The Journal of Physiology

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.

The effects of muscle disuse on the propagation of action potentials along muscle units, a key process for effective muscle activation and force production, remain poorly understood. This study aimed to investigate changes in action potential propagation and to identify biological factors influencing these changes following unilateral lower limb suspension (ULLS) and active recovery (AR). Eleven young male participants underwent 10 days of ULLS followed by 21 days of AR based on resistance exercise. Maximal force of the knee extensor muscles (MVC), High-Density surface EMG recordings and muscle biopsies of the vastus lateralis muscle were collected before ULLS, after ULLS and after AR. EMG recordings collected during submaximal isometric contractions were decomposed to estimate single motor unit conduction velocity (MU CV). Muscle biopsies were used to measure muscle fibre diameters via histochemical analysis and ion channel transcriptomic profiles via mRNA-sequencing. MVC decreased after ULLS by 29% and fully recovered after AR. MU CV decreased after ULLS and fully recovered, up to exceeding baseline values after AR. Muscle fibre diameters did not change across the interventions and showed no correlation with MU CV. Conversely, a feature importance analysis revealed that mRNA expression levels of specific ion channel genes, particularly those involved in K+ transport, correlate with MU CV at baseline and across the interventions. This study highlights the crucial role of K+ ion channels in influencing MU CV in humans, offering new insights into MU CV modulation and the mechanisms of muscle force changes after disuse and active recovery. Key pointsO_LIMuscle disuse, such as in unilateral lower limb suspension, leads to a decrease in motor unit conduction velocity (MU CV), a critical factor for muscle activation and force production. C_LIO_LIActive recovery through resistance exercise results in the full recovery of MU CV, even exceeding baseline levels. C_LIO_LIMuscle fibre diameters do not change significantly after limb suspension or active recovery and show no correlation with MU CV. C_LIO_LIConversely, ion channel mRNA expression, particularly of those related to K+ transport, correlates with MU CV and its changes following disuse and recovery. C_LIO_LIThese findings highlight K+ ion channels as a key factor in regulating MU CV in humans and provide new molecular determinants of the changes in muscle force after disuse and recovery. C_LI

Motoneuron subtypes exhibit distinct firing properties that are critical for the graded control of muscle force. A key determinant of these differences is the medium afterhyperpolarization (mAHP), which shapes discharge rate and firing gain. While subtype-specific variation in mAHP properties has traditionally been attributed to differences in small-conductance calcium-activated potassium (SK) channel expression, emerging evidence suggests that additional conductances may contribute. Here, we investigated the role of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in regulating the mAHP and excitability of mouse spinal motoneurons during postnatal development. Using whole-cell patch-clamp recordings, we show that, by the onset of the third postnatal week, an h current (Ih) is active at resting potential in fast motoneurons and is correlated with the amplitude of the mAHP. Pharmacological blockade of HCN channels with ZD7288 increased mAHP amplitude in fast but not slow motoneurons, without affecting mAHP duration, indicating a subtype-specific contribution to mAHP amplitude. In line with the mAHP regulating firing gain, ZD7288 also reduced firing gain in fast but not slow motoneurons. These findings support a contribution of HCN channel activity to the regulation of mAHP amplitude and firing gain in fast motoneurons, highlighting a potential interaction between Ih and SK channel-dependent mechanisms in shaping motoneuron excitability. Key PointsO_LIThe amplitude of the medium afterhyperpolarization (mAHP) is negatively correlated with h-current (Ih) amplitude measured near resting potential in mouse lumbar motoneurons. C_LIO_LIPharmacological blockade of HCN channels selectively increases mAHP amplitude in fast, delayed firing alpha motoneurons, with no effect observed in slow, immediate firing alpha motoneurons. C_LIO_LIInhibition of HCN channels reduces firing gain in fast motoneurons, while slow motoneurons remain unaffected. C_LIO_LIHCN channels regulate firing gain in fast motoneurons, at least in part, through modulation of mAHP amplitude. C_LI

