Microneurography: how it started and how it works

Slow touch and ultrafast pain fibres: Revisiting peripheral nerve classification

One hundred years ago, Erlanger and Gasser demonstrated that conduction velocity is correlated with the diameter of a peripheral nerve axon. Later, they also demonstrated that the functional role of the axon is related to its diameter: touch is signalled by large-diameter axons, whereas pain and temperature are signalled by small-diameter axons. Certain discoveries in recent decades prompt a modification of this canonical classification. Here, we review the evidence for unmyelinated (C) fibres signalling touch at a slow conduction velocity and likely contributing to affective aspects of tactile information. We also review the evidence for large-diameter Aβ afferents signalling pain at ultrafast conduction velocity and likely contributing to the rapid nociceptive withdrawal reflex. These discoveries imply that conduction velocity is not as clear-cut an indication of the functional role of the axon as previously thought. We finally suggest that a future taxonomy of the peripheral afferent nervous system might be based on the combination of the axońs molecular expression and electrophysiological response properties.

An open computational toolbox to analyze multi- and single-unit sympathetic nerve activity in microneurography

Microelectrode recordings from human peripheral and cranial nerves provide a means to study both afferent and efferent axonal signals at different levels of detail, from multi- to single-unit activity. Their analysis can lead to advancements both in diagnostic and in the understanding of the genesis of neural disorders. However, most of the existing computational toolboxes for the analysis of microneurographic recordings are limited in scope or not open-source. Additionally, conventional burst-based metrics are not suited to analyze pathological conditions and are highly sensitive to distance of the microelectrode tip from the active axons. To address these challenges, we developed an open-source toolbox that offers advanced analysis capabilities for studying neuronal reflexes and physiological responses to peripheral nerve activity. Our toolbox leverages the observation of temporal sequences of action potentials within inherently cyclic signals, introducing innovative methods and indices to enhance analysis accuracy. Importantly, we have designed our computational toolbox to be accessible to novices in biomedical signal processing. This may include researchers and professionals in healthcare domains, such as clinical medicine, life sciences, and related fields. By prioritizing user-friendliness, our software application serves as a valuable resource for the scientific community, allowing to extract advanced metrics of neural activity in short time and evaluate their impact on other physiological variables in a consistent and standardized manner, with the final aim to widen the use of microneurography among researchers and clinicians.

Multiple averaged records to identify Aδ-fibers in sensory nerves

BACKGROUND

Existing methods identify only ≈10 Aδ-fibers in human sensory nerves per recording. This study examines methods to increase the detection of Aδ-fibers.

NEW METHOD

Two to 20 averages of 500 replicate responses to epidermal nerve stimulation are obtained. Pairs of different averages are constructed. Each pair is analyzed with algorithms applied to amplitude and frequency to detect replication of responses to stimulation as "simultaneous similarities in two averages" (SS2AVs) at ≥99.5th percentile of control. In a pair of averages the latencies of amplitude and frequency SS2AVs for the same response to stimulation may differ by ≤0.25 ms. Therefore, Aδ-fibers are identified by the 0.25 ms moving sum of SS2AV latencies of the pairs of averages.

RESULTS

Increasing averages increases pairs of different averages and detection of Aδ-fibers: from 2 to 10 Aδ-fibers with two averages (one pair) to >50 Aδ-fibers with 12-20 averages (66-190 pairs).

COMPARISON WITH EXISTING METHOD(S)

Existing methods identify ≤10 Aδ-fibers in 10 averages/45 pairs with the medians of amplitude and frequency algorithms applied to all 45 pairs. This study identifies Aδ-fibers (i) by applying these algorithms at the 99.5th percentile of control, (ii) to each pair of averages and (iii) by the 0.25 ms sum of algorithm identified events (SS2AVs) in all pairs. These three changes significantly increase the detection of Aδ-fibers, e.g., in 10 averages/45pairs from 10 to 45.

CONCLUSIONS

Three modifications of existing methods can increase the detection of Aδ-fibers to an amount suitable (>50 with ≥12 averages) for statistical comparison of different nerves.

A century of exercise physiology: key concepts in neural control of the circulation

Early in the twentieth century, Walter B. Cannon (1871-1945) introduced his overarching hypothesis of "homeostasis" (Cannon 1932)-the ability to sustain physiological values within a narrow range necessary for life during periods of stress. Physical exercise represents a stress in which motor, respiratory and cardiovascular systems must be integrated across a range of metabolic stress to match oxygen delivery to oxygen need at the cellular level, together with appropriate thermoregulatory control, blood pressure adjustments and energy provision. Of these, blood pressure regulation is a complex but controlled variable, being the function of cardiac output and vascular resistance (or conductance). Key in understanding blood pressure control during exercise is the coordinating role of the autonomic nervous system. A long history outlines the development of these concepts and how they are integrated within the exercise context. This review focuses on the renaissance observations and thinking generated in the first three decades of the twentieth century that opened the doorway to new concepts of inquiry in cardiovascular regulation during exercise. The concepts addressed here include the following: (1) exercise and blood pressure, (2) central command, (3) neurovascular transduction with emphasis on the sympathetic nerve activity and the vascular end organ response, and (4) tonic neurovascular integration.

Human Foot Outperforms the Hand in Mechanical Pain Discrimination

Tactile discrimination has been extensively studied, but mechanical pain discrimination remains poorly characterized. Here, we measured the capacity for mechanical pain discrimination using a two-alternative forced choice paradigm, with force-calibrated indentation stimuli (Semmes-Weinstein monofilaments) applied to the hand and foot dorsa of healthy human volunteers. In order to characterize the relationship between peripheral nociceptor activity and pain perception, we recorded single-unit activity from myelinated (A) and unmyelinated (C) mechanosensitive nociceptors in the skin using microneurography. At the perceptual level, we found that the foot was better at discriminating noxious forces than the hand, which stands in contrast to that for innocuous force discrimination, where the hand performed better than the foot. This observation of superior mechanical pain discrimination on the foot compared to the hand could not be explained by the responsiveness of individual nociceptors. We found no significant difference in the discrimination performance of either the myelinated or unmyelinated class of nociceptors between skin regions. This suggests the possibility that other factors such as skin biophysics, receptor density or central mechanisms may underlie these regional differences.

PIEZO2-dependent rapid pain system in humans and mice

The PIEZO2 ion channel is critical for transducing light touch into neural signals but is not considered necessary for transducing acute pain in humans. Here, we discovered an exception - a form of mechanical pain evoked by hair pulling. Based on observations in a rare group of individuals with PIEZO2 deficiency syndrome, we demonstrated that hair-pull pain is dependent on PIEZO2 transduction. Studies in control participants showed that hair-pull pain triggered a distinct nocifensive response, including a nociceptive reflex. Observations in rare Aβ deafferented individuals and nerve conduction block studies in control participants revealed that hair-pull pain perception is dependent on Aβ input. Single-unit axonal recordings revealed that a class of cooling-responsive myelinated nociceptors in human skin is selectively tuned to painful hair-pull stimuli. Further, we pharmacologically mapped these nociceptors to a specific transcriptomic class. Finally, using functional imaging in mice, we demonstrated that in a homologous nociceptor, Piezo2 is necessary for high-sensitivity, robust activation by hair-pull stimuli. Together, we have demonstrated that hair-pulling evokes a distinct type of pain with conserved behavioral, neural, and molecular features across humans and mice.

