Peripheral signaling pathways contributing to non-histaminergic itch in humans

BACKGROUND

Chronic itch (chronic pruritus) is a major therapeutic challenge that remains poorly understood despite the extensive recent analysis of human pruriceptors. It is unclear how the peripheral nervous system differentiates the signaling of non-histaminergic itch and pain.

METHODS

Here we used psychophysical analysis and microneurography (single nerve fiber recordings) in healthy human volunteers to explore the distinct signaling mechanisms of itch, using the pruritogens β-alanine, BAM 8-22 and cowhage extract.

RESULTS

The mode of application (injection or focal application using inactivated cowhage spicules) influenced the itch/pain ratio in sensations induced by BAM 8-22 and cowhage but not β-alanine. We found that sensitizing pre-injections of prostaglandin E2 increased the pain component of BAM 8-22 but not the other pruritogens. A-fibers contributed only to itch induced by β-alanine. TRPV1 and TRPA1 were necessary for itch signaling induced by all three pruritogens. In single-fiber recordings, we found that BAM 8-22 and β-alanine injection activated nearly all CM-fibers (to different extents) but not CMi-fibers, whereas cowhage extract injection activated only 56% of CM-fibers but also 25% of CMi-fibers. A "slow bursting discharge pattern" was evoked in 25% of CM-fibers by β-alanine, in 35% by BAM 8-22, but in only 10% by cowhage extract.

CONCLUSION

Our results indicate that no labeled line exists for these pruritogens in humans. A combination of different mechanisms, specific for each pruritogen, leads to itching sensations rather than pain. Notably, non-receptor-based mechanisms such as spatial contrast or discharge pattern coding seem to be important processes. These findings will facilitate the discovery of therapeutic targets for chronic pruritus, which are unlikely to be treated effectively by single receptor blockade.

Early-onset hereditary isolated non-neurogenic orthostatic hypotension in a Swedish family

PURPOSE

Orthostatic hypotension is a common condition with heterogeneous and, in many cases, unclear underlying pathophysiology. Frequent symptoms are syncope and falls with a strong impact on daily life. A two-generation family with eight individuals segregating early-onset severe orthostatic hypotension with persistent tachycardia in upright position and repeated faints was identified. Our aim was to elucidate the underlying pathophysiology.

METHODS

One severely affected individual underwent thorough investigation with neurophysiological and blood pressure (BP) measurements, including direct recording of baroreflex-governed sympathetic nerve signalling and induction of BP rise with phenylephrine. Family members underwent parts of the examination. Genetic analysis using exome sequencing was performed.

RESULTS

Marked postural hypotension with greatly reduced cardiac preload was observed, but without signs of autonomic nervous system dysfunction: sympathetic nerve signalling was normal, as were catecholamine levels, and phenylephrine stimulation revealed a normal increase in BP. The results of the genetic analysis using exome sequencing comprising all known genes associated with the regulation of BP and catecholamine metabolism were normal.

CONCLUSION

The combined findings suggest an autosomal dominant form of early-onset orthostatic hypotension with variable clinical expression and without any additional autonomic dysfunction. It is possible that further investigation will reveal an as yet undescribed entity of orthostatic hypotension transmitted as an autosomal dominant trait.

MR microneurography of human peripheral fascicles using a clinical 3T MR scanner

BACKGROUND AND PURPOSE

Knowledge of nerve fascicular structures is essential for managing peripheral nerve disorders. This study aimed to investigate the feasibility of z-axis high-resolution magnetic resonance (MR) microneurography (zH-MRMN) in displaying the three-dimensional structures of tibial nerve fascicles in vivo using a 3T MR scanner.

MATERIALS AND METHODS

Twelve volunteers underwent z-axis conventional-resolution MR microneurography (zC-MRMN) and zH-MRMN of tibial nerves. The visibility scores of the nerve fascicles (VSNFs) on axial zC-MRMN images and axial zH-MRMN multiplanar reformation (MPR) images were compared. The nerve fascicle appearances on the longitudinal zH-MRMN MPR images were described.

RESULTS

In the nerve segments whose long axes were perpendicular to the imaging planes of both zC-MRMN and zH-MRMN, the VSNFs were not significantly different between the axial images of the two modalities (P = 0.083). In the nerve segments whose long axes were not perpendicular to the imaging planes of zC-MRMN, the VSNFs on the axial zC-MRMN images were significantly lower than those on the axial zH-MRMN MPR images that were angled perpendicular to the long axis of the tibial nerve (P < 0.001).

CONCLUSIONS

The longitudinal zH-MRMN MPR images clearly displayed the changing features of the intraneural fascicles as well as the gross morphology of the tibial nerves. zH-MRMN can clearly delineate the topography of the tibial nerve fascicles in vivo through use of a 3T MR scanner.

Normal and excessive muscle sympathetic nerve activity in heart failure: implications for future trials of therapeutic autonomic modulation

AIMS

Patients with sympathetic excess are those most likely to benefit from novel interventions targeting the autonomic nervous system. To inform such personalized therapy, we identified determinants of augmented muscle sympathetic nerve activity (MSNA) in heart failure, versus healthy controls.

METHODS AND RESULTS

We compared data acquired in 177 conventionally-treated, stable non-diabetic patients in sinus rhythm, aged 18-79 years (149 males; 28 females; left ventricular ejection fraction [LVEF] 25 ± 11% [mean ± standard deviation]; range 5-60%), and, concurrently, under similar conditions, in 658 healthy, normotensive volunteers (398 males; aged 18-81 years). In heart failure, MSNA ranged between 7 and 90 bursts·min^-1 , proportionate to heart rate (p < 0.0001) and body mass index (BMI) (p = 0.03), but was unrelated to age, blood pressure, or drug therapy. Mean MSNA, adjusted for age, sex, BMI, and heart rate, was greater in heart failure (+14.2 bursts·min^-1 ; 95% confidence interval [CI] 12.1-16.3; p < 0.0001), but lower in women (-5.0 bursts·min^-1 ; 95% CI 3.4-6.6; p < 0.0001). With spline modeling, LVEF accounted for 9.8% of MSNA variance; MSNA related inversely to LVEF below an inflection point of ∼21% (p < 0.006), but not above. Burst incidence was greater in ischaemic than dilated cardiomyopathy (p = 0.01), and patients with sleep apnoea (p = 0.03). Burst frequency correlated inversely with stroke volume (p < 0.001), cardiac output (p < 0.001), and peak oxygen consumption (p = 0.002), and directly with norepinephrine (p < 0.0001) and peripheral resistance (p < 0.001).

CONCLUSION

Burst frequency and incidence exceeded normative values in only ∼53% and ∼33% of patients. Such diversity encourages selective deployment of sympatho-modulatory therapies. