Experimental Physiology

BackgroundSkeletal muscle represents around 40% of total human body weight and exhibits remarkable plasticity. It can hypertrophy, atrophy, or regenerate in response to changes in activity, nutrient availability, or injury. The main component of striated muscle, the myofiber, is a post-mitotic, multinucleated cell that contains the muscles contractile unit, the sarcomere. The myonuclei within these fibers are specialized and differ in terms of gene expression and localization. Adult muscles also contain various other cell types, including adult muscle stem cells (MuSCs), macrophages, fibro-adipogenic progenitors (FAPs), and endothelial cells. MuSCs are central to muscle plasticity, and are capable of activation, proliferation, differentiation, and fusion to form new myofibers during regeneration, or to fuse with existing myofibers during hypertrophy. Muscle hypertrophy and myofibers enlargement involve increased protein synthesis and reduced protein degradation, as well as myonuclear accretion following satellite cell activation. Multiple signaling pathways, such as the mTOR pathway and the RhoA/SRF mechanotransduction pathway, are involved in these processes. MethodsWe performed single-nucleus RNA sequencing (snRNA-seq) on plantaris muscles of adult mice, comparing samples 7 days after hypertrophy induction (overload, 7OV) to non-hypertrophied controls (Ctl). RNAscope experiments on isolated myofibers identified the heterogeneity of myonuclei along the myofiber. ResultsSnRNA-seq analysis revealed a previously unknown population of myonuclei (UM). UM-Ctl, which is present only in the Ctl condition, and UM-7OV, only in the 7OV condition. These myonuclei are localised at the tips of myofibres. Furthermore, we determined that UM-7OV are not newly fused myonuclei from activated satellite cells. Trajectory analyses suggest that UM-Ctl transition into UM-7OV during hypertrophy, returning to a near-basal homeostatic state after 21 days of overload (21OV). Gene expression analysis showed that UM-Ctl and UM-7OV have distinct gene expression profiles compared to other myonuclei and respond differently to hypertrophy. ConclusionOur findings suggest the existence of a specific population of myonuclei with unique localization and gene expression profiles, which play distinct roles at baseline and during hypertrophy. These results highlight the differential properties of myonuclei in the myofiber and their potential specific functions in muscle homeostasis and adaptation.

Clinically, burn severity is reported as the size (and depth) of burn wounds relative to total body surface area (TBSA). This nomenclature is also often used in rodent models of burns. Accordingly, accurate determination and reporting of rodent TBSA is required to ensure the rigor and reproducibility of preclinical burn research. Rodent TBSA is typically estimated indirectly as a function of body mass. Further, empirical quantification of rodent TBSA through pelt dissection does not consider differences in rodent and human anatomy, making comparison of relative burn size in rodents and humans a challenge. Here, we compared commonly used approaches to directly determine or indirectly estimate rodent TBSA to demonstrate the impact different approaches can have on the calculation of relative burn size. A total of n=48 C57BL/6J background mice (55% male) ranging from 4 to 45 weeks of age and 17 to 40 grams were used. Mice were weighed prior to euthanasia. After euthanasia, mouse length was measured from the nose to anus. Mice were then placed into clear polypropylene sheet protectors (21.6 x 27.9 cm) to trace the areas of both the dorsal and ventral surfaces as well as all four limbs (dorsal-ventral (DV) tracing). Next, the pelt was carefully excised from the body through cutting a lateral line from the mouth to the genitalia, then again proximally to distally on all four limbs. The pelt was gently placed on a sheet protector and traced when both relaxed and stretched. The ears and tail were removed and traced separately. Photographs were taken of all tracings next to a ruler for scale and analyzed in ImageJ. Stretched pelt measurements of TBSA were 34% (79.4{+/-}7.6 vs. 57.5{+/-}7.5 cm2, P<0.001) and 30% (70.6{+/-}10.9 vs. 52.7{+/-}8.1 cm2, P<0.001) greater than relaxed pelt TBSA measurements in male and female mice respectively. TBSA estimated by DV tracing was 9% greater in males (62.5{+/-}10.9 vs. 57.5{+/-}7.5 cm2) and 15% in females (60.6{+/-}12.3 vs. 52.7{+/-}8.1 cm2) compared to TBSA measurements made on relaxed pelts. Accordingly, empirically derived Meeh constants (k) from DV tracing were greater than those derived from relaxed pelt measurements for both males (7.14{+/-}0.59 vs. 6.58{+/-}0.72) and females (7.72{+/-}0.58 vs. 6.78{+/-}0.80). In contrast k values derived from stretched pelt measures of TBSA were significantly greater than those determined in relaxed pelts for males (8.91{+/-}0.87 vs. 6.58{+/-}0.72, P<0.001) and females (8.85{+/-}1.25 vs. 6.78{+/-}0.80, P>0.001). The combined ears and tail represent approximately 7% and 8% of the TBSA measured by the relaxed pelt approach, respectively. Exclusion of the tail and ears from the calculated TBSA results in derived k values that are [~]16-17% lower. The approach used to determine TBSA in mice significantly influences measured areas and thus derived k values. We suggest that stretching the pelt prior to tracing inflates TBSA values, where measurements made from relaxed pelts or by DV tracing likely provide more accurate estimates of actual TBSA. Further, exclusion of the tail and ears (the latter of which is not typically considered in estimates of TBSA in humans) may be a useful approach relating relative burn sizes of mice to those of humans.

