Journal of Neurophysiology

The short-latency stretch reflex (SLR) is the fastest sensorimotor response in human limbs. The spinal SLR is traditionally viewed as automatic and resistant to rapid plasticity, while adaptive feedback is often attributed to transcortical mechanisms underlying the long-latency reflex. Using high-density surface electromyography (64-channel arrays) from the pectoralis major and posterior deltoid during an instructed-delay reaching task, we probed reflex gains with brief perturbations delivered during action preparation. Pre-perturbation muscle activity showed no systematic goal-directed change. After task familiarization and with sufficient preparation time, SLR gains decreased progressively (logarithmically) with experience when the planned movement was expected to stretch the homonymous muscle. This tuning occurred both with and without agonist muscle pre-loading and predicted the observed improvements in reaching performance. Early transcortical responses showed comparable tuning across load conditions. Our study shows that spinal feedback circuits can progressively adapt within a single session to support the performance of goal-directed movements. HighlightsO_LIThe short-latency stretch reflex adapts rapidly with experience in planned reaching C_LIO_LISpinal reflex tuning occurs with and without agonist muscle pre-loading C_LIO_LIReflex tuning evolves logarithmically and predicts reaching performance C_LIO_LIEarly transcortical reflex gains show comparable experience-dependent tuning C_LI

Certain features of everyday motor skills become automatic while others remain controlled. Here we use a novel keyboard typing task to investigate whether motor automaticity depends on the frequency of naturally learned motor sequences. Participants type five-letter strings that vary in their word and bigram (two-letter sequence) frequency in natural language, allowing us to examine the influence of prior exposure without laboratory training. Novel pseudo word strings are tested as well. We find greater sequence frequency in natural language is associated with faster inter-keypress intervals and lower temporal variability within the sequence. In contrast, latencies to initiate a sequence are slower for novel pseudo-word strings but are otherwise insensitive to natural word frequency. We also find individual differences in inter-keypress speed and variability are robust across frequency levels but are unrelated to conventional measures of typing skill. Our method establishes keyboard typing as a scalable, ethologically valid framework for probing features of a naturally acquired human motor skill. This research will help extend laboratory-based studies of motor sequence learning and sets the stage for future investigations of linguo-motor processes. Moreover, our findings demonstrate which features within naturally acquired motor sequences become automatic and that typing proficiency is not determined solely by automaticity.

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.

Postural control depends on the ability to adapt motor responses to changing environmental and physiological conditions. Neuromuscular fatigue (NMF) is known to alter balance and muscle activation patterns, yet its effects on motor adaptation during whole-body postural tasks and on the persistence of learned strategies remain unclear. This study investigated whether localized NMF of the ankle dorsiflexors influences adaptation to a novel postural perturbation task and whether learning under fatigue induces persistent changes during subsequent re-exposure. Twenty-five healthy young adults were assigned to either a fatigue (FAT) or no-fatigue (NoFAT) group and completed two experimental sessions separated by 48-72 h allowing recovery from acute fatigue for fatigued group. Participants adapted to repeated mechanical perturbations while standing upright, while ground reaction forces and electromyographic activity of lower-limb muscles were recorded. NMF did not impair overall adaptation performance, as both groups exhibited similar reductions in performance error across practice. However, participants exposed to fatigue exhibited altered postural recovery dynamics, characterized by a reduced return toward the initial posture following perturbation release. These differences persisted during re-exposure on the subsequent day, despite the absence of acute fatigue. In parallel, NMF modified muscle activation and coactivation patterns involving both fatigued and non-fatigued muscles, several of which were retained during re-exposure. These findings indicate that the central nervous system preserves successful adaptation to postural perturbations under fatigue by reorganizing neuromuscular coordination and stabilization strategies. Learning under fatigue therefore influences not only immediate motor execution, but also shapes the longer-term representation of postural control strategies.

