Martinez-Valdes, E. | Search bioRxiv

bioRxiv2023

The short-term acquisition of a skill learning task unveils the neural filtering of shared synaptic input to spinal motor neurons

The acquisition of a motor skill involves adaptations of spinal and supraspinal pathways to alpha motoneurons. In this study, we estimated the shared synaptic contributions of these pathways to understand the neural mechanisms underlying the short-term acquisition of a new force-matching task. High-density surface electromyography (HDsEMG) was acquired from the first dorsal interosseous (FDI; 7 males and 6 females) and tibialis anterior (TA; 7 males and 4 females) during 15 trials of an isometric force-matching task. For two selected trials (pre- and post-skill acquisition), we decomposed the HDsEMG into motor unit spike trains, tracked motor units between trials, and calculated the mean discharge rate and the coefficient of variation of inter-spike interval (CoVISI). We also quantified the post/pre ratio of motor units coherence within delta, alpha, and beta bands. Improvements in force-matching were accompanied by a significant increase in the mean discharge rate and a decrease in CoVISI for both muscles. Moreover, the area under the curve within alpha band decreased by [~]22% and [~]13% for the TA and FDI muscles, respectively, with no changes in the delta or beta bands. These reductions correlated significantly with increased coupling between force/neural drive and target oscillations. These results suggest that the short-term acquisition of a new force-matching skill is mediated by the attenuation of tremor oscillations in the shared synaptic inputs. In other words, the central nervous system acts as a matched filter to modulate the synaptic weights of shared inputs and suppress neural components unrelated to the specific task. Supported by simulations, a plausible mechanism behind these alpha band reductions may involve spinal interneurons phase-cancelling descending oscillations.

neuroscience↗

bioRxiv2021

Concurrent assessment of motor unit firing properties and fascicle length changes with high-density surface electromyography ultrasound-transparent electrodes

The integration of electromyography (EMG) and ultrasound imaging has provided important information about the mechanisms of muscle activation and contraction. Unfortunately, EMG does not allow an accurate assessment of the interplay between the neural drive received by muscles, changes in fascicle length (FL) and the force/torque produced. We aimed to assess the relationship between modulations in tibialis anterior (TA) motor unit (MU) firing rate, FL and dorsiflexion torque (DT) using ultrasound-transparent high-density EMG electrodes. EMG and ultrasound images were recorded simultaneously from TA, using a 32-electrode silicon matrix, while performing isometric dorsiflexion, at diverse ankle joint positions (0{degrees} and 30{degrees} plantar flexion) and torques (20% and 40% of maximum). EMG signals were decomposed into individual MUs and changes in FL were assessed with a fascicle-tracking algorithm. MU firings were converted into a cumulative spike train (CST) that was cross-correlated with DT (CST-DT) and FL (CST-FL). High cross-correlations were found for CST-FL, 0.60 (range: 0.31-0.85) and CST-DT 0.71 (range: 0.31-0.88). Cross-correlation lags revealed that the delay between CST-FL (~75ms) was significantly smaller than CST-DT (~150ms, p<0.001). These delays affected the interpretation of MU recruitment/de-recruitment thresholds, with FL showing similar lengths for both recruitment and de-recruitment. This study is the first to demonstrate that changes in TA FL are closely related to both modulations in MU firing frequency and DT. These relationships allow assessment of the interplay between neural drive, muscle contraction and resultant torque, thereby providing a better understanding of the mechanisms responsible for the generation of muscle force. NEW AND NOTEWORTHYBy employing ultrasound-transparent high-density surface EMG electrodes, we show that modulations in tibialis anterior motor unit discharge rate were closely related to both changes in its fascicle length and resultant torque. These relationships allowed quantifying delays between neural drive and muscle shortening as well as muscle shortening and torque during submaximal isometric contractions, providing an accurate estimation of the time required to generate muscle force and subsequent production of torque via the tendon.

physiology↗