Mass is a fundamental aspect of muscle contractile function, yet the inertial effects of inactive muscle mass is generally neglected in modeling and not quantified in studies on small muscles or isolated fibers. However, during submaximal contractions, inactive muscle tissue may take longer to be accelerated by active fibers, and may be subject to prolonged deceleration, both of which may potentially reduce force development and work output. We sought to test if inactive tissue mass imposes an inertial penalty on muscle performance, using in situ sinusoidal work-loop experiments on rat plantaris muscles. Regional fascicle dynamics, measured across supramaximal and submaximal levels of activation, showed that decreasing activation significantly reduced fascicle strain and increased both shortening and lengthening latency. Contrary to our predictions, however, reductions in work, beyond those explained by decreased fascicle strain, were negligible. Normalized work did not decline disproportionately relative to force, suggesting no clear inertial penalty on work at this muscle size. Our findings suggest that while inactive muscle mass influences the dynamics of submaximal contractions, its impact on work during submaximal contractions at small muscle sizes is limited.

PurposeThe para-cycling classification system aims to minimize the impact of impairments on competition outcomes with the help of scientific evidence. This study investigated the impact of unilateral and bilateral ankle immobility on cycling performance, quantified by the maximal average mechanical power output (AMPO) over one revolution relative to that without ankle immobility. MethodsTen well-trained non-disabled cyclists performed all-out 6-second sprints on a cycle ergometer at 120 rpm under three conditions: without ankle foot orthoses (AFOs), with 1 AFO and with 2 AFOs immobilizing the ankle joint(s). Mechanical power output, pedal forces, cycling kinematics and surface-electromyography were measured. Maximal AMPO; ankle, knee and hip joint AMPO; and the amount of muscle excitation were calculated. ResultsWith 1 AFO and 2 AFOs, respectively, maximal AMPO was 96% (p<0.05) and 91% (p<0.001) of that without AFOs (1188 W). The decrease in maximal AMPO with ankle immobilization was less than the decrease in ankle joint AMPO (126 W decrease with 2 AFOs; p<0.001), due to an increase in hip joint AMPO (69 W increase with 2 AFOs; p<0.05). The amount of muscle excitation was not significantly different across conditions. ConclusionsThese findings provide a first quantitative and mechanistic indication of the impact of ankle immobility on cycling performance, which may offer valuable evidence to support the development of an evidence-based para-cycling classification system.