Sensory neuron activation from topical treatments modulates the sensorial perception of human skin

Neural signaling of skin sensory perception from topical treatments is often reported in subjective terms such as a sensation of skin "tightness" after using a cleanser or "softness" after applying a moisturizer. However, the mechanism whereby cutaneous mechanoreceptors and corresponding sensory neurons are activated giving rise to these perceptions has not been established. Here, we provide a quantitative approach that couples in vitro biomechanical testing and detailed computational neural stimulation modeling along with a comprehensive in vivo self-assessment survey to demonstrate how cutaneous biomechanical changes in response to treatments are involved in the sensorial perception of the human skin. Strong correlations are identified between reported perception up to 12 hours post treatment and changes in the computed neural stimulation from mechanoreceptors residing deep under the skin surface. The study reveals a quantitative framework for understanding the biomechanical neural activation mechanism and the subjective perception by individuals.

Optimizing examination time and diagnostic performance of the histamine-induced axon-reflex flare response in diabetes

INTRODUCTION/AIMS

The axon-reflex flare response is a reliable method for functional assessment of small fibers in diabetic peripheral neuropathy (DPN), but broad adoption is limited by the time requirement. The aims of this study were to (1) assess diagnostic performance and optimize time required for assessing the histamine-induced flare response and (2) associate with established parameters.

METHODS

A total of 60 participants with type 1 diabetes with (n = 33) or without (n = 27) DPN participated. The participants underwent quantitative sensory testing (QST), corneal confocal microscopy (CCM), and flare intensity and area size assessments by laser-Doppler imaging (FLPI) following an epidermal skin-prick application of histamine. The flare parameters were evaluated each minute for 15 min, and the diagnostic performance compared to QST and CCM were assessed using area under the curve (AUC). Minimum time-requirements until differentiation and to achieve results comparable with a full examination were assessed.

RESULTS

Flare area size had better diagnostic performance compared with CCM (AUC 0.88 vs. 0.77, p < 0.01) and QST (AUC 0.91 vs. 0.81, p = 0.02) than mean flare intensity, and could distinguish people with and without DPN after 4 min compared to after 6 min (both p < 0.01). Flare area size achieved a diagnostic performance comparable to a full examination after 6 and 7 min (CCM and QST respectively, p > 0.05), while mean flare intensity achieved it after 5 and 8 min (CCM and QST respectively, p > 0.05).

DISCUSSION

The flare area size can be evaluated 6-7 min after histamine-application, which increases diagnostic performance compared to mean flare intensity.

Frontiers in diagnostic and therapeutic approaches in diabetic sensorimotor neuropathy (DSPN)

Diabetes sensory polyneuropathy (DSPN) is a significant complication of diabetes affecting up to 50% of patients in their lifetime and approximately 20% of patients suffer from painful diabetes neuropathic pain. DSPN - both painless and painful - leads to considerable morbidity including reduction of quality of life, increased lower limb amputations and is associated with worsening mortality. Significant progress has been made in the understanding of pathogenesis of DSPN and the last decade has seen newer techniques aimed at its earlier diagnosis. The management of painful DSPN remains a challenge despite advances made in the unravelling the pathogenesis of pain and its transmission. This article discusses the heterogenous clinical presentation of DSPN and the need to exclude key differential diagnoses. Furthermore, it reviews in detail the current diagnostic techniques involving both large and small neural fibres, their limitations and advantages and current place in the diagnosis of DSPN. Finally, the management of DSPN including newer pharmacotherapies are also discussed.

Measuring pain and nociception: Through the glasses of a computational scientist. Transdisciplinary overview of methods

In a healthy state, pain plays an important role in natural biofeedback loops and helps to detect and prevent potentially harmful stimuli and situations. However, pain can become chronic and as such a pathological condition, losing its informative and adaptive function. Efficient pain treatment remains a largely unmet clinical need. One promising route to improve the characterization of pain, and with that the potential for more effective pain therapies, is the integration of different data modalities through cutting edge computational methods. Using these methods, multiscale, complex, and network models of pain signaling can be created and utilized for the benefit of patients. Such models require collaborative work of experts from different research domains such as medicine, biology, physiology, psychology as well as mathematics and data science. Efficient work of collaborative teams requires developing of a common language and common level of understanding as a prerequisite. One of ways to meet this need is to provide easy to comprehend overviews of certain topics within the pain research domain. Here, we propose such an overview on the topic of pain assessment in humans for computational researchers. Quantifications related to pain are necessary for building computational models. However, as defined by the International Association of the Study of Pain (IASP), pain is a sensory and emotional experience and thus, it cannot be measured and quantified objectively. This results in a need for clear distinctions between nociception, pain and correlates of pain. Therefore, here we review methods to assess pain as a percept and nociception as a biological basis for this percept in humans, with the goal of creating a roadmap of modelling options.

Effect of Self-Directed Home Therapy Adherence Combined with TheraBracelet on Poststroke Hand Recovery: A Pilot Study

Hand impairment is a common consequence of stroke, resulting in long-term disability and reduced quality of life. Recovery may be augmented through self-directed therapy activities at home, complemented by the use of rehabilitation devices such as peripheral sensory stimulation. The objective of this study was to determine the effect of adherence to self-directed therapy and the use of TheraBracelet (subsensory random-frequency vibratory stimulation) on hand function for stroke survivors. In a double-blind, randomized controlled pilot trial, 12 chronic stroke survivors were assigned to a treatment or control group (n = 6/group). All participants were instructed to perform 200 repetitions of therapeutic hand tasks 5 days/week while wearing a wrist-worn device 8 hours/day for 4 weeks. The treatment group received TheraBracelet vibration from the device, while the control group received no vibration. Home task repetition adherence and device wear logs, as well as hand function assessment (Stroke Impact Scale Hand domain), were obtained weekly. Repetition adherence was comparable between groups but varied among participants. Participants wore the device to a greater extent than adhering to completing repetitions. A linear mixed model analysis showed a significant interaction between repetition and group (p = 0.01), with greater adherence resulting in greater hand function change for the treatment group (r = 0.94; R ² = 0.88), but not for the control group. Secondary analysis revealed that repetition adherence was greater for those with lower motor capacity and greater self-efficacy at baseline. This pilot study suggests that adherence to self-directed therapy at home combined with subsensory stimulation may affect recovery outcomes in stroke survivors. This trial is registered with NCT04026399.