During explicit sequence learning (ESL), micro-offline gains (MOGS) occur during brief rest periods. MOGS are calculated as the difference in keypresses-per-second (KPS) between the first sequence of one trial and the last sequence of the preceding trial. To date, all studies evaluating MOGS have calculated KPS from the motor execution time (MET) that occurs between keypresses, but this approach ignores potential contributions from motor preparation which occur prior to the first keypress. Given that ESL relies on both pre-movement motor planning and subsequent motor execution, we hypothesized that ignoring motor preparation time (MPT) neglects a critical component of skill acquisition, potentially misrepresenting the true magnitude of MOGS. To test this, we calculated MOGS with and without MPT in thirty adults who performed an ESL task. Our results show that including MPT flipped MOGS from positive to negative and significantly increased the positive correlation between early learning and a gold-standard ESL metric: the number of correct sequences performed. Our results suggest that MPT should be incorporated into MOGS calculations and that excluding it overestimates micro-offline learning.

Anticipatory postural adjustments (APAs) scale with velocity of approaching objects, with scaling magnitude depending on whether the moving object is actively foveated and tracked, processed through fixated peripheral vision, or processed through fixated central vision. Aging preferentially degrades the magnocellular pathway underlying peripheral motion processing while sparing the extraretinal signals available during smooth pursuit. We therefore asked whether the effect of aging on velocity-dependent APA scaling differs across these three visual pathways. Eighteen young and eighteen older adults stopped a virtual object approaching at four velocities (15-33 cm/s) under three gaze conditions: active foveation via smooth pursuit, central fixation, and peripheral fixation. We measured peak anticipatory force, rate of force development, and time to contact at force onset. Despite reduced smooth pursuit gain in older adults, velocity-dependent scaling was equivalent between age groups during active foveation, and minimal in both groups during central fixation. Critically, young adults scaled force rate during peripheral fixation nearly as steeply as during active foveation, whereas older adults slope was significantly lower -- a difference not observed during the other gaze conditions. Older adults achieved comparable peak force by initiating responses earlier. These results establish that age-related decline in anticipatory motor control is pathway-specific: aging selectively impairs peripheral motion processing for APAs, while extraretinal mechanisms remain capable of sustaining velocity-dependent scaling. More broadly, peripheral motion processing emerges as a candidate physiological locus of age-related postural vulnerability, raising the question of whether magnocellular-targeted training can restore APA scaling in older adults. Key PointsO_LIYoung and older adults stopped virtual objects under three visual conditions: active foveation via smooth pursuit eye movements, and stationary gaze with the object moving through either central or peripheral vision. C_LIO_LIVelocity-dependent force rate scaling was preserved during active foveation in both age groups, minimal during fixated central vision in both age groups, and selectively impaired in older adults during fixated peripheral vision. C_LIO_LIWe found an age-induced vulnerability in peripheral visual motion processing for anticipatory posture stabilization. C_LI

We investigated the dimensions of output zones affecting specific combinations of forelimb muscles in the precentral "motor" cortex of macaque monkeys. Single-pulse intracortical microstimulation (S-ICMS) was used to evoke subthreshold effects in multiple wrist and finger muscles. Results indicate that each motor cortex site represents a different combination of muscles. The effects evoked from cortical sites separated by several hundred microns invariably involved different profiles of muscle activity. The muscle fields of remote CM cells were rarely identical, while the fields of neighboring CM cells were often similar. Given the number of unrecorded muscles, we conclude that primate motor cortex is a mosaic of output sites representing forelimb muscles in different combinations.

Humans consistently decrease the amount of force they produce during isometric finger pressing in the absence of visual feedback, a phenomenon often called force drift. This decrease in force production has been attributed to limitations in working memory and/or adaptive neural control processes that minimize energy consumption. In this study, we investigated a potential peripheral reason for such force drifts: increases in the coefficient of friction between the fingertip and the surface it contacts due to changes in fingertip contact area as the fingertip hydrates under prolonged pressure. We investigated this possibility by eliciting force drifts from participants performing isometric pressing tasks against smooth glass, which shows the phenomenon of increasing contact area during prolonged contact, and a polymer which does not exhibit this phenomenon. We confirmed that the coefficient of friction only increased on the glass plate, however we did not observe a difference in force drifts between these two surfaces, although we found some evidence that force drift could be associated with coefficient of friction. Our findings suggest that factors other than peripheral changes in coefficient of friction are the primary drivers of force drifts.

