Coherence at 16-32 Hz Can Be Caused by Short-Term Synchrony of Motor Units

Abstract

of discharge times for pairs of motor units are used to infer the proportion of common synaptic input received by motor neurons. The physiological mechanisms that can produce the experimentally observed peaks in the cross-correlation histogram and the coherence spectrum are uncertain. The present study used a computational model to impose synchronization on the discharge times of motor units. Randomly selected discharge times of a unit that was being synchronized to a reference unit were aligned with some of the discharge times of the reference unit, provided the original discharge time was within 30 ms of the discharge by the reference unit. All time-domain measures (indexes CIS, E, and kЈ) were sensitive to changes in the level of imposed motor-unit synchronization (P Ͻ 0.01). In addition, synchronization caused a peak between 16 and 32 Hz in the coherence spectrum. The shape of the cross-correlogram determined the frequency at which the peak occurred in the coherence spectrum. Further, the magnitude of the coherence peak was highly correlated with the time-domain measures of motor-unit synchronization (r 2 Ͼ 0.80), with the highest correlation occurring for index E (r 2 ϭ 0.98). Thus the peak in the 16-to 32-Hz band of the coherence spectrum can be caused by the time that individual discharges are advanced or delayed to produce synchrony. Although the in vivo processes that adjust the timing of motor-unit discharges are not fully understood, these results suggest that they may not depend entirely on an oscillatory drive by the CNS.

Coherence between motor unit discharges in response to shared neural inputs

“…A similar relationship between synchronization and coherence was also observed by (Moritz et al, 2005) when motor unit synchronization was simulated by adjusting motor unit firing times by a specified delay. Adjustment of the individual motor unit discharge times to induce synchronization as in (Moritz et al, 2005) imposed a frequency modulation on the discharge times, similar to the effect of applying a shared motoneuron input signal as in the results presented here. In this study, coherence between motor units is introduced by the application of a common signal to two or more motor units, thereby enabling the observed coherence to be directly related to properties of the shared motoneuron input signals.…”

“…Experimental studies have reported a significant positive correlation between motor unit synchronization and coherence between motor unit discharge times in the 15-30 Hz frequency range (Farmer et al, 1993;Kilner et al, 2002;Semmler et al, 2004). A similar relationship between synchronization and coherence was also observed by (Moritz et al, 2005) when motor unit synchronization was simulated by adjusting motor unit firing times by a specified delay. Adjustment of the individual motor unit discharge times to induce synchronization as in (Moritz et al, 2005) imposed a frequency modulation on the discharge times, similar to the effect of applying a shared motoneuron input signal as in the results presented here.…”

“…Increased beta-band coherence between motor units has been associated with increased synchronization in experimental studies (Farmer et al, 1997;Halliday et al, 1999;Kilner et al, 2002;Semmler et al, 2002Semmler et al, , 2004. Similarly, the adjustment of motor unit firings to synchronize simulated motor unit firing times has been found to impose a frequency modulation upon the motor unit discharge times that appears as coherence in the 16-32 Hz range (Moritz et al, 2005). The results of a previous simulation study (Lowery and Erim, 2005) indicate that the amount of synchronization between motor units varies both with the frequency of the common input and the motor unit mean firing rates.…”

Increase in heterogeneity of biceps brachii activation during isometric submaximal fatiguing contractions: a multichannel surface EMG study

“…Semmler and Nordstrom 1998). Moreover, a computational model demonstrated that the imposition of motor unit synchronization causes a peak between 16 and 32 Hz in the coherence spectrum derived from the discharge times of pairs of motor units (Moritz et al 2005). The increase of heterogeneity of the spatiotemporal distribution of BB muscle activity, therefore, likely indicates a decrease in the strength of common input associated with motor unit synchronization.…”

Motor Unit Synchronization Is Increased in Biceps Brachii After Exercise-Induced Damage to Elbow Flexor Muscles

“…For voluntary isometric contractions of fresh muscle, the prevailing view is that motor unit synchronization arises from common synaptic input received by motor neurons from either branched last-order interneurons or cortical efferents that receive common input (Datta et al 1991;Farmer et al 1990Farmer et al , 1993bMantel and Lemon 1987;Moritz et al 2005;Schmied et al 1999), but does not seem to be influenced by peripheral afferents (Farmer et al 1993a(Farmer et al , 1997. However, the mechanisms responsible for an increase in motor unit synchronization after eccentric exercise are less clear, because the exerciseinduced muscle damage is likely to cause physiological adjustments within the muscle that influence both the spinal and cortical control of movement.…”