Perinatal opioid exposure is a prevalent clinical concern linked to respiratory instability and adverse infant outcomes. The opioid buprenorphine is prescribed as a medication for opioid use disorder during pregnancy and used to treat neonatal opioid withdrawal syndrome, yet its direct effects on neonatal control of breathing have not been examined. Here, we asked how acute buprenorphine exposure affects breathing at rest, and during chemoreceptor stimulation. Using dual-chamber head-out plethysmography, we measured pulmonary ventilation rate ([V]I) and metabolic rate in awake male and female Sprague-Dawley neonatal rats on postnatal days 4-5 (P4-5) during eupnea and a hypoxic-hypercapnic (HH) challenge. The effects of buprenorphine and two opioid receptor antagonists, naloxone hydrochloride, or peripherally restricted naloxone methiodide, were assessed using a repeated measures design. [V]I during eupnea and HH were markedly depressed following buprenorphine administration. Buprenorphine reduced [V]O2 and [V]CO2 and produced ventilatory equivalents for O2 and CO2 consistent with frank hypoventilation, driven by reduced breathing frequency and tidal volume (VT). When administered after buprenorphine, neither naloxone hydrochloride nor naloxone methiodide could rescue the buprenorphine-mediated hypoventilation in eupnea or during HH. In contrast, pre-treatment with either naloxone hydrochloride or naloxone methiodide attenuated buprenorphine-induced hypoventilation by preserving VT. These findings demonstrate that neonatal protective chemoreceptor reflexes are depressed by buprenorphine and suggest that pre-treatment with a peripheral opioid receptor antagonist could mitigate buprenorphine-induced hypoventilation without inducing opioid withdrawal. Key PointsO_LIAcute buprenorphine exposure significantly depressed pulmonary ventilation rate ([V]I) during eupnea and hypoxic hypercapnia (HH) in awake neonatal rats. C_LIO_LIBuprenorphine-induced hypoventilation was driven by reduced tidal volume (VT) and breathing frequency. C_LIO_LIBuprenorphine also reduced oxygen consumption ([V]O2) and carbon dioxide production ([V]CO2). C_LIO_LINaloxone given after buprenorphine failed to reverse hypoventilation. C_LIO_LIIn contrast, pre-treatment with either naloxone hydrochloride or peripherally restricted naloxone methiodide mitigated buprenorphine-induced hypoventilation by preserving VT. C_LI

Elite athletes exhibit sport-specific neural adaptations, yet it remains unclear whether such changes reflect general effects of training or the unique demands of individual sports. Skiing requires postural control and whole-body coordination under dynamically unstable environments, placing high demands on somatosensory processing and sensorimotor integration. The present study aimed to identify structural brain characteristics specific to elite skiers by comparing them with athletes from other sports disciplines and non-athletes. T1-weighted MRI data were analyzed using voxel-based morphometry in 13 skiers, 23 non-ski control athletes and 25 non-athletes. Whole-brain analysis comparing skiers with non-ski athletes revealed a significant cluster showing greater gray matter volume in skiers compared with non-ski athletes in the left postcentral gyrus, extending into the superior parietal lobule. The identified cluster primarily encompassed cytoarchitectonic Areas 2 and 5L. These regions are involved in higher-order somatosensory processing and multisensory integration. Importantly, region-of-interest analysis demonstrated that gray matter volume within this cluster was greater in skiers compared with non-ski athletes and non-athletes, with no difference between non-ski athletes and non-athletes. These findings highlight the relative prominence of structural adaptations within somatosensory-parietal networks, reflecting the unique integration of proprioceptive and other sensory information required for elite skiing. Overall, these findings provide evidence for sport-specific structural brain differences in elite athletes and highlight the importance of somatosensory and parietal regions in sensorimotor integration relevant to skiing. These findings may have implications for understanding neural markers of expertise and may inform future approaches to training and performance evaluation in skiing.

Mental imagery is known to be accompanied by autonomic responses, traditionally viewed as merely downstream consequences of imagery. Recent theoretical work has challenged this view, proposing that mental imagery requires the integration of cortical sensory representations with ascending interoceptive signals supplied by the autonomic nervous system. These two views make opposite predictions: if autonomic activity is only a consequence of imagery, then the responsiveness of the autonomic nervous system should not predict imagery vividness. If instead autonomic input shapes the generation of mental images, individuals with greater autonomic responsiveness should experience more vivid imagery. The present study tested these competing predictions by examining whether individual differences in cardiac vagal reactivity (indexed by the magnitude of HRV change in response to a paced breathing manipulation) predict self-reported visual imagery vividness. Imagery vividness was assessed using the Vividness of Visual Imagery Questionnaire (VVIQ) at a separate time point from the paced breathing protocol, ensuring that any observed relationship between cardiac vagal capacity cannot reflect autonomic activation driven by imagery itself. The key result was that cardiac vagal reactivity (indexed by RMSSD change normalized by mean R-R interval), significantly predicted higher VVIQ scores (r = .30, p = .031). These findings demonstrate that vividness of mental imagery is not exclusively central in origin but also shaped by the capacity of the autonomic nervous system to enter a high-parasympathetic state. Imagery thus likely involves bidirectional autonomic-cortical interaction, with descending pathways triggering the intention to generate an image and ascending interoceptive signals contributing to its generation.