The prefrontal cortex (PFC) plays a critical role in maintaining working memory (WM) representations while filtering irrelevant distractors. In macaques, PFC neurons exhibit persistent delay period activity that is robust to distractor interference. The common marmoset has emerged recently as a complementary primate model for investigating the neural basis of cognitive processes including WM, in part because the relatively lissencephalic cortex of this species enables laminar recordings which could provide substantial insight into the microcircuit basis of these functions. It remains unknown however, whether marmoset WM performance is robust to distractors presented during delay periods of WM tasks, and how such distractor filtering may be implemented in PFC circuits. Here, we addressed this gap by conducting wireless recordings of PFC in freely moving marmosets performing a touchscreen-based delayed-match-to-location (DML) task in which a salient visual distractor was presented during the delay period on a subset of trials. Marmosets maintained WM performance on distractor trials, showing a decrease in accuracy of only 5%. Consistent with prior observations in both the macaque and marmoset models, we found that many PFC neurons exhibited activity related to the stimulus sample, during the delay period, and around the time of the behavioural response. In a subset of neurons, we observed distractor-mediated modulations of persistent delay period activity which were associated with a greater incidence of performance errors on the DML task. These findings reveal that marmoset WM is robust to distractor interference, and that the PFC mechanisms instantiating WM and distractor filtering are conserved in this primate species. Taken together, they support the common marmoset as a complementary model for investigating the contribution of PFC circuits to mnemonic and attentional processes.

Descending corticospinal and afferent pathways underlying spinally evoked motor potential both contribute to motor output, yet how their interaction at the spinal and peripheral levels is organized spatially within a muscle remains unclear. This study investigated the spatiotemporal characteristics of descending modulation of spinally evoked motor potentials by combining subthreshold transcranial magnetic stimulation (TMS) with transcutaneous spinal cord stimulation (tSCS). In Experiment 1, spinally evoked motor potentials were recorded from multiple lower-limb muscles at various interstimulus intervals (ISIs) defined relative to central conduction time (CCT). Subthreshold TMS facilitated spinally evoked motor potentials from CCT onward across all recorded muscles, with additional bilateral facilitation observed at longer ISIs. In Experiment 2, high-density surface electromyography (HDsEMG) revealed distinct intramuscular activation patterns in the tibialis anterior. The center of gravity (CoG) of TMS-induced motor evoked potentials was located more proximally than that of spinally evoked motor potentials. Notably, the CoG of facilitation maps was shifted further proximally than that of both single-stimulus responses. These findings suggest that descending and afferent inputs preferentially recruit partially distinct motoneuron pools within the same muscle. The proximal bias of facilitation indicates recruitment of additional motoneurons rather than uniform amplification of existing activity. Together, these results demonstrate that the interaction between descending and afferent inputs is both timing-dependent and spatially non-uniform, providing new insight into sensorimotor integration in the human lower limb.

The human sensorimotor system is remarkably effective at automatically parsing total movement error into its constituent parts, the error component due to a perturbation, or externally-generated error (EGE), versus the error component due to motor noise, or internally-generated error (IGE). Participants robustly, and implicitly, adapt to minuscule (2{degrees}) EGEs in the form of randomized visuomotor rotations while ignoring identically-sized errors caused by IGE. This error parsing, and its associated perceptual processes, directly contrasts previous work showing that humans must observe rotations that are > 1.5x the standard deviations of their motor variability, or [≥] 4{degrees}, before explicitly reporting their presence. While the combined results suggest a dissociation between perception for action--which allows for precise and automatic error parsing--and perception for conscious detection, this must be inferred across studies using different methodologies. Here, we combined a within-subjects study design and computational modeling to shed light on the principles underlying implicit adaptation to a perturbation and explicit perturbation detection. Neuro-typical adults participated in two experiments consisting of pseudo-randomized rotations during reaches to a single target, with one session requiring explicit reports after each reach of whether a perturbation was detected. Participants demonstrated a clear dissociation between implicit responses to a perturbation and explicit detection, with robust adaptation to 1{degrees} EGEs, but an inability to reliably report the presence of an EGE until it reached [~] 4{degrees}. For the adaptation task, a model that assumes the participant compares proprioceptive and visual cues to detect a perturbation and corrects for a proportion of this error best fit the data. For signal-detection, a Bayesian causal-inference model in which sensory cues are optimally integrated with a prior on their cause best fit those data. These results indicate that implicit adaptation is dissociated from explicit perturbation detection and the sensorimotor system applies distinct computational strategies to these behaviors.