Orexin, Sleep, Sympathetic Neural Activity, and Cardiovascular Function

Inadequate sleep duration and quality are associated with reduced cardiovascular health and increased mortality. Experimental evidence points to the sympathetic nervous system as a key mediator in the observed relationship between poor sleep and cardiovascular dysfunction. However, brain mechanisms underpinning the impaired sympathetic function associated with poor sleep remain unclear. Recent evidence suggests the central orexin system, particularly orexins A and B and their receptors, have a key regulatory role for sleep in animal and human models. While orexin system activity has been observed to significantly impact sympathetic regulation in animals, the extension of these findings to humans has been difficult due to an inability to directly assess orexin system activity in humans. However, direct measures of sympathetic activity in populations with narcolepsy and chronic insomnia, 2 sleep disorders associated with deficient and excessive orexin neural activity, have allowed indirect assessment of the relationships between orexin, sleep, and sympathetic regulation. Further, the recent pharmaceutical development of dual orexin receptor antagonists for use in clinical insomnia populations offers an unprecedented opportunity to examine the mechanistic role of orexin in sleep and cardiovascular health in humans. The current review assesses the role of orexin in both sleep and sympathetic regulation from a translational perspective, spanning animal and human studies. The review concludes with future research directions necessary to fully elucidate the mechanistic role for orexin in sleep and sympathetic regulation in humans.

Inhibition of NaV1.7: the possibility of ideal analgesics

The selective inhibition of Na_V1.7 is a promising strategy for developing novel analgesic agents with fewer adverse effects. Although the potent selective inhibition of Na_V1.7 has been recently achieved, multiple Na_V1.7 inhibitors failed in clinical development. In this review, the relationship between preclinical in vivo efficacy and Na_V1.7 coverage among three types of voltage-gated sodium channel (VGSC) inhibitors, namely conventional VGSC inhibitors, sulphonamides and acyl sulphonamides, is discussed. By demonstrating the PK/PD discrepancy of preclinical studies versus in vivo models and clinical results, the potential reasons behind the disconnect between preclinical results and clinical outcomes are discussed together with strategies for developing ideal analgesic agents.

Neuroplasticity after upper-extremity rehabilitation therapy with sensory stimulation in chronic stroke survivors

This study investigated the effect of using subthreshold vibration as a peripheral sensory stimulation during therapy on cortical activity. Secondary analysis of a pilot triple-blinded randomized controlled trial. Twelve chronic stroke survivors underwent 2-week upper-extremity task-practice therapy. Half received subthreshold vibratory stimulation on their paretic wrist (treatment group) and the other half did not (control). EEG connectivity and event-related de-/resynchronization for the sensorimotor network during hand grip were examined at pre-intervention, post-intervention and follow-up. Statistically significant group by time interactions were observed for both connectivity and event-related spectral perturbation. For the treatment group, connectivity increased at post-intervention and decreased at follow-up. Event-related desynchronization decreased and event-related resynchronization increased at post-intervention, which was maintained at follow-up. The control group had the opposite trend for connectivity and no change in event-related spectral perturbation. The stimulation altered cortical sensorimotor activity. The findings complement the clinical results of the trial in which the treatment group significantly improved gross manual dexterity while the control group did not. Increased connectivity in the treatment group may indicate neuroplasticity for motor learning, while reduced event-related desynchronization and increased event-related resynchronization may indicate lessened effort for grip and improved inhibitory control. EEG may improve understanding of neural processes underlying motor recovery.

Quantification of cardiac and respiratory modulation of axonal activity in the human vagus nerve

We recently documented the first microelectrode recordings from the cervical vagus nerve in awake humans. Here we aimed to quantify cardiac and respiratory modulation of vagal activity to assess the feasibility of targeting axons supplying the heart and airways. Multi-unit activity was recorded from 43 sites in 19 healthy participants in the left (n = 10) and right (n = 9) vagus nerves with ECG, continuous non-invasive blood pressure and respiration. Cross-correlation histograms were constructed between axonal spikes and the R-waves or the peaks of inspiration. The latencies for the peak in cardiac modulation showed a bimodal distribution: while the majority of sites (72%) had peak latencies that preceded the R-wave by up to 550 ms (mean ± SD, -300 ± 178 ms), 12 sites had latencies of up to 250 ms following the R-wave (64 ± 87 ms). Interestingly, the majority of sites with negative latencies (68%) were found in the left nerve whereas most of those with positive latencies (75%) were found in the right. Conversely, on average the peak of respiratory modulation straddled the peak of inspiration. Sites showing respiratory modulation were more prevalent and showed stronger modulation than those with cardiac modulation: calculated for sites with modulation indices ≥15%, the median cardiac and respiratory modulation indices were 23.4% (n = 17) and 44.5% (n = 35), respectively. We conclude that, despite the fact that much of the vagus nerve supplies the gut, cardiac and respiratory modulation of vagal nerve activity can be identified through invasive recordings in awake humans. KEY POINTS: Intraneural recordings from the cervical vagus were obtained in awake humans via tungsten microelectrodes inserted into the nerve through ultrasound guidance. Cross-correlation analysis of multi-unit vagal activity revealed cardiac and respiratory modulation, from which the amplitude and latency of the peaks could be computed. The magnitude of the cardiac modulation (23%) was weaker than that of the respiratory modulation (45%). The latencies for the peak in cardiac modulation showed a bimodal distribution: the majority of sites (72%) had peak latencies that preceded the R-wave, while the remainder had latencies that followed the R-wave. The majority of sites with negative latencies (68%) were found in the left nerve whereas most of those with positive latencies (75%) were found in the right. On average the peak of respiratory modulation coincided with the peak of inspiration.

Slowly-adapting type II afferents contribute to conscious touch sensation in humans: evidence from single unit intraneural microstimulation

NEW & NOTEWORTHY

Slowly-adapting type II mechanoreceptive afferents (SA-II) in glabrous hand skin encode touch force, direction, and velocity, as well as skin stretch/tension. Using single unit intraneural microstimulation, via microneurography in humans, a single mechanoreceptive afferent can be electrically-stimulated, producing a clear percept, yet SA-II stimulation has produced ambiguous results. We show that selective SA-II stimulation produces large pressure sensations, which has implications for their role in perceived touch and generating realistic touch feedback from prosthetics.

KEY POINTS

Slowly adapting type II mechanoreceptors (SA-IIs) are primary sensory neurons in humans that respond to pressure and stretch applied to the skin. To date, no specific conscious correlate of touch has been linked to SA-II activation Using microneurography and intraneural microstimulation to stimulate single sensory neurons in human subjects, we find a specific sensation linked to the activation of single SA-II afferents. This sensation of touch was reported as gentle pressure and subjects could detect this with a high degree of accuracy. Methods of artificial tactile sensory feedback and computational models of touch should include SA-II s as meaningful contributors to the conscious sensation of touch.