Although muscle atrophy may partially account for age-related strength decline, it is further influenced by alterations of neural input to muscle. Persistent inward currents and the level of common synaptic inputs to motoneurons influence neuromuscular function. However, these have not yet been described in aged human quadriceps. High density surface electromyography (HDsEMG) signals were collected from the vastus lateralis of 15 young (mean{+/-}SD, 23 {+/-} 5 y) and 15 older (67 {+/-} 9 y) men during submaximal sustained and 20-s ramped contractions. HDsEMG signals were decomposed to identify individual motor unit discharges, from which delta F and intramuscular coherence were estimated. Older participants produced significantly lower knee extensor torque (p<0.001) and poorer force tracking ability (p<0.001) than young. Older participants also had lower delta F (p=0.001) and coherence estimates in the alpha frequency band (p<0.001) during ramp contractions when compared to young. Persistent inward currents and common synaptic inputs are lower in the vastus lateralis of older males when compared to young. These data highlight altered neural input to the clinically and functionally important quadriceps, further underpinning age-related loss of function which may occur independently of the loss of muscle mass. Key PointsO_LIThe age-related loss of muscle mass is exceeded by the loss of function, which is influenced by structural and functional alterations of the nervous system. C_LIO_LIMotoneuronal persistent inward currents and common synaptic inputs play an important role in the activation of motor units and subsequent force generation and control ability. C_LIO_LIHere we show reduced estimates of persistent inward currents and lower common synaptic inputs to older vastus lateralis, potentially contributing to observed lower strength and poorer force tracking. C_LIO_LIThese findings highlight decrements of the aged human motor system, accompanied by muscle atrophy in functionally relevant muscle groups, which should be considered in the application of interventions targeting aged human muscle. C_LI

The inherited muscle-wasting disease, Duchenne muscular dystrophy (DMD), renders skeletal muscle fibers (SMFs) Na+-overloaded, ischemic, membrane-damaged, cation-leaky, depolarized, and prone to myogenic firing. DMD fibers nevertheless survive up to 3 decades before succumbing to Ca2+-necrosis. The Ca2+-necrosis is explicable, the longevity is not. Modeling here shows that SMFs ion homeostasis strategy, a low-cost resilient Pump-Leak/Donnan feedback process we term "Donnan dominated", underpins that longevity. Together, SMFs huge chloride-permeability and tiny sodium-permeability minimize excitability and pump costs, facilitating the outsized SMF pump-reserve that lets DMD fibers withstand deep ischemia and leaky channels. We illustrate how, as these impairments intensify, patients chronic Na+-overload (now non-invasively evident via Na23-MRI) would change. In simulations, prolonged excitation ([-&gt;]physiological Na+-overloading) and/or intense ischemia ([-&gt;]too little Na+-pumping) and accumulated bleb-damage ([-&gt;]too much Na+-leaking) eventually trigger Ca2+-overloading conditions. Our analysis implies an urgent need to identify SMFs pivotal small PNa, thereby opening new therapeutic remediation routes.

PurposeRecent research has explored region-specific responses within the biceps femoris long head. However, evidence on regional muscle activation remains controversial, primarily because information derived solely from surface electromyograms (sEMG) amplitude does not necessarily provide an accurate estimate of neural drive to the muscle. To address this limitation, this study investigated whether there are proximodistal differences in motor unit properties of the biceps femoris long head during isometric hip extension and knee flexion tasks. MethodsSeventeen resistance-trained males performed isometric knee flexion and hip extension tasks at 20% and 40% of maximal voluntary contraction. High-density sEMG were recorded from proximal and distal regions of the biceps femoris long head and decomposed into individual motor units. Central motor unit properties (mean discharge rate, discharge rate variability, recruitment and de-recruitment thresholds) and action potential properties (amplitude and conduction velocity) were analyzed. Bipolar sEMG amplitude was also calculated for each region to simulate traditional sEMG measurements. ResultsBipolar sEMG amplitude, motor unit action potential amplitude and conduction velocity were significantly greater in the distal region during both tasks. In contrast, no proximodistal differences were observed in central motor unit properties. ConclusionThese findings suggest that increased bipolar sEMG amplitude in the distal region of the biceps femoris long head is driven by motor unit action potential properties rather than differences in central modulation, likely influenced by intra-muscular variations in muscle mechanics and geometry. This emphasizes limitations of relying solely on sEMG amplitude to infer neural control strategies in the biceps femoris long head.

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 ([&ge;]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 ([&le;]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.

