Synchronization of motor unit activity during voluntary contraction in man.

Abstract

SUMMARY1. Motor unit synchronization has been studied in human first dorsal interosseous muscle.2. Two needle electrodes were inserted into the muscle and the activity of pairs of motor units recorded.3. Pre-and post-stimulus histograms of the firing of unit pairs showed a narrow central peak of duration 1-3-9-3 ms (88% of sample in the range 1-6 ms; mode 3-0 ms), together with a variable amount of synchronization of somewhat longer duration.4. For the duration of the whole synchronization peak (85% sample in range 5-15 ms; mode between 6-1 and 8-0 ms (31 % of sample)), units fired between 8 and 485% times more often than would have been expected had the units been firing independently of one another. Amplitudes of the peak of the recorded histograms expressed as a proportion of control ranged from 1-8 to 10-9 (mean 3*9; bin width 160 ,ts).5. The strength of synchronization between the firing of motor unit pairs was inversely related to differences in recruitment threshold. The largest amount of synchronization was observed for pairs of units in which both had recruitment thresholds < 0 5 N or > 1P0 N. Less synchronization was found between pairs of units in which one had a recruitment threshold < 0-5 N and the other a threshold > 10ON.6. The time course of synchronization was well matched by the predictions of a theoretical model based on the hypothesis that underlying the observed synchronization is the joint arrival of EPSPs from branched last-order input fibres.

How computational technique and spike train properties affect coherence detection

“…direct comparison of near-synchronous spike times used to measure short-term synchronization. Whereas short-term synchronization indices express the incidence of nearsynchronous spike times, coherence reveals the strength and frequency of common inputs that drive these synchronous discharges, which are thought to be either oscillatory neural activity in the sensorimotor cortex (de Luca et al, 1993) or a subcortical region (Farmer et al, 1993), or branched common inputs at the spinal cord level (Sears and Stagg, 1976;Datta and Stephens, 1990). However, these two measures are inextricably linked mathematically since the crosscorrelation used to find temporal synchrony indices is directly related to the CPSD by the Fourier transform.…”

“…For the computational synchronization index (SI comp ), we used a cross-correlogram identical to those used for experimental synchrony indices (Bremner et al, 1991a;Datta and Stephens, 1990;Ellaway and Murthy, 1985;Logigian et al, 1988;Nordstrom et al, 1992;Wiegner and Wierzbicka, 1987), with lag times from −100 ms to 100 ms and 1-ms bins, with lag times centered at each bin. An event is counted each time a response spike from one train is within ±100 ms of a reference spike in the other train.…”

Role of across-muscle motor unit synchrony for the coordination of forces

“…This increases the probability of simultaneous discharge in the target neurons sharing these inputs. Later work in cats (Kirkwood and Sears 1978) and humans (Datta and Stephens 1990) gave further quantitative support to this hypothesis. Synchrony of broader duration is likely due to synchronization of separate pre-synaptic inputs to the motoneurons (Kirkwood 1979).…”

“…Early observations of motor unit firing patterns suggested a higher than chance tendency for pairs of motor units to fire synchronously (Sears and Stagg 1976) particularly among muscles that control the digits (Datta and Stephens 1990;Bremner et al 1991a;Nordstrom et al 1992). Sears and Stagg (1976) proposed that the above-chance coincident discharges of motor units occurring within a few milliseconds of each other (shortterm synchronization) arises from shared inputs from branched axons of last-order neurons.…”

Effect of knee joint angle on motor unit synchronization

“…40 Therefore, when using the traditional method of evaluating motor unit synchronization by crosscorrelating the firing of single motor unit pairs, a large number of motor unit pairs must be recorded to reliably estimate the degree of synchronization within and between muscles. This method provides an index of synchronization by evaluating the probability that randomly selected single motor unit pairs are synchronized.…”