Freediving in rats has emerged as a relevant model to study physiology and neural adaptation underlying submersion mechanisms. However, despite well-established strain-dependent differences in behaviour and physiological responses, most studies about freediving rely on Sprague Dawley rats. As the choice of strain could significantly shape experimental results depending on the field of research, we conducted a behavioural comparative study between Long Evans (LE) rats, genetically closer to the Wild Norway rat, with the commonly used Sprague Dawley (SD) strain. We developed an 11-week progressive voluntary freediving protocol involving four distances (from 5 to 11 meters), and assessed the rats natural willingness to dive and swim, and identified several parameters for evaluation of their confidence (waiting time before diving, speed), performance capacity (freediving time) and population variability. We found that Long Evans rats were naturally more willing to dive and more confident, compared to Sprague Dawley rats: they showed better performance with longer time underwater and slower diving speed. We also uncover differences in their variability, at trial-to-trial intra-individual and population inter-individual levels, which can guide the choice of one strain over the other, depending on the aim of the scientific inquiry. HighlightsO_LILong Evans rats were naturally more willing and confident at the beginning of the freediving training. C_LIO_LILong Evans freedivers showed greater ease in the water during the course of training compared to Sprague Dawleyfreedivers. C_LIO_LILong Evans freedivers demonstrated greater inter- and intra-individual variability. C_LI

ObjectiveN-acetylcysteine (NAC) is a clinically available antioxidant with potential applications in trauma-induced hypermetabolic states, including burn injury and crush syndrome. However, its effects on heat-stressed skeletal muscle cells remain incompletely characterized. This study conducted a secondary analysis of a publicly available dataset to quantify NACs protective effects against heat-stress-induced cellular damage. MethodsWe re-analyzed a publicly available dataset (Lu J, 2024, Mendeley Data, doi:10.17632/wffrtcgbnx.1) containing 21 observations across three conditions: Control (n=3), Heat Stress only (HS, n=3), and HS with NAC at five doses (0.5-8.0 mM, n=3 per dose). The primary outcome was the protective ratio [(HS+NAC - HS) / (Control - HS)], where 1.0 indicates complete protection. Statistical analyses included one-way ANOVA, post-hoc t-tests with Bonferroni correction, Cohens d effect sizes, and bootstrap confidence intervals. ResultsHeat stress significantly reduced cell viability by 56.3% (Control: 100.0 {+/-} 12.2 vs HS: 43.7 {+/-} 5.1; t(4)=7.37, p=0.002, Cohens d=6.02). NAC demonstrated a biphasic dose-response with maximal protection at 2.0 mM (66.7 {+/-} 14.4), yielding a protective ratio of 0.409 (95% CI: 0.146-0.675), representing 40.9% protection against heat stress damage. The comparison between HS and HS+NAC (2.0 mM) showed a large effect size (Cohens d = 2.12) but did not reach statistical significance (p = 0.060) due to the small sample size. One-way ANOVA confirmed overall group differences (F(2,18)=32.39, p<0.001, 2=0.783). ConclusionsNAC provides partial protection against heat stress-induced skeletal muscle cell damage at 2.0 mM, with a large effect size suggesting clinical relevance despite limited statistical power. These preliminary findings support further investigation of NAC as an adjunct therapy in trauma-induced hypermetabolic states. All analysis code is provided for reproducibility.

Gait asymmetry is a common manifestation of walking impairment among clinical populations. We recently developed a novel treadmill walking approach called dynamic treadmill walking that can provide asymmetric gait training by changing the treadmill speed between fast and slow speeds within a single stride. Here, we studied the energy expenditure associated with a variety of dynamic treadmill walking conditions. We hypothesized that the metabolic power required for dynamic treadmill walking in all conditions would approximate the metabolic power associated with conventional walking at the mean of the fast and slow speeds employed in the task. Eleven young adults without gait impairment walked on an instrumented treadmill and breathed into a metabolic measurement system. During dynamic treadmill walking, the treadmill fluctuated between 0.75m/s and 1.50m/s, each for 50% of an individuals stride time. We used a metronome to synchronize participants right heel-strikes with four different timing conditions. Net metabolic power during dynamic treadmill walking was significantly greater than normal walking at the mean speed of the task (1.125m/s) and generally lower than walking at the fast speed (1.5m/s). We did not observe any significant associations between net metabolic power and several measures of gait asymmetry during dynamic treadmill walking. These findings establish dynamic treadmill walking as a promising technique for improving gait symmetry in individuals who cannot tolerate fast treadmill walking, a common gait rehabilitation approach. Future work will assess the feasibility, metabolic demands, and clinical efficacy of using dynamic treadmill walking to improve gait symmetry in clinical populations. Key Points SummaryO_LIDynamic treadmill walking (i.e., walking with oscillating treadmill speeds) has previously been shown to drive gait asymmetries, but little is known about the energy expenditure required to complete the task. C_LIO_LIOur hypothesis was that dynamic treadmill walking would have similar metabolic power requirements to normal walking at a speed that is intermediate between the two dynamic treadmill walking speeds. C_LIO_LIWe found that dynamic treadmill walking actually requires metabolic power that is greater than the average of the two belt speeds, but less than that used for fast walking. C_LIO_LIDynamic treadmill walking is a promising and clinically translatable technique for rehabilitating populations with gait asymmetries that is not more energetically costly than fast treadmill walking, a common gait rehabilitation approach. C_LI