ABSTRACT

Slowly-adapting type II (SA-II, Ruffini) mechanoreceptive afferents respond well to pressure and stretch, and are regularly encountered in human microneurography studies. Despite an understanding of SA-II response properties, their role in touch perception remains unclear. Specific roles of different myelinated Aβ mechanoreceptive afferents in tactile perception have been revealed using single unit intraneural microstimulation (INMS), via microneurography, recording from and then electrically stimulating individual afferents. This method directly links single afferent artificial activation to perception, where INMS produces specific 'quantal' touch percepts associated with different mechanoreceptive afferent types. However, SA-II afferent stimulation has been ambiguous, producing inconsistent, vague sensations or no clear percept. We physiologically characterized hundreds of individual Aβ mechanoreceptive afferents in the glabrous hand skin and examined the subsequent percepts evoked by trains of low amplitude INMS current pulses (<10 μA). We present 18 SA-II afferents where INMS resulted in a clear, electrically evoked sensation of large (∼36 mm² ) diffuse pressure, which was projected precisely to their physiologically-defined receptive field in the skin. This sensation was felt as natural, distinctive from other afferents, and showed no indications of multi-afferent stimulation. Stimulus frequency modulated sensation intensity and even brief stimuli (4 pulses, 60 ms) were perceived. These results suggest SA-II afferents contribute to perceived tactile sensations, can signal this rapidly and precisely, and are relevant and important for computational models of touch sensation and artificial prosthetic feedback. Abstract figure legend Using microneurography, recordings were made from single mechanoreceptive afferents in the median nerve of human subjects. After fiber classification, low amplitude (<10 μA) intraneural microstimulation was delivered to evoke sensations of touch. Varied sensations were evoked that could be attributed to selective activation of the recorded afferents. We identify a consistent link between type II slowly adapting mechanoreceptive afferents (SA-IIs) and a specific sensation (light pressure). These sensations matched the afferent properties precisely, indicated sensations were evoked by stimulating single SA-II afferents, and were modified by stimulus train modulations. This article is protected by copyright. All rights reserved.

Concomitant sensory stimulation during therapy to enhance hand functional recovery post stroke

Background

Post-stroke hand impairment is prevalent and persistent even after a full course of rehabilitation. Hand diminishes stroke survivors’ abilities for activities of daily living and independence. One way to improve treatment efficacy is to augment therapy with peripheral sensory stimulation. Recently, a novel sensory stimulation, TheraBracelet, has been developed in which imperceptible vibration is applied during task practice through a wrist-worn device. The objective of this trial is to determine if combining TheraBracelet with hand task practice is superior to hand task practice alone.

Methods

A double-blind randomized controlled trial will be used. Chronic stroke survivors will undergo a standardized hand task practice therapy program (3 days/week for 6 weeks) while wearing a device on the paretic wrist. The device will deliver TheraBracelet vibration for the treatment group and no vibration for the control group. The primary outcome is hand function measured by the Wolf Motor Function Test. Other outcomes include the Box and Block Test, Action Research Arm Test, upper extremity use in daily living, biomechanical measure of the sensorimotor grip control, and EEG-based neural communication.

Discussion

This research will determine clinical utility of TheraBracelet to guide future translation. The TheraBracelet stimulation is delivered via a wrist-worn device, does not interfere with hand motion, and can be easily integrated into clinical practice. Enhancing hand function should substantially increase stroke survivors' independence and quality of life and reduce caregiver burden.

Trial registration

NCT04569123. Registered on September 29, 2020

Sensory Feedback for Upper-Limb Prostheses: Opportunities and Barriers

The addition of sensory feedback to upper-limb prostheses has been shown to improve control, increase embodiment, and reduce phantom limb pain. However, most commercial prostheses do not incorporate sensory feedback due to several factors. This paper focuses on the major challenges of a lack of deep understanding of user needs, the unavailability of tailored, realistic outcome measures and the segregation between research on control and sensory feedback. The use of methods such as the Person-Based Approach and co-creation can improve the design and testing process. Stronger collaboration between researchers can integrate different prostheses research areas to accelerate the translation process.

Small Fiber Neuropathy in Diabetes Polyneuropathy: Is It Time to Change?

Diabetes polyneuropathy is an important complication of diabetes polyneuropathy, and its notable sequelae of foot ulceration, autonomic dysfunction, and neuropathic pain are associated with significant morbidity and mortality. Despite the major impact on quality of life and health economic costs, it remains underdiagnosed until late in its natural history, and there is lack of any intervention that can reverse its clinical progress. Assessment of small fiber neuropathy (SFN) in diabetes offers an opportunity to detect abnormalities at an early stage so that both interventional studies and preventative measures can be enacted to prevent progression to the devastating complications of foot ulceration and cardiac dysautonomic death. Over the last two decades, significant advances have been made in understanding the pathophysiology of diabetes neuropathy and its assessment. In this review, we discuss limitations of the screening methods recommended in current clinical guidelines which are based on large nerve fiber assessments. Thereafter, we discuss in detail the various methods currently available to assess small fiber structure and function and examine their individual strength and limitations. Finally, we discuss the reasons why despite the considerable body of evidence available, legislators and global experts have yet to incorporate the assessment of SFN as routine clinical surveillance in diabetes management. We hope that these insights will stimulate further discussion and be instrumental in the early adoption of these methods so as to reduce the burden of complications arising due to diabetes polyneuropathy.

Henry Head's lifelong studies of cutaneous sensation

In 1900 research on cutaneous sensation was defined by histological techniques defining sensory receptors in skin, leading to undetermined conceptual problems when considered in relation to Brown-Séquard's startling finding that there were two qualitatively different afferent pathways in the spinal cord. Four modalities were considered to function as the determinants of sensory input. In 1903 Rivers and Head carried out the first interventional study of human cutaneous sensation, and analysed the return of sensation following section and immediate suture of the dorsal cutaneous branch of Head's left radial nerve. This resulted in the revolutionary idea summarised in his description of protopathic and epicritic sensory systems in peripheral sensory nerve. Although this concept was at best seen as controversial and even ridiculed by some of his many contemporaneous critics, more recently this concept has proven a fundamentally important stimulus to understanding the physiology of cutaneous sensation. His writings show him to have been capable of deeply instructive thought, based on his clinical experience and his admiration of Hughlings Jackson's teaching concerning the hierarchical organisation of brain function. First and foremost a clinician neuroscientist, his ideas were ahead of their time and not understood.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na+ and Ca2+ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K+ channels. Because they display limited central side effects, peripherally restricted Na+ and Ca2+ channel blockers and K+ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Nav1.3, Nav1.7, Nav1.8, Cav3.2, and HCN2 and activators of Kv7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K+ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na+ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca2+ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing "pain" as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Multisite silicon probes enable simultaneous recording of spontaneous and evoked activity in multiple isolated C-fibres in rat saphenous nerve

BACKGROUND

Recordings of electrical activity in nerves have provided valuable insights into normal function and pathological behaviours of the nervous system. Current high-resolution techniques (e.g. teased fibre recordings) typically utilise electrodes with a single recording site, capturing the activity of a single isolated neuron per recording.

