Biology subjects

Nazaroff, B. M.

Publications and source records attributed to Nazaroff, B. M..

2 recordsLinked to original sources

bioRxiv2026

Arm locomotor behaviour affects motor unit discharge characteristics in the stationary leg

Persistent inward currents (PICs), which are facilitated by monoaminergic inputs such as serotonin (5-HT), amplify synaptic drive and strongly influence motoneuron excitability. Although rhythmic locomotor activity increases serotonergic drive in animal models, its effects on intrinsic motoneuron properties in humans remain unclear. We examined whether rhythmic arm cycling alters motoneuron excitability of the non-exercising tibialis anterior during submaximal contractions. Twelve healthy adults (8 males, 4 females) performed triangular isometric dorsiflexion contractions at 25% and 50% MVC under four conditions: resting arm (CONTROL), finger tapping (TAP), arm cycling at 50-60 RPM (LOW), and arm cycling at 80-90 RPM (HIGH). Motor unit activity was identified from high-density surface electromyography that was decomposed into spike trains. Recruitment thresholds of identified and tracked motor units were consistent across conditions, but {Delta}F (i.e., an estimate of the PIC-related contributions to motor unit discharge) decreased during high-cadence arm cycling at stronger contraction intensities, which may reflect either reduced neuromodulation and/or increased or altered patterns of inhibition. In contrast, ascending discharge rate modulation deviated from linearity to a greater extent (i.e., brace height was larger) during both low- and high-cadence cycling, indicating greater neuromodulatory influence on the ascending discharge rate pattern. Self-sustained discharge was also elevated during cycling tasks, reflecting prolonged motor unit discharge. Taken together, these findings suggest that rhythmic activity of the arms modulates the discharge characteristics of motoneuron pools in the legs via unique combinations of excitatory, neuromodulatory and inhibitory inputs, which advances our understanding on the mechanisms of interlimb neural coupling.

neuroscience↗

bioRxiv2025

Training-induced alterations in the modulation of human motoneuron discharge patterns with contraction force

Motoneurons adapt to both resistance and endurance training in reduced animal preparations, with adaptations seemingly more apparent in higher threshold neurons, but similar evidence in humans is lacking. Here, we compared the identified motor unit (MU) discharge patterns from decomposed electromyography signals acquired during triangular dorsiflexion contractions up to 70% of maximal voluntary force (MVF) between resistance-trained, endurance-trained, and untrained individuals (n=23 in each group). We then estimated intrinsic motoneuron properties and garnered insight about the proportion of excitatory, inhibitory, and neuromodulatory inputs contributing to motor commands across contraction intensities in each group. Participants also performed a task where a triangular contraction was superimposed onto a sustained one designed to challenge inhibitory control of dendritic persistent inward currents (PICs). Both trained groups demonstrated greater MU discharge rates with greater ascending discharge rate modulation during higher contraction forces ([≥]50% MVF), which were accompanied by more linear MU discharge patterns and greater post-acceleration attenuation slopes of the ascending discharge rates. No differences in discharge rate hysteresis or the discharge rate characteristics during the sombrero tasks between groups, suggesting no differences in neuromodulatory input. Conversely, resistance-compared to endurance-trained individuals exhibited greater acceleration slopes during lower contractions forces ([≤]50% MVF), indicating the possibility of enhanced initial activation of PICs. Collectively, the greater and more linear MU discharge patterns in the trained groups either suggests a more reciprocal (i.e., push-pull) excitation-inhibition coupling during higher contraction forces or enhanced excitatory synaptic input to the motor pool, which might underpin greater force production of trained individuals.

neuroscience↗