The intrinsic properties of spinal motoneurons support flexible movement, including the maintenance of postural tone. Motoneurons can produce sustained action potential output that outlasts synaptic input, a phenomenon traditionally attributed to persistent inward currents (PICs) mediated by sodium and calcium channels. Using whole-cell patch clamp electrophysiology, we examined how specific ion channels contribute to PIC maturation and non-linear firing dynamics that allow motoneurons to sustain their output in fast and slow lumbar motoneurons across postnatal development in mice. PIC amplitude and non-linear firing dynamics increased after weight bearing in fast but not slow motoneurons. Blocking Nav1.6 channels reduced PIC amplitude at both pre- and post-weight-bearing stages, whereas L-type calcium channel blockade only reduced PICs after weight bearing emerged. However, reducing PIC amplitude--either individually or in combination--did not abolish sustained firing hysteresis. Unexpectedly, activation of muscarinic receptors increased PIC amplitude while promoting adaptive firing dynamics, suggesting that PICs alone do not drive this behavior. Instead, pharmacological manipulation of potassium currents mediated by KCNQ and Kv1.2 channels, which oppose PICs, produced substantial changes in firing dynamics. Strikingly, blocking HCN channels promoted sustained firing dynamics and led to the emergence of self-sustained firing in fast motoneurons. These results indicate that while PICs and non-linear firing dynamics mature together, sustained firing relies on mechanisms beyond PICs, with potassium and HCN channels playing key modulatory roles. Key PointsO_LIPersistent inward currents (PICs) and recruitment-derecruitment hysteresis increase in parallel in fast, but not slow, motoneurons following the onset of hindlimb weight bearing. C_LIO_LIIncreased expression or function of L-type calcium channels may contribute to enhanced PICs in fast motoneurons after weight bearing emerges. C_LIO_LINeither Nav1.6 nor L-type calcium channels are required for sustained firing hysteresis in fast motoneurons. C_LIO_LIKCNQ channels attenuate PICs and, together with Kv1.2 channels, shape recruitment-derecruitment asymmetry, thereby modulating firing hysteresis in fast motoneurons. C_LIO_LIHCN channels generate a resting H-current that delays recruitment, modulates firing hysteresis, and prevents the emergence of self-sustained firing in fast motoneurons. C_LI

AimDespite men exhibiting greater muscle strength and fatigibility than women, it remains unclear if there are sex-based differences in muscle recruitment strategies e.g. motor unit (MU) recruitment and modulation of firing rate (FR) at normalised forces and during progressive increases in force. MethodsTwenty-nine healthy male and thirty-one healthy female participants (18-35 years) were studied. Intramuscular electromyography was used to record individual motor unit potentials (MUPs) and near fibre MUPs from the vastus lateralis (VL) during 10% and 25% maximum isometric voluntary contractions (MVC), and spike-triggered averaging was used to obtain motor unit number estimates (MUNE) of the VL. Multilevel mixed-effects linear regression models were used to investigate the effects of sex at each contraction level. ResultsMen exhibited greater muscle strength (p<0.001) and size (p<0.001) than women, with no difference in force steadiness at 10% or 25% MVC. Women had smaller MUs and higher FR at 10% MVC (both p<0.02), similar to that at 25% MVC in MU size (p=0.062) and FR (p=0.031). However, both sexes showed similar increases in MU size and FR when moving from low-to mid-level contractions. There were no sex differences in any near fibre MUP parameters or in MUNE. ConclusionIn the vastus lateralis, women produce muscle force via different neuromuscular recruitment strategies to men which is characterised by smaller MUs discharging at higher rates. However, similar strategies are employed to increase force production from low to moderate contractions. These findings of similar proportional increases between sexes support the use of mixed sex cohorts in studies of this nature. Key pointsO_LIIncreases in muscle force production are mediated by motor unit (MU) recruitment, and MU firing rate (FR). C_LIO_LIWomen are underrepresented in studies of human neuromuscular research and markedly differ to men in a number of aspects of neuromuscular function, yet little is known of the recruitment strategies of each. C_LIO_LIHere we demonstrate men and women have similar vastus lateralis MU number estimates, yet women recruit smaller MUs with higher FR than men at normalised contraction levels. However, increases in force are achieved via similar trajectories of MU recruitment and MU FR in men and women. C_LIO_LIAlthough men and women exhibit divergent neuromuscular recruitment strategies to achieve normalised forces, increases in force are achived similarly and support the inclusion of mixed sex cohorts in studies of this nature. C_LI