The present study investigates how grip force (GF) and load force (LF) dynamics reorganize under varying task constraints, focusing on the fractal and entropic properties of motor output. Twenty healthy adults performed precision grip tasks across five force conditions: two spontaneous conditions (pre, post) without visual feedback and three target-driven conditions (natural, -10%, +10% of natural). The temporal and informational structure of GF and LF signals were quantified using the Hurst exponent (H) and Sample Entropy (SampEn), capturing long-range temporal organization and local irregularity. Constrained conditions reduced GF coefficient of variation but increased both H and SampEn relative to spontaneous pre-trials, while LF indices remained largely unchanged. Thus, grip control under constraint became more temporally persistent and locally irregular, suggesting a more structured temporal organization rather than a simple loss of complexity. Intra-trial analyses further revealed an increase in H from the first to the second half of spontaneous trials, consistent with progressive self-organization in the absence of explicit constraints. Across conditions, H and SampEn were positively correlated for both GF and LF, suggesting that predictability and complexity are not necessarily inversely related in this context. Overall, these findings suggest that the human motor system adapts to force constraints not by suppressing variability, but by reorganizing it across scales, combining temporal persistence with local flexibility. This multidimensional characterization of variability may help refine theoretical models of optimal movement variability and may inform clinical or training approaches aimed at assessing or enhancing neuromotor adaptability.

Strikingly, highly trained athletes engaged in vertiginous activities (e.g., dance and slacklining) and patients with bilateral vestibular loss show a similar pattern of neural plasticity, likely resulting from reduced vestibular sensory processes. However, unlike patients, these athletes show no balance impairments, quite the opposite. This suggests that the attenuation of vestibular processing represents an adaptive recalibration to excessive vestibular stimulation rather than a sign of dysfunction. Concurrently, tactile processing increases as vestibular processing attenuates. Our findings indicate that effective adaptation extends beyond simple tactile compensation: it involves a strengthened tactile-brain pathway. Indeed, following unexpected base-of-support translations, the coupling between plantar shear forces (i.e., a proxy of plantar sole tactile afferents) and cortical responses over the somatosensory areas was markedly enhanced in Athletes. Cross-correlation analysis revealed stronger (r = 0.71) and faster (36 ms) tactile-brain coupling in Athletes (n = 25) compared with age- and gender-matched Controls (n = 18). This enhancement occurred within the first 180 ms following translation, that is, during the critical early phase of skin-surface interaction. Notably, artistic swimmers, who undergo intense vestibular stimulation in a weightless underwater environment without balance equilibrium constraints, also exhibit enhanced tactile-brain coupling. This suggests that strengthening the tactile-brain coupling is not merely a byproduct of balance expertise, but rather a broader adaptive response to sustained vestibular stimulation. Multimodal neurons integrating vestibular and somatosensory inputs, such as those in the somatosensory cortex and thalamus, may increase their responsiveness to foot tactile afferents when vestibular inputs become excessive. In such contexts, the somatosensory system may assume a dominant role in providing gravity-related information for balance control.

Neurons in motor cortex can be engaged not only in motor execution but also during observation of movements performed by other anthropomorphic agents (i.e. humans or monkeys). However, it is unknown how motor cortical neurons respond during observation of the range of assistive or prosthetic devices controlled by people using intracortical brain-computer interfaces (iBCIs). We recorded single-unit activity in the precentral gyrus while iBCI users viewed grasp-like movements performed by a spectrum of virtual effectors that included human, robotic, and hand-like dot stimuli. We found a relationship between neural modulation and effector anthropomorphicity (i.e. human-likeness) that existed on an ensemble-wide and individual neuron level, suggesting that human motor cortex activity incrementally increases in response to the visually observed agents human-likeness. Both solicited and spontaneous feedback from the participant indicated a relationship between neural activity and subjective assessments of anthropomorphicity, revealing a powerful contribution of context on observation-induced activity in motor cortex. The activity of motor cortex remained similar during attempted hand movements while different effectors were being observed, suggesting that intuitive external device control via iBCIs may not be overtly affected by the anthropomorphicity of the effector. SIGNIFICANCE STATEMENTThe tendency for neurons in motor cortex to respond during movement observation has been proposed to underlie cognitive processes from motor learning and language development to empathy and theory of mind. Understanding how the motor cortex is engaged during observation of abstract and anthropomorphic agents informs our understanding of these processes and may guide development of neural prostheses which harness the activity of motor cortical neurons to restore lost neurologic function. Here we provide unique neuron-level evidence that human motor cortex activity is gradually modulated by how human-like an observed agent appears and moves. This finding advances our interpretation of "mirror" activity in the brain and could help guide the design of brain-controlled prostheses used by people with tetraplegia.