NEW METHOD

We conducted proof-of-principle C-fibre recordings in the saphenous nerve of urethane-anaesthetised adult Wistar rats using 32-channel multisite silicon electrodes. Data was acquired using the OpenEphys recording system and clustered offline with Kilosort 2.5.

RESULTS

In single recordings in 5 rats, 32 units with conduction velocities in the C-fibre range (< 1 m/s) were identified via constant latency responses and classified using activity dependent slowing. In two animals, 6 C-fibres (5 classified as nociceptors) were well isolated after clustering. Their activity could be tracked throughout the recording - including during periods of spontaneous activity. Axonal conduction velocities were calculated from spontaneous activity and/or low frequency electrical stimulation using only the differences in action potential latency as it propagated past multiple probe sites.

COMPARISON WITH EXISTING METHODS

Single electrode approaches have a low data yield and generating group data for specific fibre types is challenging as it requires multiple experimental subjects and recording sessions. This is particularly true when the experimental targets are the small, unmyelinated C-fibres carrying nociceptive information.

CONCLUSIONS

We demonstrate that multisite recordings can greatly increase experimental yields and enhance fibre identification. The approach is of particular utility when coupled with clustering analysis. Multisite probes and analysis approaches constitute a valuable new toolbox for researchers studying the peripheral nervous system.

Cardiovascular responses to low-level transcutaneous vagus nerve stimulation

AIMS

The aim was to determine cardiovascular responses to an arbitrary protocol of transcutaneous low-level vagus nerve electrical stimulation (tVNS).

METHODS

Study was performed in 15 male volunteers, mean age 23 years. Data were collected during two sessions - sham stimulation (no stimulation) and stimulation. Each session included one-hour resting phase followed by 15-min autonomic nervous system testing phase (Valsalva, deep breathing, wet-cold face tests), all in supine position. The right tragus stimulation parameters were: 20 Hz, constant current at sensation threshold, 1 ms rectangular pulse width. The ECG, noninvasive arterial blood pressure and thoracic impedance cardiography measurements were recorded and analyzed continuously with the Task Force® Monitor (CNSystems Medizintechnik GmbH, Graz, Ver. 2.2.10.0). t-Test for paired samples, paired Wilcoxon signed-rank, and one-way ANOVA for repeated measurements were carried out. P < 0.05 was considered significant.

RESULTS

We demonstrated significant reductions of left ventricular contractility and output parameters, a trend for heart rate reduction, and resulting beneficial reduction of left ventricular work load. However, significant increases of blood pressure and total peripheral resistance were recognized, possibly as a reflex response.

CONCLUSION

It seems that our tVNS protocol has a potential for cardiac autonomic modulation. This gives us opportunity to advance our stimulation parameters with participant-specific adjustments. Further studies are however needed to prove the therapeutic potential of such approach in different patient groups.

Nav1.7 target modulation and efficacy can be measured in nonhuman primate assays

Humans with loss-of-function mutations in the Na_v1.7 channel gene (SCN9A) show profound insensitivity to pain, whereas those with gain-of-function mutations can have inherited pain syndromes. Therefore, inhibition of the Na_v1.7 channel with a small molecule has been considered a promising approach for the treatment of various human pain conditions. To date, clinical studies conducted using selective Na_v1.7 inhibitors have not provided analgesic efficacy sufficient to warrant further investment. Clinical studies to date used multiples of in vitro IC₅₀ values derived from electrophysiological studies to calculate anticipated human doses. To increase the chance of clinical success, we developed rhesus macaque models of action potential propagation, nociception, and olfaction, to measure Na_v1.7 target modulation in vivo. The potent and selective Na_v1.7 inhibitors SSCI-1 and SSCI-2 dose-dependently blocked C-fiber nociceptor conduction in microneurography studies and inhibited withdrawal responses to noxious heat in rhesus monkeys. Pharmacological Na_v1.7 inhibition also reduced odor-induced activation of the olfactory bulb (OB), measured by functional magnetic resonance imaging (fMRI) studies consistent with the anosmia reported in Na_v1.7 loss-of-function patients. These data demonstrate that it is possible to measure Na_v1.7 target modulation in rhesus macaques and determine the plasma concentration required to produce a predetermined level of inhibition. The calculated plasma concentration for preclinical efficacy could be used to guide human efficacious exposure estimates. Given the translatable nature of the assays used, it is anticipated that they can be also used in phase 1 clinical studies to measure target modulation and aid in the interpretation of phase 1 clinical data.

Absence of reliable physiological signature of illusory body ownership revealed by fine-grained autonomic measurement during the rubber hand illusion

The neural representation of a 'biological self' is linked theoretically to the control of bodily physiology. In an influential model, selfhood relates to internal agency and higher-order interoceptive representation, inferred from the predicted impact of efferent autonomic nervous activity on afferent viscerosensory feedback. Here we tested if an altered representation of physical self (illusory embodiment of an artificial hand) is accompanied by sustained shifts in autonomic activity. Participants (N = 37) underwent procedures for induction of the rubber hand illusion (synchronous stroking of own unseen hand and observed stroking of artificial hand) and a control condition (asychronous stroking). We recorded electrocardiography, electrodermal activity, and a non-invasive measure of multiunit skin sympathetic nerve activity (SKNA) from the chest. We compared these autonomic indices between task conditions, and between individuals who did and did not experience the illusion. Bayes factors quantified the strength of evidence for and against null hypotheses. Observed proprioceptive drift and subjective reports confirmed the efficacy of the synchronous (vs asynchronous) condition in inducing illusory hand ownership. Stringent discriminant analysis classified 24/37 individuals as experiencing the rubber hand illusion. Surprisingly, heart rate, heart rate variability, electrodermal activity, and SKNA measures revealed no autonomic differences between synchronous vs asynchronous conditions, nor between individuals who did or did not experience the rubber hand illusion. Bayes factors indicated substantial evidence for no physiological differences. In contrast to earlier reports, our autonomic data show the absence of a reliable change in physiological state during the rubber hand illusion. More encompassing perturbations of self-experience, for example in full body illusions, may nevertheless be coupled to, or facilitated by, changes in efferent autonomic activity and afferent viscerosensory feedback. Our findings suggest that such changes in bodily physiology are not sustained as an obligatory component of the rubber hand illusion.