Resistance training lasting a few weeks increases maximal force mainly through neural adaptations that enhance the drive from the nervous system to muscle. While these adaptations have been well documented at the motor unit (MU) level during submaximal force contractions, the mechanisms underlying force increases during maximal voluntary contractions are poorly understood. This is due to a classic technical limitation in tracking MUs longitudinally during maximal force tasks. Here, we solved this technical challenge, enabling the investigation of MU adaptations during MVCs in both the trained and untrained limbs following unilateral resistance training. High-density surface electromyography was recorded from the biceps brachii of both limbs before and after a 4-week unilateral resistance-training intervention, and the same MUs were longitudinally tracked across sessions during MVCs by concatenation of three MVC trials of ~5-s each.Unilateral training increased maximal force in the trained limb (+16%) and induced strength transfer to the untrained limb (+8%). In both limbs, maximal contractions after training were characterized by greater EMG amplitude, faster muscle-fiber conduction velocity, and higher MU discharge rates, indicating enhanced neural drive to the motoneuron pool. These adaptations were strongly associated with improvements in maximal force (R2 > 0.7 for all). Importantly, longitudinal MU tracking revealed a non-uniform adaptation across the MU pool: MUs with higher baseline conduction velocity, indicative of higher recruitment threshold, exhibited the largest pre-post increases in discharge rate, whereas lower-threshold units showed smaller changes. Collectively, these findings demonstrate that gains in maximal force and their transfer to the untrained limb are primarily mediated by enhanced rate coding of higher-threshold MUs during MVCs.

In humans the skin is a primary thermoregulatory organ, with vasodilation leading to rapid body cooling, whereas in Rodentia the tail performs an analogous function. Many thermodetection mechanisms are likely to be involved including transient receptor potential vanilloid-type 4 (TRPV4), a widely distributed ion channel with both mechanical and thermosensitive properties. Previous studies have shown that TRPV4 can act as a vasodilator by local action in blood vessels, and in this study, we investigated whether TRPV4 activity effects mus muscularis tail vascular tone and thermoregulation. We measured tail blood flow by pressure plethysmography in lightly sedated mus muscularis (CD1 strain) at a range of ambient temperatures, with and without intraperitoneal administration of the blood brain barrier crossing TRPV4 antagonist GSK2193874. We also measured heart rate and blood pressure with and without GSK2193874. As expected for a thermoregulatory organ, we found that tail blood flow increased with temperature. However, unexpectedly we found that the TRPV4 antagonist GSK2193874 increased tail blood flow at all temperatures, and we observed changes in heart rate variability. Since TRPV4 activation stimulates the relaxation of peripheral resistance arteries (vasodilation) that would increase tail blood flow, these data suggest that increases in tail blood flow resulting from the TRPV4 antagonist may arise from a site other than the blood vessels themselves, perhaps in central cardiovascular control centres such as the hypothalamus.

There is a lack of consensus in the literature regarding the effects of dystrophin deficiency on the Ca2+-handling properties of the SR in mdx mice, an animal model of Duchenne muscular dystrophy. One possible reason for this is that only a few studies control for the presence of branched fibres. Fibre branching, a consequence of degenerative-regenerative processes such as muscular dystrophy, has in itself a significant influence on the function of the SR. In our present study we attempt to detect early effects of dystrophin deficiency on SR Ca2+ handling by using unbranched fibres from the immediate post-necrotic stage in mdx mice (just regenerated following massive necrosis). Using kinetically-corrected Fura-2 fluorescence signals measured during twitch and tetanus, we analysed the amplitude, rise time and decay time of {Delta}[Ca2+]i in unfatigued and fatigued fibres. Decay was also resolved into SR pump and SR leak components. Fibres from mdx mice were similar in all respects to fibres from wt littermates apart from: (i) a longer rise time and slower rate of rise of [Ca2+]i during a tetanus; and (ii) a mitigation of the fall in {Delta}[Ca2+]i amplitude during the course of fatigue. Our findings suggest that the early effects of a loss of dystrophin on SR Ca2+ handling are only slight, and differ from the widely held view that there is significant Ca2+ pathology in mdx mice. It may be that Ca2+ pathology is magnified by progressive branching and degeneration. New findingsCentral question: What are the early effects of dystrophin deficiency on SR Ca2+ handling in the mdx mouse? Main finding: In the mdx mouse, Ca2+ handling by the SR is little affected by the absence of dystrophin when looking at fibres without branches that have just regenerated following massive myonecrosis. This has important implications for the traditional view that Ca2+ pathology is significant in the mdx mouse.