Multiple learning signals can shape motor output, including reward and punishment (via value-based reinforcement learning) and sensorimotor error (via motor adaptation). However, it is unclear if action values, learned via reinforcement learning, interact with error-based motor learning. Here, we asked if the learned value of competing motor goals influences how the motor system learns from sensorimotor errors linked to those goals. We designed a paradigm that required participants (N = 85) to learn about the value associated with different movement targets prior to undergoing implicit visuomotor adaptation at or nearby those same targets. We observed two classes of related effects: repulsion and suppression. When adaptation brought the limb toward targets associated with low-value, learning was attenuated compared to when adaptation brought the limb away from targets associated with low-value, indicating a "repelling" effect of low-value actions. Moreover, adaptation was suppressed in all directions when low-value targets were themselves the goal of movement. These findings were asymmetric with respect to valence: we did not observe comparable attraction effects when adaptation brought the limb toward targets associated with high-value, nor did we see overall enhancement of adaptation when targets associated with high-value acted as movement goals. Additional analyses and experiments demonstrated that these effects did not reflect generic biases and were driven by relative rather than absolute goal values. Repulsion and suppression effects were tied to movement directions rather than the targets themselves. These results point to a novel interaction between reinforcement learning and motor memory -- a kind of "instrumental-motor transfer." Significance StatementIn order to ensure our motor behavior yields our desired outcomes, we must not only learn which actions lead to which outcomes (via reinforcement learning) but also how to precisely execute those actions to achieve the desired outcome (by calibrating our actions via implicit motor adaptation. While it is clear that reinforcement learning and motor adaptation operate at different levels of an action selection hierarchy, it is unclear if they interact. Knowing if and how these processes interact is critical for understanding the fundamental algorithms and various neural circuits underlying real-world motor behavior. Here, we show a novel interaction between the two processes where implicit adaptation is suppressed when adaptation would bring the hand toward a previously punished movement goal.

Motor learning is shaped by motivational context: punishment can accelerate initial learning, whereas reward enhances memory retention. Yet it remains unclear whether the dissociable effects of reward and punishment observed in laboratory tasks generalize to complex real-world skills. Here, we tested this idea using a naturalistic motor task--basketball free-throw shooting. Sixty-eight participants trained under four motivational contexts that differed only in how points were awarded for each pair of consecutive shots: control (standard scoring), reward (bonus points for two consecutive successful shots), punishment (penalty for two consecutive missed shots), or mixed (both bonus and penalty). Performance was assessed before training, immediately after, and 1 and 3 days later. Punishment and mixed schedules significantly improved early acquisition, resulting in higher accuracy immediately after training compared to the control and reward conditions. This advantage emerged during the first training block, indicating a rapid motivational influence on performance. In contrast, reward selectively enhanced offline consolidation: three days after training, the reward group showed the largest gains in accuracy, outperforming both the control and punishment groups. The mixed schedule produced quick early gains similar to punishment, but achieved smaller long-term improvements than reward. Consistent with these findings, individual punishment sensitivity was associated with gains in acquisition, while reward sensitivity correlated with offline improvements. Together, these findings demonstrate dissociable effects of motivational valence on the acquisition and consolidation of a complex real-world motor skill. More generally, they position motivational interventions as simple and cost-effective strategies to enhance rehabilitation and sports training.