From Brain to Blood Vessel: Insights From Muscle Sympathetic Nerve Recordings: Arthur C. Corcoran Memorial Lecture 2020

Multiunit recordings of postganglionic sympathetic outflow to muscle yield otherwise imperceptible insights into sympathetic neural modulation of human vascular resistance and blood pressure. This Corcoran Lecture will illustrate the utility of microneurography to investigate neurogenic cardiovascular regulation; review data concerning muscle sympathetic nerve activity of women and men with normal and high blood pressure; explore 2 concepts, central upregulation of muscle sympathetic outflow and cortical autonomic neuroplasticity; present sleep apnea as an imperfect model of neurogenic hypertension; and expose the paradox of sympathetic excitation without hypertension. In awake healthy normotensive individuals, resting muscle sympathetic nerve activity increases with age, sleep fragmentation, and obstructive apnea. Its magnitude is not signaled by heart rate. Age-related changes are nonlinear and differ by sex. In men, sympathetic nerve activity increases with age but without relation to their blood pressure, whereas in women, both rise concordantly after age 40. Mean values for muscle sympathetic nerve activity burst incidence are consistently higher in cohorts with hypertension than in matched normotensives, yet women's sympathetic nerve traffic can increase 3-fold between ages 30 and 70 without causing hypertension. Thus, increased sympathetic nerve activity may be necessary but is insufficient for primary hypertension. Moreover, its inhibition does not consistently decrease blood pressure. Despite a half-century of microneurographic research, large gaps remain in our understanding of the content of the sympathetic broadcast from brain to blood vessel and its specific individual consequences for circulatory regulation and cardiovascular, renal, and metabolic risk.

Cardiovascular autonomic nervous system dysfunction in chronic kidney disease and end-stage kidney disease: disruption of the complementary forces

PURPOSE OF REVIEW

Several nontraditional risk factors have been the focus of research in an attempt to understand the disproportionately high cardiovascular morbidity and mortality in chronic kidney disease (CKD) and end-stage kidney disease (ESKD) populations. One such category of risk factors is cardiovascular autonomic dysfunction. Its true prevalence in the CKD/ESKD population is unknown but existing evidence suggests it is common. Due to lack of standardized diagnostic and treatment options, this condition remains undiagnosed and untreated in many patients. In this review, we discuss current evidence pointing toward the role of autonomic nervous system (ANS) dysfunction in CKD, building off of crucial historical evidence and thereby highlighting the areas in need for future research interest.

RECENT FINDINGS

There are several key mediators and pathways leading to cardiovascular autonomic dysfunction in CKD and ESKD. We review studies exploring the mechanisms involved and discuss the current measurement tools and indices to evaluate the ANS and their pitfalls. There is a strong line of evidence establishing the temporal sequence of worsening autonomic function and kidney function and vice versa. Evidence linking ANS dysfunction and arrhythmia, sudden cardiac death, intradialytic hypotension, heart failure and hypertension are discussed.

SUMMARY

There is a need for early recognition and referral of CKD and ESKD patients suspected of cardiovascular ANS dysfunction to prevent the downstream effects described in this review.There are many unknowns in this area and a clear need for further research.

Recording and quantifying sympathetic outflow to muscle and skin in humans: methods, caveats and challenges

The development of microneurography, in which the electrical activity of axons can be recorded via an intrafascicular microelectrode inserted through the skin into a peripheral nerve in awake human participants, has contributed a great deal to our understanding of sensorimotor control and the control of sympathetic outflow to muscle and skin. This review summarises the different approaches to recording muscle sympathetic nerve activity (MSNA) and skin sympathetic nerve activity (SSNA), together with discussion on the issues that determine the quality of a recording. Various analytical approaches are also described, with a primary emphasis on those developed by the author, aimed at maximizing the information content from recordings of postganglionic sympathetic nerve activity in awake humans.

Slowly conducting potentials in human sensory nerves

BACKGROUND

In clinical practice, small myelinated sensory fibers, Aδ-fibers, conveying mainly pain and temperature sensations, cannot be examined with available nerve conduction study techniques. Currently, these fibers can only be examined with experimental or very specialized and not commonly available nerve conduction techniques, or only indirectly with cerebral evoked potentials.

NEW METHOD

This study uses equipment and methods available in clinical neurophysiology laboratories to record from human sensory nerves ≥1000 averaged responses to focal, non-painful stimuli applied by a special electrode to epidermal nerves. The averaged responses to odd numbered stimuli are compared to the averaged responses to even numbered stimuli. An algorithm identifies potentials common in both averages. The 99^th and 99.9^th percentiles for this algorithm are obtained from control records without stimulation and applied to records with stimulation to identify potentials resulting from stimulation of intraepidermal nerves.

RESULTS

The algorithm identifies numerous negative and positive potentials as being different from controls at the 99th and 99.9^th percentile levels. The conduction velocities of the potentials range from of 1.3-29.9 m/s and are compatible with conduction velocities of Aδ-fibers.

COMPARISON WITH EXISTING METHOD(S)

No existing methods.

CONCLUSIONS

The stimulation, recording and data analysis methods used in this study can be applied in the clinical EMG laboratory to identify Aδ-fibers in human sensory nerves.

Sympathetic Effect of Auricular Transcutaneous Vagus Nerve Stimulation on Healthy Subjects: A Crossover Controlled Clinical Trial Comparing Vagally Mediated and Active Control Stimulation Using Microneurography

Introduction: Auricular low-level transcutaneous vagus nerve stimulation (aLL-tVNS) has emerged as a promising technology for cardiac arrhythmia management but is still experimental. In this physiological study, we hypothesized that aLL-tVNS modulated the autonomic nervous balance through a reduction of sympathetic tone and an increase in heart rate variability (HRV). We investigated the muscle sympathetic nerve activity (MSNA) recorded by microneurography during vagally mediated aLL-tVNS and active control on healthy volunteers. Methods: In this crossover, double-blind controlled study, healthy men (N = 28; 27 ± 4 years old) were assigned to aLL-tVNS applied to cymba and lobe (active control) of the right ear. Each participant was randomly allocated to the three sequences (5 Hz, 20 Hz, and active control-5 Hz) during one session. MSNA signal was recorded at rest, during voluntarily apnea and aLL-tVNS. Sympathetic activity was expressed as: 1) number of bursts per minute (burst frequency, BF) and 2) MSNA activity calculated as BF x mean burst amplitude and expressed as changes from baseline (%). RR intervals, HRV parameters and sympathetic activity were analyzed during 5 min-baseline, 10 min-stimulation, and 10 min-recovery periods. Mixed regression models were performed to evaluate cymba-(5—20 Hz) effects on the parameters with stimulation. Results: During apnea and compared to baseline, BF and MSNA activity increased (p = 0.002, p = 0.001, respectively). No stimulation effect on RR intervals and HRV parameters were showed excepted a slightly increase of the LF/HF ratio with stimulation in the cymba-5Hz sequence (coef. ± SE: 0.76 ± 0.32%; p = 0.02). During stimulation, reductions from baseline in BF (Coef. ± SE: −4.8 ± 1.1, p < 0.001) was observed but was not statistically different from that one in the active control. Reduction of MSNA activity was not significantly different between sequences. Conclusion: Acute right cymba aLL-tVNS did not induce any overall effects neither on heart rate, HRV nor MSNA variables on healthy subjects when compared to active control. Interestingly, these findings questioned the role of active controls in medical device clinical trials that implied subjective endpoints.