Incomplete spinal cord injury disrupts voluntary movement, in part through motoneuron dysfunction, yet the mechanisms underlying this dysfunction remain poorly understood. Using a non-invasive approach to decode the spiking activity of large populations of spinal motoneurons, we quantified the relative contributions of excitatory, inhibitory, and neuromodulatory inputs to motoneuron rate coding after chronic incomplete spinal cord injury. Eighteen participants with incomplete spinal cord injury and 18 age- and sex-matched control participants performed submaximal isometric plantar flexion tasks while high-density surface electromyography was recorded from the soleus and gastrocnemius medialis muscles. Motoneuron firing behaviour was analysed to estimate neuromodulatory drive and the balance between inhibitory and excitatory inputs. Participants with incomplete spinal cord injury exhibited lower rate coding, characterised by lower firing rates at recruitment, lower firing rate modulation, and lower peak firing rates compared with healthy controls. Although estimates of neuromodulatory drive did not differ between groups, individuals with spinal cord injury showed a shift in the inhibition-excitation balance toward greater inhibition compared with controls. Furthermore, increasing inhibitory input through muscle length changes and antagonist tendon vibration modulated motoneuron firing in controls, but not in individuals with incomplete spinal cord injury. Together, these findings suggest that impaired rate coding after incomplete spinal cord injury arises from an altered inhibitory-excitatory balance rather than reduced neuromodulatory drive. Taking advantage of methodological advances to decode spinal motor neuron activity during voluntary contraction, this study identified excessive inhibitory input to spinal motoneurons as a key neural mechanism contributing to muscle weakness and impaired motor function in individuals with incomplete spinal cord injury.

White adipocyte adiponectin exocytosis is triggered by cAMP and a concomitant increase of cytosolic Ca2+ potentiates its release. White adipose tissue is richly innervated by sympathetic nerves co-releasing noradrenaline (NA) and ATP that may act on receptors in the adipocyte plasma membrane to increase cAMP via adrenergic receptors and Ca2+ via purinergic receptors, respectively. Here we determine the importance of NA and ATP for the regulation of white adipocyte adiponectin exocytosis, at the cellular and molecular level, and we specifically detail the ATP signalling pathway. Immunohistochemical staining demonstrates that tyrosine hydroxylase (enzyme involved in catecholamine synthesis) is dramatically reduced in inguinal white adipose tissue (IWAT) isolated from mice with diet-induced obesity; this is associated with diminished levels of NA in IWAT and with lowered serum adiponectin. Adiponectin exocytosis (measured as increase in plasma membrane capacitance and as secreted product) is triggered by NA or ATP alone in cultured and primary mouse IWAT adipocytes, and enhanced by a combination of the two secretagogues. The ATP-induced adiponectin exocytosis is largely Ca2+-dependent and activated via P2Y2 receptors (P2Y2Rs) and the Gq11/PLC pathway. Adiponectin release induced by the nucleotide is abrogated in adipocytes isolated from obese/diabetic mice and this is associated with [~]70% reduced abundance of P2Y2Rs. The NA-triggered adiponectin exocytosis is likewise abolished in "obese adipocytes", concomitant with a 50% lower gene expression of beta 3 adrenergic receptors ({beta}3ARs). The NA-stimulated adiponectin secretion does not contain Ca2+-dependent components. Collectively, our data suggest that sympathetic innervation is a principal regulator of adiponectin exocytosis and that disruptions of this control are associated with the obesity-associated reduction of circulating levels of HMW adiponectin. Key point listO_LIWhite adipose tissue is richly innervated by sympathetic nerves that co-release noradrenaline (NA) and ATP. C_LIO_LIProtein levels of tyrosine hydroxylase and NA are dramatically decreased in white adipose tissue from obese/diabetic mice, concomitant with reduced serum levels of high-molecular weight (HMW) adiponectin. C_LIO_LINA and ATP stimulate white adipocyte adiponectin exocytosis via beta adrenergic and purinergic receptors respectively. The ATP-induced adiponectin secretion is chiefly Ca2+-dependent and activated via the P2Y2/Gq11/PLC pathway. C_LIO_LIThe purinergic signalling is abrogated in adipocytes from obese/diabetic mice, due to reduced abundance of P2Y2Rs. The response to NA is likewise abolished in "obese adipocytes", associated with lowered gene expression of beta 3 adrenergic receptors ({beta}3ARs). C_LIO_LIWe propose that sympathetic innervation is central in regulation of adiponectin exocytosis via co-secretion of NA and ATP and that this control is disrupted in obesity-associated diabetes, leading to lower circulating levels of HMW adiponectin. C_LI