Sympathetic preganglionic neurons (SPNs) distribute signals widely across paravertebral ganglia, yet the reliability of spike propagation along their predominantly unmyelinated axons remains poorly defined. We examined temperature- and activity-dependent modulation of SPN axonal conduction using an ex vivo adult mouse thoracic sympathetic chain preparation. Population compound action potentials (CAPs) were evoked by supramaximal stimulation of T10 ventral roots and recorded from branching axons in interganglionic compared to unbranching axons in the splanchnic nerve. At physiological temperature (36{degrees}C), scaled CAP magnitude was reduced by [~]50% relative to 22{degrees}C, with preferential loss of slower-conducting axonal components. These reductions are consistent with substantial temperature-dependent decreases in effective axonal recruitment, likely reflecting conduction failure in a large fraction of SPNs. Losses were more pronounced in interganglionic pathways, suggesting increased vulnerability in branching projections. To assess activity-dependent effects, stimuli were delivered at 1, 5, and 20 Hz with focus on 5 and 20 Hz stimulus trains (20s duration). The overall time-course of train-evoked depression was similar across temperatures; however, the underlying axonal populations differed. At 22{degrees}C, slower-conducting axons exhibited marked frequency-dependent depression, whereas at 36{degrees}C the remaining faster-conducting axons displayed facilitation, particularly at 20 Hz. Slower-conducting responses also showed post-train potentiation at physiological temperature. These findings indicate that SPN axonal conduction is not uniformly reliable and is strongly modulated by temperature and activation history. Preferential vulnerability of slow-conducting, likely small-diameter and branching axons identifies axonal conduction as a physiologically regulated site of gain control in sympathetic output.

Cybersickness is a major barrier to the widespread adoption of virtual reality (VR), yet its underlying neurophysiological mechanisms remain poorly understood. This study investigated the relationship between vestibulomotor weighting and cybersickness. Vestibulomotor weighting was quantified using electrical vestibular stimulation (EVS), with coherence and gain between the EVS input and medial-lateral center-of-pressure (ML-CoP) responses indexing the contribution of vestibular input to postural control. Thirty-eight healthy young adults (females n=21, males n=17) completed a standing VR rollercoaster task while receiving continuous stochastic EVS (0-25 Hz; {+/-}4.5 mA), with ML-CoP responses recorded using a force plate. Cybersickness was assessed using the Fast Motion Sickness Scale (FMS) and Simulator Sickness Questionnaire, and participants were classified as non-sick (FMS < 5), medium-sick (FMS [&ge;] 5), or high-sick (terminated the VR exposure early due to intolerance). Baseline EVS-ML-CoP coherence across 2.5-8 Hz was significantly greater in high-sick than in non-sick participants, indicating elevated vestibulomotor weighting in individuals who developed symptoms. During VR exposure, coherence declined over time in symptomatic groups (mean slope = -0.0027 for medium-sick), whereas non-sick participants maintained consistently low coherence (mean slope = -0.0005). Despite this reduction in vestibular coupling, postural sway increased in the high-sick group relative to the medium-and non-sick groups (+29% vs. -7% and -30% change in ML-CoP RMS, respectively), while vestibular-evoked response amplitude decreased (gain reduced by 64% across 2.5-3.5 Hz). These findings indicate that greater baseline vestibulomotor weighting was associated with increased susceptibility to cybersickness, whereas reductions in vestibular contributions during VR with EVS reflected adaptive reweighting that was insufficient to prevent instability and symptom progression. Together, the results highlight baseline sensory reliance as a key determinant of cybersickness vulnerability and suggest that reweighting during exposure plays a secondary, mitigating role. New and NoteworthyWe provide the first evidence that baseline vestibulomotor weighting predicts susceptibility to cybersickness in virtual reality and is dynamically reduced during exposure. Using electrical vestibular stimulation, we show that symptomatic individuals begin with greater reliance on vestibular input for postural control and progressively downweight these signals in response to sensory conflict.