Despite extensive research on vestibular function in microgravity, particularly during orbital and parabolic flight exposure, several gaps remain regarding the spontaneous behavior of vestibular organs under non-terrestrial gravitoinertial conditions. In particular, semicircular canal function, typically assessed through vestibulo-ocular reflex (VOR) recordings, has yielded inconsistent findings, with reports describing either no effect or reduced performance in microgravity. Moreover, many of these studies are limited by methodological constraints that reduce the interpretability of their conclusions. To clarify these discrepancies, we evaluated horizontal and vertical VOR responses during parabolic flights to assess semicircular canal function under transient weightlessness. Participants were passively rotated at a constant frequency and amplitude during normogravity and microgravity phases, centered along the head vertical or inter-aural axis. Eye movements were recorded binocularly using infrared eye-tracking in darkness to eliminate visual influences, while participants were tightly restrained to minimize proprioceptive variability. Results show a reduction in VOR gain during microgravity in both axes, despite consistent rotational stimulation across gravity conditions. In addition, VOR gain remained reduced after parabolas in the horizontal plane, whereas vertical VOR performance was preserved. These are the first results to demonstrate an immediate alteration of semicircular canal function in weightlessness. Possible sources of the reduction in VOR performance in 0g are discussed. We also propose that the observed post-flight effects reflect a down-weighting of semicircular canal inputs during multisensory integration.

ObjectiveAging is associated with changes in cortical excitability and altered responsiveness to benzodiazepines, but the effects of benzodiazepine challenge on motor cortical paired-pulse physiology in older adults remain incompletely understood. We examined whether intravenous midazolam differentially modulates corticospinal excitability and short-interval paired-pulse transcranial magnetic stimulation (TMS) responses in younger and older adults. MethodsFifteen younger adults (18-35 years) and fifteen older adults (50-69 years) underwent single-pulse and paired-pulse TMS of the left primary motor cortex at baseline and during intravenous midazolam administration. Single-pulse motor evoked potential (MEP) amplitude was used to assess corticospinal excitability. Short-interval paired-pulse responses were quantified as the ratio of conditioned to unconditioned MEP amplitude. ResultsAt baseline, younger adults showed greater corticospinal excitability than older adults, reflected by larger single-pulse MEP amplitudes (adjusted p = 0.04). Younger adults demonstrated paired-pulse inhibition at baseline, reflected by a conditioned/unconditioned MEP ratio below 1.0 (ratio = 0.73; adjusted p < 0.01), whereas older adults did not show inhibition and instead had a mean ratio above 1.0 (ratio = 1.25). Midazolam reduced single-pulse MEP amplitudes in both groups. During midazolam administration, paired-pulse inhibition was no longer observed in younger adults, and older adults continued to show no evidence of inhibition. ConclusionsYounger and older adults differed in baseline corticospinal excitability and in short-interval paired-pulse TMS responses. Intravenous midazolam reduced corticospinal excitability and altered paired-pulse response patterns, eliminating baseline paired-pulse inhibition in younger adults while producing little measurable change in older adults. These findings suggest that aging may modify the net motor cortical response to benzodiazepine challenge. The results should be interpreted in relation to the paired-pulse stimulation parameters used and support further studies using complementary approaches to characterize age-related differences in inhibitory and facilitatory motor cortical circuits.

The glymphatic system mediates brain-wide cerebrospinal fluid (CSF) transport and is highly sensitive to brain state. Experimental studies show that 2-adrenergic agonist- based anesthesia enhances glymphatic CSF influx, whereas isoflurane markedly suppresses it. However, it has been suggested that the reduced tracer influx observed during isoflurane anesthesia may reflect rapid clearance of tracer from the basal cisterns rather than genuine inhibition of glymphatic transport. To address this question, we compared conventional short-duration cisterna magna tracer injections with prolonged low-rate infusion while maintaining identical total tracer dose and anesthesia duration. Across both paradigms, ketamine/dexmedetomidine anesthesia consistently produced substantially greater perivascular CSF influx than isoflurane. In contrast, tracer accumulation in blood and cervical lymph nodes remained largely unchanged between conditions. These findings demonstrate that suppression of glymphatic influx during isoflurane anesthesia is independent of tracer injection duration and support the conclusion that 2-adrenergic agonist-based anesthesia promotes glymphatic transport through mechanisms linked to sleep-like brain states.

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.