Diurnal Modulation of Persistent Inward Current Contribution to Spinal Motor Neuron Behaviour

Abstract

Despite the critical role of persistent inward currents (PICs) in modulating motor neuron output, and thus neuromuscular performance, it remains unknown whether their contribution to motor neuron discharge behaviour varies throughout the day. This study aimed to determine whether PIC-related effects on motor neuron activity during submaximal dorsiflexion tasks differ between the early morning and late afternoon. Eighteen healthy adults (4 females; 27.4{+/-}5.6 years) performed triangular isometric contractions at two time-points: early morning (7:00-8:30 a.m.) and late afternoon (5:00-7:30 p.m.). Two conditions were tested: (1) a relative condition, where the target force corresponded to 40% of the maximal voluntary force (MVF) measured during that session, and (2) an absolute condition, where the target force was 40% of the MVF recorded during the first session. High-density surface electromyography signals were recorded from the tibialis anterior and decomposed into motor unit spike trains. The prolongation effect of PICs, estimated via {Delta}F, was significantly greater in the late afternoon in both the relative and absolute force conditions. The amplification effect of PICs, estimated by the acceleration phase of the discharge trajectory, was also higher in the late afternoon, but only in the relative force condition. No time-of-day differences were found for brace height, while attenuation was reduced in the late afternoon in the relative force condition. Collectively, these findings provide evidence for a diurnal modulation of the influence of PICs on motor neuron discharge behaviour, likely mediated by reduced inhibitory input in the late afternoon rather than by changes in neuromodulatory drive. Key pointsO_LI- Human neuromuscular performance fluctuates throughout the day, with higher force output typically observed in the late afternoon. C_LIO_LI- While circadian effects on peripheral determinants of muscle function (e.g., muscle- tendon properties, metabolic efficiency) are well documented, the neural mechanisms underlying these diurnal variations remain poorly understood. C_LIO_LI- Persistent inward currents (PICs) play a critical role in regulating motor neuron behaviour, yet their potential diurnal modulation has not been previously investigated. C_LIO_LI- Our results provide novel evidence for a diurnal modulation of the influence of PICs on motor neuron discharge behaviour, likely mediated by reduced inhibitory input in the late afternoon rather than by changes in neuromodulatory drive. C_LIO_LI- These findings suggest that intrinsic motor neuron excitability (and therefore spinal gain modulation) fluctuates across the day. C_LI