Short-term synchronization of motor units in human extensor digitorum communis muscle: relation to contractile properties and voluntary control

Abstract

Synchronous activity was studied in relation to the contractile properties of pairs of motor units (MUs) recorded with independent microelectrodes in the right extensor digitorum communis muscle (EDC) of human subjects during isometric finger extension. MU contractile properties were characterized in terms of the rise time and amplitude of twitch tensions extracted by spike-triggered averages of the extension force. Synchronization of MU discharges appeared in the form of narrow central peaks in the cross-correlograms of 35 of 50 pairs of MUs, suggesting the contribution of common last-order neurons. Synchronization peaks tended to be briefer and higher among pairs of MUs with slower and smaller twitches than among pairs of MUs with faster and larger twitches. The higher peaks of slow-contracting MUs suggest a greater effectiveness of the common synaptic inputs. The broader peaks of fast-contracting MUs might reflect an additional synchronization of the inputs to fast MUs at high force levels. The areas of the cross-correlogram peaks were similar for both groups and suggest that under our conditions, about three motoneurons would discharge synchronously for a given motoneuron spike. To test whether the amount of MU synchronization could be altered voluntarily, four subjects attempted to increase or decrease synchrony, using as feedback clicks triggered by coincident firings of the recorded MUs. In nine of 15 conditioning sessions, the magnitudes of the synchronization peaks showed significant changes in the intended direction. These results imply that supraspinal centers can control the relative amount of inputs that contribute to the synchronization of motoneuron discharges during voluntary contraction of EDC.

Common Input across Motor Nuclei Mediating Precision Grip in Humans

“…Substantial synchrony has been shown previously to exist for most pairs of motor units within intrinsic (Datta and Stephens, 1990;Bremner et al, 1991a;Nordstrom et al, 1992) and within extrinsic hand muscles (Bremner et al, 1991b;Schmied et al, 1993;Keen and Fuglevand, 2004). These findings support the idea that presynaptic fibers branch extensively to provide broadly distributed input across motor neurons comprising a single motor nucleus (Mantel and Lemon, 1987).…”

“…The baseline mean was calculated as the mean count in the first and last 60 ms of the cross-correlogram. The peak boundaries in cross-correlograms were determined as the bins corresponding to 10 and 90% of the maximum rise in the cusum near time 0 (Schmied et al, 1993). The magnitude of peaks in cross-correlograms were quantified as the total number of counts within the boundaries of the peak above the baseline mean divided by the duration of the recording (Nordstrom et al, 1992).…”

“…Therefore, for cases of nonsignificant peaks in the cross-correlogram, CIS was automatically calculated for an 11 ms region of the cross-correlogram centered at time 0 (Semmler and Nordstrom, 1995). The significance of the cross-correlogram peak was determined according to the method described by Schmied et al (1993), in which the average number of counts in the peak region must be Ͼ3 SDs above the mean count (z score Ն1.96) to be considered significant.…”

Control of motor units in human flexor digitorum profundus under different proprioceptive conditions

“…The strength of short-term synchronization that we observed in FDP lies within the range of values reported for other hand muscles. Our values of k' (from 1·1 to 2·01) for FDP are somewhat less than those for extensor digitorum communis, for which k' values of 1·2 to 3·49 have been reported (Schmied, Ivarsson & Fetz, 1993), and for first dorsal interosseous (FDI) in which Nordstrom et al (1992) found k' values of 0·79-3·44. Earlier studies have shown that the strength of short-term synchrony can change with different motor tasks.…”

Tension Distribution of Single Motor Units in Multitendoned Muscles: Comparison of a Homologous Digit Muscle in Cats and Monkeys

“…Electromyographic studies of extrinsic finger muscles in humans, for example, typically describe a single MU as optimally activated during movement of a particular finger (Schmied et al, 1993;Kilbreath and Gandevia, 1994). Studies examining the discharge of single MUs during individuated movements of each finger reveal, however, that a MU optimally activated during movement of a particular finger typically discharges during movements of adjacent fingers as well, albeit at a lower discharge frequency or at a higher recruitment threshold (Schieber, 1993;Kilbreath and Gandevia, 1994).…”