Biology subjects

Valencic, T.

Publications and source records attributed to Valencic, T..

4 recordsLinked to original sources

bioRxiv2026

Reduction in Ia afferent input via ischaemia alters motor unit discharge characteristics and estimates of persistent inward currents

Persistent inward currents (PICs) govern motoneuron output and are influenced by diffuse neuromodulation and local inhibition. When large diameter afferent feedback is lost, as in some neurological conditions, PICs might additionally amplify and prolong synaptic inputs. Here, we examined whether reducing Ia afferent transmission via ischaemic nerve block alters PIC contribution to tibialis anterior (TA) motor unit (MU) discharge. Across two experiments 12 adults (5 female) performed triangular-shaped isometric dorsiflexion to 30% (Experiments 1 and 2) and 50% (Experiment 2) maximum voluntary force (MVF) at baseline, after a 20-minute rest (control), and during occlusion after inducing an ischaemic nerve block, confirmed by abolition of the soleus H-reflex. TA myoelectrical activity measured during contractions was decomposed into MU spike trains, and from smoothed MU discharges, discharge rate hysteresis ({Delta}F) and ascending non-linearity (brace height) were quantified. Results from Experiment 1 involving contractions matched to absolute force levels revealed increased peak discharge rate, {Delta}F, and brace height post-occlusion. However, {Delta}F normalised to maximal theoretical hysteresis did not change across time points. In Experiment 2, where MVF was reassessed at each timepoint and contractions were matched to relative force, peak discharge rate, normalised {Delta}F and brace height increased post-occlusion compared to pre-, across both contraction intensities. {Delta}F only increased post-occlusion at 50% MVF, with no changes at 30% MVF. These results show that ischaemic block of large-diameter axons, likely reducing reciprocal inhibition, increases PIC contribution to discharge rate modulation, highlighting the role of Ia afferent input in shaping motoneuron output in humans.

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↗

bioRxiv2024

Motor unit discharge properties of the vastii muscles and their modulation with contraction level depend on the knee-joint angle

This study examined the effect of the knee-joint angle on motor unit (MU) discharge properties of the vastii muscles and their modulation with contraction level. Twelve young adults performed unilateral isometric knee-extension contractions during three experimental sessions at either 25, 55, and 85{degrees} of knee flexion (full extension: 0{degrees}) in a randomised order. Each session involved maximal voluntary contractions (MVCs) followed by submaximal trapezoidal and triangular contractions at different levels relative to maximal voluntary torque (MVT). High-density surface electromyograms were recorded from vastus lateralis and medialis muscles and, subsequently, decomposed to obtain discharge timings of individual MUs. MVT was the greatest, whereas MU discharge rate (DR) during MVCs and submaximal contraction levels ([≥]30% MVT) was the lowest at the intermediate joint angle (55{degrees}). The highest DR during MVCs and high-level contractions (70% MVT), however, was at the most flexed knee position (85{degrees}), which was due to a greater DR increase 50-70% MVT compared to 25{degrees} and 55{degrees}. The onset-offset DR hysteresis ({Delta}F), an estimate of persistent inward current contribution to motoneuron discharge, decreased with knee flexion and increased with contraction level, whereas the degree of motoneuron input-output nonlinearity (brace height) did not vary with joint angle but decreased with contraction level. At 85{degrees}, {Delta}F increased more and brace height decreased less with contraction level compared to 25{degrees} and 55{degrees}. These findings indicate that vastii MU DR and its modulation with contraction level vary with knee-joint angle, which could be partly explained by the modulation of motoneuron intrinsic electrical properties. NEW & NOTEWORTHYThis study explored the relationship between motoneuron output to the vastii muscles at different knee-joint angles (quadriceps lengths) and isometric contraction levels. We showed that the motor unit discharge rate was lowest at the angle of the greatest absolute torque capacity, whereas the contraction-level-induced increases in discharge rate and motoneuron excitability were the greatest in the flexed position. These findings suggest that joint-angle-dependent adjustments in sensory feedback modulate motor control of the knee-extensor muscles.

physiology↗

bioRxiv2023

Motor unit discharge rate modulation during isometric contractions to failure is intensity and task dependent

The nature of neuromuscular decrements associated with contractions to task failure is known to dependent on task demands. Task-specificity of the associated adjustments in motor unit discharge rate (MUDR) behaviour, however, remains unclear. This study examined MUDR adjustments during different submaximal isometric knee-extension tasks to failure. Participants performed a sustained and an intermittent task at 20 and 50% of maximal voluntary torque (MVT), respectively (Experiment 1). High-density surface electromyography signals were recorded from vastus lateralis (VL) and medialis (VM) and decomposed into individual MU discharge timings, with the identified MUs tracked from recruitment to task failure. MUDR was quantified and normalised to intervals of 10% of contraction time (CT). MUDR of both muscles exhibited distinct modulation patterns in each task. During 20% MVT sustained task, MUDR decreased until [~]50% CT, after which it gradually returned to baseline. Conversely, during the 50% MVT intermittent task, MUDR remained stable until [~]40-50% CT, after which it started to continually increase until task failure. To explore the effect of contraction intensity on the observed patterns, VL and VM MUDR was quantified during sustained contractions at 30 and 50% MVT (Experiment 2). During the 30% MVT sustained task, MUDR remained stable until [~]80-90% CT in both muscles, after which it continually increased until task failure. During the 50% MVT sustained task the increase in MUDR occurred earlier, after [~]70-80% CT. Our results suggest that adjustments in MUDR during submaximal isometric contractions to failure are task- and intensity-dependent.

neuroscience↗