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.

We investigated effects of three aerobic exercise interventions, varying in amount and intensity with durations of 8-9-months on small RNA (smRNA) expression and regulatory pathways in skeletal muscle and plasma from 120 participants. Using untargeted smRNA sequencing focused on miRNAs and piRNAs, adjusting for demographics and bodyweight, we identified 124 muscle smRNAs altered by exercise amount and 15 by intensity, and 47 plasma smRNAs altered by intensity and one by amount. These smRNAs were enriched in metabolic, transcriptional, translational, and cell cycle pathways. Exercise-induced changes in several smRNAs-six from muscle and five from plasma-and exercise-induced reduction in body weight, aligned with improvement in insulin sensitivity (p<0.05). These findings demonstrate tissue-specific regulation of smRNAs by exercise and identify potential candidates for exercise mimetics to modulate muscle insulin sensitivity.

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

Pleasurable urge to move to music is often referred to as groove. Although previous studies have shown an association between the supplementary motor area (SMA) and the groove experience, its causal role remains unclear. Here, we investigated whether the SMA is causally involved in groove experience during music listening using repetitive transcranial magnetic stimulation. Fifteen healthy participants completed three sessions on separate days: excitatory stimulation (intermittent theta burst stimulation; iTBS) over the SMA, inhibitory stimulation (continuous theta burst stimulation; cTBS) over the SMA, and sham stimulation (iTBS or cTBS) over the vertex. After each stimulation session, participants listened to five high-groove and five low-groove musical excerpts and rated urge-to-move and pleasure on a 0-100 scale. Heart rate was additionally recorded as an exploratory physiological measure during music listening. Linear mixed-effects models (LMM) showed that pleasure ratings, but not urge-to-move ratings, were higher following both iTBS and cTBS compared with sham stimulation. In exploratory LMMs, reduced log-transformed heart rate variability (HRV) significantly predicted higher pleasure ratings. These findings suggest that SMA stimulation modulates the pleasurable component of the groove experience, likely via network-level mechanisms rather than a simple linear relationship between SMA excitability and pleasure. They also raise the possibility that reduced parasympathetic activity, reflected by lower HRV, mediates the stimulation-related increase in musical pleasure. Future studies should investigate the causal roles of other brain regions as well as clarify the directionality between autonomic changes and the groove experience.

Motor skills such as bicycle riding are considered robust and transferable across bicycle types. However, when the steering direction is inverted (reversed bicycle) control is disrupted to the extent that the bicycle cannot be ridden. With sufficient practice, the reversed bicycle can be learned, but this learning appears to produce impairment of normal bicycle riding suggesting modification of this long-established motor memory. Here we investigate the learning process of riding a reversed bicycle over four days of practice, while repeatedly assessing normal bicycle performance to measure any potential interference. Introduction of the reversed bicycle disrupted predictive control, reflected in a consistently increased time lag in the steering-roll coupling during reversed bicycle trials. This increase in delay suggests that predictive behavior in normal bicycle riding cannot be transferred to the reversed bicycle. With training, some participants successfully learned to ride the reversed bicycle by gradually reorganizing this coupling, whereas others failed to acquire this inverted coupling. Notably, even short-term exposure to the reversed bicycle interfered with normal bicycle riding, reducing distance ridden and increasing variability in steering rate. Together, we show that even a highly practiced whole-body motor skill is susceptible to rapid interference when control dynamics are altered.

