Motor unit firing pattern, synchrony and coherence in a deafferented patient

Abstract

The firing of spinal motoneurons (MNs) is controlled continuously by inputs from muscle, joint and skin receptors. Besides altering MN synaptic drive, the removal of these inputs is liable to alter the synaptic noise and, thus, the variability of their tonic activity. Sensory afferents, which are a major source of common and/or synchronized inputs shared by several MNs, may also contribute to the coupling in the time and frequency domains (synchrony and coherence, respectively) observed when cross-correlation and coherence analyses are applied to the discharges of MN pairs. Surprisingly, no consistent changes in firing frequency, nor in synchrony and coherence were reported to affect the activity of 3 pairs of motor units (MUs) tested in a case of sensory polyradiculoneuropathy (SPRNP), leading to an irreversible loss of large diameter sensory afferents (Farmer et al., 1993). Such a limited sample, however, precludes a definite conclusion about the actual impact that a chronic loss of muscle and cutaneous afferents may have on the firing properties of human MUs. To address this issue, the firing pattern of 92 MU pairs was analyzed at low contraction force in a case of SPRNP leading similarly to a permanent loss of proprioceptive inputs. Compared with 8 control subjects, MNs in this patient tended to discharge with slightly shorter inter-spike intervals but with greater variability. Synchronous firing tended to occur more frequently with a tighter coupling in the patient. There was no consistent change in coherence in the 15–30 Hz frequency range attributed to the MN corticospinal drive, but a greater coherence was observed below 5 Hz and between 30 and 60 Hz in the patient. The possible origins of the greater irregularity in MN tonic discharges, the tighter coupling of the synchronous firing and the changes in coherence observed in the absence of proprioceptive inputs are discussed.

Beta-band motor unit coherence and nonlinear surface EMG features of the first dorsal interosseous muscle vary with force

“…5). This complements the results of previous studies that have found greater shortterm synchronization between high-threshold motor unit pairs (Datta and Stephens 1990;Schmied et al 2014). High-threshold motor units, but not low-threshold units, also exhibited increased coherent beta-band activity when execution errors were amplified during a force tracking task (Hwang et al 2017).…”

Peer Review #1 of "motoRneuron: an open-source R toolbox for time-domain motor unit analyses (v0.1)"

“…If there are too few, an accurate picture of peak significant cannot be made. This can also been seen in the current experimental data (Fig 2.) The number of recurrence intervals used to generate histograms in previous studies has been reported in the range of approximately 300 -3000 (Schmied & Descarreaux 2010;Defreitas et al 2014;Schmied et al 2014). While no specific limit has been provided within the literature, it is recommended that visual inspections of the histograms be made to ensure adequate data quality.…”

motoRneuron: an open-source R toolbox for time-domain motor unit analyses

“…2). The number of recurrence intervals used to generate histograms in previous studies has been reported in the range of approximately 300–3,000 (Schmied & Descarreaux, 2010; Defreitas et al, 2014; Schmied, Forget & Vedel, 2014). While no specific limit has been provided within the literature, it is recommended that visual inspections of the histograms be made to ensure adequate data quality.…”