Influence of sympathetic activation on myocardial contractility measured with ballistocardiography and seismocardiography during sustained end-expiratory apnea

Ballistocardiography (BCG) and seismocardiography (SCG) assess vibrations produced by cardiac contraction and blood flow, respectively, through micro-accelerometers and micro-gyroscopes. BCG and SCG kinetic energies (KE) and their temporal integrals (iK) during a single heartbeat are computed in linear and rotational dimensions. Our aim was to test the hypothesis that iK from BCG and SCG are related to sympathetic activation during maximal voluntary end-expiratory apnea. Multiunit muscle sympathetic nerve traffic [burst frequency (BF), total muscular sympathetic nerve activity (tMSNA)] was measured by microneurography during normal breathing and apnea (n = 28, healthy men). iK of BCG and SCG were simultaneously recorded in the linear and rotational dimension, along with oxygen saturation ([Formula: see text]) and systolic blood pressure (SBP). The mean duration of apneas was 25.4 ± 9.4 s. SBP, BF, and tMSNA increased during the apnea compared with baseline (P = 0.01, P = 0.002,and P = 0.001, respectively), whereas [Formula: see text] decreased (P = 0.02). At the end of the apnea compared with normal breathing, changes in iK computed from BCG were related to changes of tMSNA and BF only in the linear dimension (r = 0.85, P < 0.0001; and r = 0.72, P = 0.002, respectively), whereas changes in linear iK of SCG were related only to changes of tMSNA (r = 0.62, P = 0.01). We conclude that maximal end expiratory apnea increases cardiac kinetic energy computed from BCG and SCG, along with sympathetic activity. The novelty of the present investigation is that linear iK of BCG is directly and more strongly related to the rise in sympathetic activity than the SCG, mainly at the end of a sustained apnea, likely because the BCG is more affected by the sympathetic and hemodynamic effects of breathing cessation. BCG and SCG may prove useful to assess sympathetic nerve changes in patients with sleep disturbances.NEW & NOTEWORTHY Ballistocardiography (BCG) and seismocardiography (SCG) assess vibrations produced by cardiac contraction and blood flow, respectively, through micro-accelerometers and micro-gyroscopes. Kinetic energies (KE) and their temporal integrals (iK) during a single heartbeat are computed from the BCG and SCG waveforms in a linear and a rotational dimension. When compared with normal breathing, during an end-expiratory voluntary apnea, iK increased and was positively related to sympathetic nerve traffic rise assessed by microneurography. Further studies are needed to determine whether BCG and SCG can probe sympathetic nerve changes in patients with sleep disturbances.

A Systematic Review of the Diagnostic Methods of Small Fiber Neuropathies in Rehabilitation

This systematic review describes the several methods to diagnose and measure the severity of small fiber neuropathies and aims to guide the physician to define all the diagnostic approaches for adopting the best strategies described in the current literature. The search was conducted in PubMed, EMBASE, Cochrane Library and Web of Science. Two reviewers independently reviewed and came to consensus on which articles met inclusion/exclusion criteria. The authors excluded all the duplicates, animals' studies, and included the English articles in which the diagnostic measures were finalized to assess the effectiveness of rehabilitation and pharmacologic treatment of patients with small fiber neuropathies. The search identified a total of 975 articles with the keywords "small fiber neuropathy" AND "rehabilitation" OR "therapy" OR "treatment". Seventy-eight selected full-text were analyzed by the reviewers. Forty-one publications met the inclusion criteria and were included in the systematic review. Despite the range of diagnostic tools for the assessment of small fiber neuropathy, other robust trials are needed. In addition, always different diagnostic approaches are used, a unique protocol could be important for the clinicians. More research is needed to build evidence for the best diagnostic methodologies and to delineate a definitive diagnostic protocol.

Neural signal recording and processing in somatic neuroprosthetic applications. A review

Neurointerfaces have acquired major relevance as both rehabilitative and therapeutic tools for patients with spinal cord injury, limb amputations and other neural disorders. Bidirectional neural interfaces are a key component for the functional control of neuroprosthetic devices. The two main neuroprosthetic applications of interfaces with the peripheral nervous system (PNS) are: the refined control of artificial prostheses with sensory neural feedback, and functional electrical stimulation (FES) systems attempting to generate motor or visceral responses in paralyzed organs. The results obtained in experimental and clinical studies with both, extraneural and intraneural electrodes are very promising in terms of the achieved functionality for the neural stimulation mode. However, the results of neural recordings with peripheral nerve interfaces are more limited. In this paper we review the different existing approaches for PNS signals recording, denoising, processing and classification, enabling their use for bidirectional interfaces. PNS recordings can provide three types of signals: i) population activity signals recorded by using extraneural electrodes placed on the outer surface of the nerve, which carry information about cumulative nerve activity; ii) spike activity signals recorded with intraneural electrodes placed inside the nerve, which carry information about the electrical activity of a set of individual nerve fibers; and iii) hybrid signals, which contain both spiking and cumulative signals. Finally, we also point out some of the main limitations, which are hampering clinical translation of neural decoding, and indicate possible solutions for improvement.

Phase I Safety Trial: Extended Daily Peripheral Sensory Stimulation Using a Wrist-Worn Vibrator in Stroke Survivors

Peripheral sensory stimulation augments post-stroke upper extremity rehabilitation outcomes. Most sensory stimulations interfere with natural hand tasks and the stimulation duration is limited. We developed TheraBracelet, low-level random-frequency vibration applied via a wristwatch, to enable stimulation during hand tasks and potentially extend stimulation durations. To determine safety of prolonged exposure to TheraBracelet. Single-site double-blind crossover randomized controlled trial. Chronic stroke survivors were instructed to wear a device on the affected wrist for > 8 h/day everyday for 2 months while coming to the laboratory weekly for evaluations, with a 2-week break between each month. The device applied vibration at 60% and 1% of the sensory threshold for the real and sham month, respectively. The order of the real and sham months was randomized/balanced. Adverse events (AEs) were assessed weekly, including worsening of hand sensation, dexterity, grip strength, pain, or spasticity and occurrence of skin irritation or swelling. Device-related AE rates were compared between the real and sham month. Twenty-five participants completed the study. Six participants (24%) experienced mild AEs involving worsened sensory scores that may be related to the intervention with reasonable possibility. Two experienced them in the real stimulation month only, 3 in the sham month only, and 1 in both months. Therefore, less participants experienced device-related AEs in the real than sham month. Daily stimulation using the device for a month is safe for chronic stroke survivors. Future studies examining the efficacy of pairing TheraBracelet with therapy for increasing neurorehabilitation outcomes are a logical next step. Trial registration: NCT03318341.