Sensory and corticospinal (CST) pathways activate spinal GABAergic interneurons with axo-axonic connections onto proprioceptive (Ia) afferents that depolarize these afferents (termed primary afferent depolarization, PAD). In rodents sensory-evoked PAD is produced by GABAA receptors at nodes of Ranvier in Ia-afferents, rather than at presynaptic terminals, and facilitates action potential propagation to motoneurons by preventing branch point failures, rather than causing presynaptic inhibition. Here we examined if PAD likewise facilitates the Ia-afferent mediated H-reflex in humans by evoking PAD with both sensory and CST stimulation. H-reflexes in several lower limb muscles were facilitated by prior conditioning from low-threshold proprioceptive, cutaneous or CST pathways, with a similar time course ([~]200 ms) to the PAD measured in rodent Ia-afferents. Long trains of repeated cutaneous or proprioceptive afferent stimulation produced long-lasting facilitation of the H-reflex for up to 2 minutes, consistent with the tonic depolarization of rodent Ia-afferents mediated by nodal 5-GABA receptors for similar stimulation trains. Facilitation of the conditioned H-reflexes was not mediated by direct facilitation of the motoneurons because isolated stimulation of sensory or CST pathways did not modulate the firing rate of tonically activated motor units in tested muscles. Furthermore, cutaneous conditioning increased the firing probability of a single motor unit during the H-reflex without increasing its firing rate at this time, indicating that the underlying excitatory postsynaptic potential (EPSP) was more probable, but not larger. These results are consistent with sensory and CST pathways activating nodal GABAA receptors that reduce intermittent failure of action potentials propagating into Ia-afferent branches. Key Points SummaryO_LIThe control of posture and movement requires peripheral sensory feedback, which was previously thought to be inhibited by specialized GABAergic neurons in the spinal cord. C_LIO_LIBased on new findings in rodents, we provide evidence in humans that sensory and corticospinal pathways that likely activate these GABAergic pathways facilitate, rather than inhibit, the flow of sensory feedback in afferents that carry information about body position, movement and effort. C_LIO_LIThese new findings of how sensory and descending pathways facilitate this sensory feedback to spinal motor neurons can now be applied to people with injury to the brain or spinal cord where these GABA neurons are affected, allowing us to understand how altered sensory control may affect residual motor function and the production of involuntary muscle spasticity. C_LI

1The energetic cost of skeletal muscle contraction is a fundamental driver in the selection of locomotor strategies. Muscle energy consumption depends on muscle-fibre typology, neural drive, and mechanical state. While the influence of fibre-type and mechanical state on energy use has been extensively characterised in isolated muscle preparations, these experiments fail to capture the influence of realistic in vivo motor unit recruitment strategies. Hence, this study aims to capture neural drive in the tibialis anterior, in particular motor unit recruitment thresholds and rate coding, and the corresponding changes in energy use across a range of mechanical demands. Fixed ankle angle dorsiflexion contractions were performed at varying rates of torque development, while high density electromyography characterized motor unit spiking activity, B-mode ultrasound captured muscle fascicle dynamics, and indirect calorimetry measured energetic rates. Faster rates of torque development required earlier recruitment of motor units with increased motor unit firing rates which coincided with increased fascicle strain rates. At higher rates of torque development, these altered motor unit discharge patterns and muscle mechanics coincided with increased muscle energy use. Together, these findings highlight that in vivo muscle energetics emerge from the dynamic interplay between neural control and mechanical demands.

