Search

5‐HT2 receptors on motoneurones play a critical role in facilitating persistent inward currents (PICs). Although facilitation of PICs can enhance self‐sustained firing after periods of excitation, the relationship between 5‐HT2 receptor activity and self‐sustained firing in human motor units (MUs) has not been resolved. MU activity was assessed from the tibialis anterior of 10 healthy adults (24.9 ± 2.8 years) during two contraction protocols. Both protocols featured steady‐state isometric contractions with constant descending drive to the motoneurone pool. However, one protocol also included an additional phase of superimposed descending drive. Adding and then removing descending drive in the middle of steady‐state contractions altered MU firing behaviour across the motor pool, where newly recruited units in the superimposed phase were unable to switch off (P = 0.0002), and units recruited prior to additional descending drive reduced their discharge rates (P < 0.0001, difference in estimated marginal means (∆) = 2.24 pulses/s). The 5‐HT2 receptor antagonist, cyproheptadine, was then administered to determine whether changes in MU firing were mediated by serotonergic mechanisms. 5‐HT2 receptor antagonism caused reductions in MU discharge rate (P < 0.001, ∆ = 1.65 pulses/s), recruitment threshold (P = 0.00112, ∆ = 1.09% maximal voluntary contraction) and self‐sustained firing duration (P < 0.0001, ∆ = 1.77s) after the additional descending drive was removed in the middle of the steady‐state contraction. These findings indicate that serotonergic neuromodulation plays a key role in facilitating discharge and self‐sustained firing of human motoneurones, where adaptive changes in MU recruitment must occur to meet the demands of the contraction. Key points: Animal and cellular preparations indicate that somato‐dendritic 5‐HT2 receptors regulate the intrinsic excitability of motoneurones.5‐HT2 receptor antagonism reduces estimates of persistent inward currents in motoneurones, which contribute to self‐sustained firing when synaptic inputs are reduced or removed.This human study employed a contraction task that slowly increased (and then removed) the additional descending drive in the middle of a steady‐state contraction where marked self‐sustained firing occurred when the descending drive was removed.5‐HT2 receptor antagonism caused widespread reductions in motor unit (MU) discharge rates during contractions, which was accompanied by reduced recruitment threshold and attenuation of self‐sustained firing duration after the removal of the additional descending drive to motoneurones.These findings support the role that serotonergic neuromodulation is a key facilitator of MU discharge and self‐sustained firing of human motoneurones, where adaptative changes in MU recruitment must occur to meet the demands of the contraction. [ABSTRACT FROM AUTHOR]

Several methods are in use to record and analyze neuronal activation, each with specific advantages and challenges. New developments like the decomposition of high-density surface electromyography (HDsEMG) have enabled novel insights into discharge characteristics noninvasively in laboratory settings but face certain challenges to be applied in sports physiology in a broader scope. Several challenges can be accounted for by methodological considerations, others require further technological developments to allow this technology to be used in more applied settings. This paper aims to describe the developments of surface electromyography and identify the challenges and perspectives of HDsEMG in the context of an application in sports physiology. We further discuss methodological possibilities to overcome some of the challenges to investigate specific research questions and identify areas that require further advancements. [ABSTRACT FROM AUTHOR]

The purpose of our study was to investigate the influence of a stretch intervention on the common modulation of discharge rate among motor units in the calf muscles during a submaximal isometric contraction. The current report comprises a computational analysis of a motor unit dataset that we published previously (Mazzo et al., 2021). Motor unit activity was recorded from the three main plantar flexor muscles while participants performed an isometric contraction at 10% of the maximal voluntary contraction force before and after each of two interventions. The interventions were a control task (standing balance) and static stretching of the plantar flexor muscles. A factorization analysis on the smoothed discharge rates of the motor units from all three muscles yielded three modes that were independent of the individual muscles. The composition of the modes was not changed by the standing‐balance task, whereas the stretching exercise reduced the average correlation in the second mode and increased it in the third mode. A centroid analysis on the correlation values showed that most motor units were associated with two or three modes, which were presumed to indicate shared synaptic inputs. The percentage of motor units adjacent to the seven centroids changed after both interventions: Control intervention, mode 1 decreased and the shared mode 1 + 2 increased; stretch intervention, shared modes either decreased (1 + 2) or increased (1 + 3). These findings indicate that the neuromuscular adjustments during both interventions were sufficient to change the motor unit modes when the same task was performed after each intervention. Key points: Based on covariation of the discharge rates of motor units in the calf muscles during a submaximal isometric contraction, factor analysis was used to assign the correlated discharge trains to three motor unit modes.The motor unit modes were determined from the combined set of all identified motor units across the three muscles before and after each participant performed a control and a stretch intervention.The composition of the motor unit modes changed after the stretching exercise, but not after the control task (standing balance).A centroid analysis on the distribution of correlation values found that most motor units were associated with a shared centroid and this distribution, presumably reflecting shared synaptic input, changed after both interventions.Our results demonstrate how the distribution of multiple common synaptic inputs to the motor neurons innervating the plantar flexor muscles changes after a brief series of stretches. [ABSTRACT FROM AUTHOR]