Conventional diagonal stride skiing traditionally includes a glide phase, characterised by a period of relatively passive gliding on one ski. While the glide phase may take advantage of low ski-snow friction, it does not exhibit the same whole-cycle mechanical energy fluctuations seen in running or walking on foot. A new sub-technique, known as running style, substantially reduces the glide phase and may alter the role of elastic tissues, making the movement pattern more similar to uphill running on foot in its temporal organisation. We examined knee extensor and plantar flexor muscle-tendon behaviour in eight competitive skiers performing conventional diagonal and running techniques on a treadmill inclined at 10{degrees}. Using synchronised ultrasonography, 3D kinematics, ski forces and EMG, we quantified gastrocnemius medialis and vastus lateralis fascicle and muscle-tendon unit (MTU) dynamics in both the running (RUN) and conventional (CON) styles. Shorter glide and total cycle durations during RUN shifted MTU peak length and velocity earlier during the kick phase. Fascicles in both muscles operated at similar velocities across techniques, showing MTU-fascicle decoupling. Vastus lateralis fascicles shortened at higher absolute peak velocities than gastrocnemius in both conditions, while normalised velocities were similar. RUN increased preactivation and advanced EMG timing, while integrated EMG during the kick was lower compared to CON. These findings suggest that, despite large shifts in external mechanics between glide-based and more running-like skiing, elastic tissues may help stabilise fascicle behaviour and preserve a similar contractile strategy across muscles and techniques.

Background: C-terminal binding protein 2 (CtBP2) has been implicated in metabolic regulation, but its association with specific measures of adiposity and lipid profiles in humans remains unclear. This study examined the relationship between circulating CtBP2 levels and key components of metabolic syndrome, focusing on body fat distribution and lipid markers. Methods: Data from 508 participants (259 men, 249 women) from a publicly available dataset were analyzed. Serum CtBP2 concentrations were measured using ELISA. Associations with obesity markers (BMI, waist circumference, waist-to-hip ratio) and lipid profiles (triglycerides, HDL cholesterol) were assessed using Spearman correlation and linear regression, adjusting for age and sex. Results: CtBP2 levels showed weak but statistically significant positive correlations with all measures of adiposity, with the strongest association observed for waist circumference ({rho} = 0.150, p < 0.001), followed by BMI ({rho} = 0.120, p = 0.007) and waist-to-hip ratio ({rho} = 0.098, p = 0.027). No significant correlations were found with triglycerides or HDL cholesterol. In the regression model predicting BMI, age, and sex were significant predictors, while CtBP2 demonstrated a trend toward association ({beta} = 0.080, p = 0.052). Conclusion: Circulating CtBP2 appears to be modestly associated with measures of adiposity, particularly abdominal fat, but not with lipid abnormalities. These findings suggest a potential role for CtBP2 in obesity-related metabolic dysregulation and underscore the need for further mechanistic studies to clarify its clinical relevance.

Rhesus macaques (Macaca mulatta) are widely used as non-human primate models for biomedical research. When housed in captivity, it is essential to provide an environment that supports their natural behaviours; otherwise, they risk developing mood disorders, stereotypies, and other behavioural issues that may lead to physical harm. The objective of this preliminary study was to monitor the behaviour of three aged rhesus macaques ([&ge;] 20 y.o.), relocated from a laboratory to a Rescue Centre for Exotic Animals (Italy), and to assess the impact of novel food enrichments. Behavioural data were collected over 18 weeks, beginning at their arrival, using continuous focal sampling from video recordings. Simultaneously, faecal samples were gathered for cortisol analysis. The study was divided into three phases: a control phase without enrichments, a feeding enrichment phase (divided into two periods), and a final control phase without enrichments. Each phase comprised 900 minutes of observations for each subject. Data were analysed using generalized linear mixed models. Results showed an increase in locomotion during the enrichment and final phase compared to the initial phase. Additionally, a reduction in scratching and body-shaking behaviours was observed in the final phase compared to the initial phase. These findings suggest that implementing an enrichment program can enhance the welfare of aged non-human primates and can be considered a valuable tool in the rehabilitation of non-human primates previously housed in laboratories. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=113 SRC="FIGDIR/small/719840v1_ufig1.gif" ALT="Figure 1"> View larger version (50K): org.highwire.dtl.DTLVardef@152a3a1org.highwire.dtl.DTLVardef@74b53forg.highwire.dtl.DTLVardef@275b21org.highwire.dtl.DTLVardef@1d004d8_HPS_FORMAT_FIGEXP M_FIG C_FIG RESEARCH HIGHLIGHTSO_LIEnvironmental enrichment positively affected activity and stress indicators in aged ex-laboratory rhesus macaques. C_LIO_LILocomotion rates increased while scratching, body-shaking, and cortisol levels decreased. C_LIO_LIEnrichment enhance welfare during rehabilitation, even in older individuals. C_LI