ObjectiveTo investigate the relationship between rate of torque development (RTD) and corticospinal excitability (denoted by motor-evoked potential; MEP) during explosive voluntary contractions. Also, to assess differences in MEP and silent period duration between different phases of explosive contraction and at maximum voluntary contraction (MVC) plateau. MethodsIn 14 adults, quadriceps muscle MEP and silent period duration were measured at [~]45 (early), [~]115 (middle), and [~]190 ms (late) from EMG onset during knee-extensor isometric explosive contractions, and at MVC plateau with superimposed transcranial magnetic stimulation (TMS). RTD was measured immediately prior to early-phase MEP during the same contractions, and these two variables were correlated across separate contractions, within participants, via repeated measures correlation (RmCorr). RTD was also measured in explosive contractions without TMS over early-, middle-, and late-phases and correlated to MEP averaged across the corresponding and preceding phases, via Pearsons r correlations, assessing relationships across participants. MEP and silent period duration were compared (ANOVA) between the different phases and at MVC plateau. ResultsMEP and RTD were correlated across separate contractions within participants (RmCorr r=0.43), and in the middle phase across participants (Pearsons r=0.56). MEP and RTD were not correlated in other phases (r[&ge;]0.09). Silent period duration increased throughout the different phases of contraction and up to MVC plateau (ANOVA, p=0.001) but MEP remained constant (ANOVA, p=0.42). ConclusionThe correlations between MEP and RTD suggests corticospinal excitability is an important determinant of RTD. During rapid torque development and up to MVC plateau, corticospinal inhibition increases but excitability remains constant.

In this study, we aimed to determine the impact of an increase in motor activity during the highly plastic period of development of the motor spinal cord and hindlimb muscles in newborn mice. A swim training regimen, consisting of two sessions per day for two days, was conducted in 1 and 2-day-old (P1, P2) pups. P3-trained pups showed a faster acquisition of a four-limb swimming pattern, accompanied by dysregulated gene expression in the lateral motor column, alterations in the intrinsic membrane properties of motoneurons (MNs) and synaptic plasticity, as well as increased axonal myelination in motor regions of the spinal cord. Network-level changes were also observed, as synaptic events in MNs and spinal noradrenaline and serotonin contents were modified by training. At the muscular level, slight changes in neuromuscular junction morphology and myosin subtype expression in hindlimb muscles were observed in trained animals. Furthermore, the temporal sequence of acquiring the adult-like swimming pattern and postural development in trained pups showed differences persisting until almost the second postnatal week. A very short motor training performed just after birth is thus able to induce functional adaptation in the developing neuromuscular system that could persist several days. This highlights the vulnerability of the neuromuscular apparatus during development and the need to evaluate carefully the impact of any given sensorimotor procedure when considering its application to improve motor development or in rehabilitation strategies.

The carotid body (CB) is the key peripheral oxygen sensor. CB mitochondria are hypothesised to be uniquely adapted with unusually low intrinsic oxygen affinity which, in association with nitric oxide (NO) and reactive oxygen species signalling, enables acute responsiveness to hypoxia. However, CB mitochondrial physiology or intrinsic oxygen affinity have never been measured directly. We sought to address this key gap by isolating sheep CB mitochondria and comprehensively characterising their phenotype and contrasting them to a non-oxygen sensing tissue, left ventricular myocardium (LV). High resolution respirometry, liquid chromatography mass spectrometry, enzymatic assays and in silico modelling were used to characterise mitochondrial content, aerobic capacity, oxygen affinity, complex subunit abundance and activity, H2O2 production and NO sensitivity in ovine CB and LV. Mitochondrial oxygen affinity (P50 = 0.089 mmHg) was lower in the CB than the LV (P50 = 0.058 mmHg; p = 0.005). Whilst mitochondrial content was lower in the CB, CB mitochondria had higher respiratory rates and enzymatic activity than LV. H2O2 production and NO sensitivity were similar in the two tissues. While intrinsic mitochondrial oxygen affinity is slightly lower in the oxygen sensing CB than in the non-oxygen sensing LV, this difference is small. Hence, any role of mitochondria in CB oxygen sensing is not due to an intrinsic difference in the O2 affinity of cytochrome oxidase due to differential expression of its subunits. Instead, this work suggests that differences in O2 affinity in vivo are secondary to other factors, perhaps including NO, that alter mitochondrial O2 affinity.

Analyses of mitochondrial adaptations in human skeletal muscle have mostly used whole-muscle samples, where results may be confounded by the presence of a mixture of type I and II muscle fibres. Using our adapted mass spectrometry-based proteomics workflow, we provide new insights into fibre-specific mitochondrial differences in human skeletal muscle before and after training. Our findings challenge previous conclusions regarding the extent of fibre-type-specific remodelling of the mitochondrial proteome and highlight that most baseline differences in mitochondrial protein abundances between fibre types reported by us, and others, might be due to differences in total mitochondrial content or a consequence of adaptations to habitual physical activity (or inactivity). Most training-induced changes in different mitochondrial functional groups, in both fibre types, were stoichiometrically linked to changes in markers of mitochondrial content.