Motor unit synchronization in physiologic, enhanced physiologic, and voluntary tremor in man.

Synchronization between pairs of single motor units simultaneously recorded from wrist extensor muscles was quantitated in 3 normal subjects during physiologic tremor (PT), beta-adrenergically enhanced physiologic tremor (EPT), and fast voluntary wrist flexion-extension movements mimicking tremor (VT). Cross-correlation histograms generated from the two spike trains of each motor unit pair demonstrated central or paracentral peaks in 13/19 recordings during PT, 22/36 during EPT, and 6/7 during VT. Relative peak area was used as a quantitative index of synchronization between the two motor units of each pair. It was lowest in PT, progressively increased in EPT as tremor amplitude increased, and highest in VT. In PT and lower amplitude EPT, the synchronization indexes were higher between motor units that discharged at the same or nearly the same frequency. In contrast, in higher amplitude EPT and VT, motor units with different firing frequencies were sometimes strongly synchronized as a consequence of double discharges in faster-firing motor units that had burst repetition rates in the range of slower-firing motor units discharging as singlets. Greater motor unit synchronization with increasing tremor amplitude in EPT may be secondary to a simultaneous increase in muscle spindle afferent activity from the tremulous muscle. Greatest synchronization in VT presumably reflects near maximal supraspinal and segmental common synaptic input onto motoneurons that generate VT. These results support a longstanding hypothesis that synchronization of motor units is the physiological basis for higher amplitude tremor.