Chemotherapy acutely impairs neurovascular and hemodynamic responses in women with breast cancer

The purpose of the present study was to test the hypothesis that doxorubicin (DX) and cyclophosphamide (CY) adjuvant chemotherapy (CHT) acutely impairs neurovascular and hemodynamic responses in women with breast cancer. Sixteen women (age: 47.0 ± 2.0 yr; body mass index: 24.2 ± 1.5 kg/m) with stage II-III breast cancer and indication for adjuvant CHT underwent two experimental sessions, saline (SL) and CHT. In the CHT session, DX (60 mg/m2) and CY (600 mg/m2) were administered over 45 min. In the SL session, a matching SL volume was infused in 45 min. Muscle sympathetic nerve activity (MSNA) from peroneal nerve (microneurography), calf blood flow (CBF; plethysmography) and calf vascular conductance (CVC), heart rate (HR; electrocardiography), and beat-to-beat blood pressure (BP; finger plethysmography) were measured at rest before, during, and after each session. Venous blood samples (5 ml) were collected before and after both sessions for assessment of circulating endothelial microparticles (EMPs; flow cytometry), a surrogate marker for endothelial damage. MSNA and BP responses were increased ( P < 0.001), whereas CBF and CVC responses were decreased ( P < 0.001), during and after CHT session when compared with SL session. Interestingly, the vascular alterations were also observed at the molecular level through an increased EMP response to CHT ( P = 0.03, CHT vs. SL session). No difference in HR response was observed ( P > 0.05). Adjuvant CHT with DX and CY in patients treated for breast cancer increases sympathetic nerve activity and circulating EMP levels and, in addition, reduces muscle vascular conductance and elevates systemic BP. These responses may be early signs of CHT-induced cardiovascular alterations and may represent potential targets for preventive interventions.

NEW & NOTEWORTHY It is known that chemotherapy regimens increase the risk of cardiovascular events in patients treated for cancer. Here, we identified that a single cycle of adjuvant chemotherapy with doxorubicin and cyclophosphamide in women treated for breast cancer dramatically increases sympathetic nerve activity and circulating endothelial microparticle levels, reduces the muscle vascular conductance, and elevates systemic blood pressure.

Neuromodulation by combined sensory and motor stimulation in the peripheral nerve: tendon organ afferent activity

OBJECTIVE

Neuromuscular stimulation is a therapeutic approach to treat impairments such as stroke or pain, although the resulting inputs to the nervous system due to the stimulation are not well quantified. Stimulation activates both afferents and efferents, and the resulting neural activity is a mix of the effects of both: the changes in afferent activity due to efferent physiological actions plus the alterations due to afferent stimulation. This study quantitatively describes the resulting Golgi tendon organ Ib afferent activity in response to mixed afferent and efferent stimulation.

APPROACH

Neural and stimulated action potentials interact by means of collision, neural resetting, and refractory block. We simulated the action potential patterns of Ib afferents in the human first dorsal interosseous during constant voluntary, stimulated, and combined contractions, varying both stimulation rate and location, and both with and without simultaneous Ib stimulation.

MAIN RESULTS

Efferent stimulation affects the firing rate versus force relationships of individual tendon organs, but the effects on the population mean are very small. In contrast, afferent stimulation changes the firing rate versus force relationship of individual afferents by increasing the firing rate, decreasing the force resolution, broadening the distribution of action potential rates, and temporally phase locking a portion of the action potentials. Force resolution is retained in the population mean. The effects of afferent stimulation change with the stimulation rate and location, and with receptor firing rates.

SIGNIFICANCE

This is the first quantitative description of the changes in afferent feedback during combined efferent and afferent nerve stimulation. The small effects of efferent stimulation on the population response implies that tendon organs could provide accurate force feedback during stimulated contractions. The effects of afferent stimulation can be altered by choice of stimulus rate and site and are generalizable to other afferents.

Microneurography as a tool to study the function of individual C-fiber afferents in humans: responses from nociceptors, thermoreceptors, and mechanoreceptors

The technique of microneurography-recording neural traffic from nerves in awake humans-has provided us with unrivaled insights into afferent and efferent processes in the peripheral nervous system for over 50 years. We review the use of microneurography to study single C-fiber afferents and provide an overview of the knowledge gained, with views to future investigations. C-fibers have slowly conducting, thin-diameter, unmyelinated axons and make up the majority of the fibers in peripheral nerves (~80%). With the use of microneurography in humans, C-fiber afferents have been differentiated into discrete subclasses that encode specific qualities of stimuli on the skin, and their functional roles have been investigated. Afferent somatosensory information provided by C-fibers underpins various positive and negative affective sensations from the periphery, including mechanical, thermal, and chemical pain (C-nociceptors), temperature (C-thermoreceptors), and positive affective aspects of touch (C-tactile afferents). Insights from microneurographic investigations have revealed the complexity of the C-fiber system, methods for delineating fundamental C-fiber populations in a translational manner, how C-fiber firing can be used to identify nerve deficits in pathological states, and how the responses from C-fibers may be modified to change sensory percepts, including decreasing pain. Understanding these processes may lead to future medical interventions to diagnose and treat C-fiber dysfunction. NEW & NOTEWORTHY The technique of microneurography allows us to directly investigate the functional roles of single C-fiber afferents in awake human beings. Here we outline and discuss the current field of C-fiber research on this heterogeneous population of afferents in healthy subjects, in pathological states, and from a translational perspective. We cover C-fibers encoding touch, temperature, and pain and provide perspectives on the future of C-fiber microneurography investigations in humans.

Ultrasound-guided, open-source microneurography: Approaches to improve recordings from peripheral nerves in man

OBJECTIVE

Microneurography is the only method for recording from single neurons in intact human nerves. It is challenging - requiring technical expertise, investment in specialised equipment and has sparse data yields.

METHODS

We assessed whether ultrasound guidance in combination with an 'open access' amplifier and data capture system (Open-Ephys) would simplify and expand the scope of microneurographic recordings in humans.

RESULTS

In 32 healthy consenting volunteers, ultrasound-guidance improved success rates for obtaining cutaneous C-fibres and reduced "Skin to Nerve" times from 28.5 min to 4.5 min for recordings of the peroneal nerve (P < 0.0001). We illustrate the potential utility of ultrasound-guided microneurography for difficult to access nerves with phrenic nerve recording during a Valsalva manoeuvre. We show that Open Ephys is a viable alternative to commercially available recording systems and offers advantages in terms of cost and software customisability.

CONCLUSIONS

Ultrasound guidance for microneurography with Open Ephys facilitates cutaneous C nociceptor recordings and allows recordings to be made from nerves previously considered inaccessible.

SIGNIFICANCE

We anticipate that the adoption of these techniques will improve microneurography experimental efficiency, adds an important visual learning aid and increases the generalisability of the approach.