Biology subjects

Pearcey, G.

Publications and source records attributed to Pearcey, G..

4 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↗

bioRxiv2026

Muscle-specific motor unit firing characteristics in elbow flexors and extensors after cervical spinal cord injury

Individuals with cervical spinal cord injury (SCI) often exhibit asymmetric recovery of upper-limb function, with greater weakness in elbow extensors than flexors. To determine whether muscle-specific changes in motor unit (MU) behavior contribute to this disparity, we identified MU firing instants from high-density surface electromyography to characterize MU firing characteristics in the biceps brachii (BIC) and triceps brachii (TRI) of individuals with cervical SCI (n = 20) and non-injured controls (n = 18). We quantified rate-coding behavior and metrics related to persistent inward currents (PICs), including onset-offset hysteresis ({Delta}F), ascending firing rate nonlinearity, and self-sustained firing. At the group level, BIC MUs in SCI participants showed reduced rate coding and altered ascending firing rate nonlinearity relative to controls. In contrast, TRI MUs showed no clear group-level differences. However, subgroup analysis revealed that SCI participants with low-strength during extension (n = 9) exhibited lower {Delta}F and longer self-sustained firing durations in TRI MUs than those with high-strength (n = 6). In BIC, SCI participants with low-strength during flexion (n = 8) showed reduced rate-coding behavior relative to high-strength SCI participants (n = 9), with no differences in PIC-related metrics. Together, these results demonstrate muscle-specific alterations in MU firing after cervical SCI that may relate to strength recovery or preservation and underscore the need for nuanced analyses in heterogeneous SCI populations. Key pointsO_LIRate coding and nonlinear firing behaviors are significantly altered in the biceps brachii, but not triceps brachii, of participants with cervical spinal cord injury. C_LIO_LIStrength based subgroup analyses revealed muscle-specific differences in motor unit behaviors that may be associated with strength preservation or recovery following spinal cord injury. C_LIO_LIFunctional heterogeneity following spinal cord injury may mask group differences in motor unit behaviors and warrants careful interpretation of results of future studies. C_LI

neuroscience↗

bioRxiv2026

Competing Effects of Agonist and Antagonist Vibration on the Proprioceptive Sense of Force

Motor unit (MU) firing is affected by motoneuronal persistent inward currents (PICs), which heavily contribute to gain control of motor output. PICs are highly sensitive to inhibition; for instance, Ia reciprocal inhibition via antagonist muscle vibration drastically reduces discharge rate hysteresis ({Delta}F), an estimate of PIC magnitude. A direct link between sensitivity of PICs to inhibition and voluntary force control, however, has not been established. To determine whether force control is altered with inhibition of PICs, we recorded high-density surface EMG from the tibialis anterior, while 11 participants (5F; 6M) completed and isometric force reproduction task. Tendon vibration was applied to the agonist or antagonist muscle during the first (with visual feedback) or second contraction (without visual feedback) and participants were asked to match percieved effort across contractions, in an attempt to match neural drive to the motor pool. In support of our hypothesis, torque and MU firing rates were reduced when vibration was applied to the antagonist (torque: p < .0001; MU firing rate: p < .0001), but not agonist (torque: p = .9980; MU firing rate: p = .312) muscle tendon in the second contraction, compared to control. Conversely, when vibration was applied during the first contraction, opposite effects were observed. These results suggest that PICs play a role in the proprioceptive sense of force, offering a potential link between PICs and voluntary force control, which may be important for understanding and treatment of motor impairments. KEY POINTSO_LIMotoneuronal persistent inward currents amplify synaptic currents and therefore heavily contribute to motor output, however they are extremely sensitive to Ia reciprocal inhibition induced by muscle tendon vibration. C_LIO_LIWe show that modulation of PICs severely impacts human force sense using an effort-based force reproduction paradigm which enabled us to manipulate combinations of tendon vibration and visual feedback. C_LIO_LIThese findings provide a link between PICs and functional motor output, which may be important for understanding neurological impairments and informing rehabilitation strategies. C_LI

neuroscience↗

bioRxiv2022

The effects of biological sex on estimates of persistent inward currents in the human lower limb

Non-invasive recordings of motor unit (MU) spike trains help us understand how the nervous system controls movement and how it adapts to various physiological conditions. The majority of study participants in human and non-human animal physiology studies are male, and it is assumed mechanisms uncovered in these studies are shared between males and females. However, sex differences in neurological impairment and physical performance warrant the study of sex as a biological variable in human physiology and performance. To begin addressing this gap in the study of biophysical properties of human motoneurons, we quantified MU discharge rates and estimates of persistent inward current (PIC) magnitude in both sexes by quantifying {Delta}F. We decomposed MU spike trains from the tibialis anterior (TA), medial gastrocnemius (MG), and soleus (SOL) using high-density surface electromyography and blind source separation algorithms. Ten participants of each sex performed slow triangular (10s up and down) isometric contractions to a peak of 30% of their maximum voluntary contraction. We then used linear mixed effects models to determine if peak discharge rate and {Delta}F were predicted by the fixed effects of sex, muscle, and their interaction. Despite a lack of significant sex-differences in peak discharge rates across all muscles, {Delta}F was larger ({chi}2(1) = 6.26, p = 0.012) in females (4.73 {+/-} 0.242 pps) than males (3.81 {+/-} 0.240 pps). These findings suggest that neuromodulatory drive, inhibitory input, and/or biophysical properties of motoneurons differ between the sexes and may contribute to differences in MU discharge patterns. KEY POINTS- Sex differences in motor unit studies have been revealed with greater inclusion of female participants, however, mechanisms for these differences remain unclear. - Estimates of persistent inward currents (i.e., {Delta}F) were greater in females than males in the tibialis anterior, medial gastrocnemius, and soleus muscles. - This suggests that neuromodulatory drive, monoaminergic signaling, or descending control may differ between the sexes. - Therefore, sex differences in estimates of PICs may provide a mechanism behind previously reported sex differences in motoneuron discharge patterns..

neuroscience↗