a large number of counts. However, synchrony was widespread within the motoneurone pool; of the unit pairs whose cross-correlation histograms contained more than 100 counts per 1 ms bin, 89% were significantly synchronized. 4. The strength of synchrony between pairs of units did not change consistently during 15 min of continuous activity. 5. It was concluded that most, if not all, low-threshold masseter motoneurones have some common synaptic inputs, although the origin of those inputs remains unclear.