Transcranial alternating current stimulation (TACS) is commonly used to synchronize a cortical area and its outputs to the stimulus waveform, but gathering evidence for this based on brain recordings in humans is challenging. The corticospinal tract transmits beta oscillations (∼21 Hz) from the motor cortex to tonically contracted limb muscles linearly. Therefore, muscle activity may be used to measure the level of beta entrainment in the corticospinal tract due to TACS over the motor cortex. Here, we assessed whether TACS is able to modulate the neural inputs to muscles, which would provide indirect evidence for TACS‐driven neural entrainment. In the first part of the study, we ran simulations of motor neuron (MN) pools receiving inputs from corticospinal neurons with different levels of beta entrainment. Results suggest that MNs are highly sensitive to changes in corticospinal beta activity. Then, we ran experiments on healthy human subjects (N = 10) in which TACS (at 1 mA) was delivered over the motor cortex at 21 Hz (beta stimulation), or at 7 Hz or 40 Hz (control conditions) while the abductor digiti minimi or the tibialis anterior muscle were tonically contracted. Muscle activity was measured using high‐density electromyography, which allowed us to decompose the activity of pools of motor units innervating the muscles. By analysing motor unit pool activity, we observed that none of the TACS conditions could consistently alter the spectral contents of the common neural inputs received by the muscles. These results suggest that 1 mA TACS over the motor cortex given at beta frequencies does not entrain corticospinal activity. Key points: Transcranial alternating current stimulation (TACS) is commonly used to entrain the communication between brain regions.It is challenging to find direct evidence supporting TACS‐driven neural entrainment due to the technical difficulties in recording brain activity during stimulation.Computational simulations of motor neuron pools receiving common inputs in the beta (∼21 Hz) band indicate that motor neurons are highly sensitive to corticospinal beta entrainment.Motor unit activity from human muscles does not support TACS‐driven corticospinal entrainment. [ABSTRACT FROM AUTHOR]

Movements are reportedly controlled through the combination of synergies that generate specific motor outputs by imposing an activation pattern on a group of muscles. To date, the smallest unit of analysis of these synergies has been the muscle through the measurement of its activation. However, the muscle is not the lowest neural level of movement control. In this human study (n = 10), we used a purely data‐driven method grounded on graph theory to extract networks of motor neurons based on their correlated activity during an isometric multi‐joint task. Specifically, high‐density surface electromyography recordings from six lower limb muscles were decomposed into motor neurons spiking activity. We analysed these activities by identifying their common low‐frequency components, from which networks of correlated activity to the motor neurons were derived and interpreted as networks of common synaptic inputs. The vast majority of the identified motor neurons shared common inputs with other motor neuron(s). In addition, groups of motor neurons were partly decoupled from their innervated muscle, such that motor neurons innervating the same muscle did not necessarily receive common inputs. Conversely, some motor neurons from different muscles‐including distant muscles‐received common inputs. The study supports the theory that movements are produced through the control of small numbers of groups of motor neurons via common inputs and that there is a partial mismatch between these groups of motor neurons and muscle anatomy. We provide a new neural framework for a deeper understanding of the structure of common inputs to motor neurons. Key points: A central and unresolved question is how spinal motor neurons are controlled to generate movement.We decoded the spiking activities of dozens of spinal motor neurons innervating six muscles during a multi‐joint task, and we used a purely data‐driven method grounded on graph theory to extract networks of motor neurons based on their correlated activity (considered as common input).The vast majority of the identified motor neurons shared common inputs with other motor neuron(s).Groups of motor neurons were partly decoupled from their innervated muscle, such that motor neurons innervating the same muscle did not necessarily receive common inputs. Conversely, some motor neurons from different muscles, including distant muscles, received common inputs.The study supports the theory that movement is produced through the control of groups of motor neurons via common inputs and that there is a partial mismatch between these groups of motor neurons and muscle anatomy. [ABSTRACT FROM AUTHOR]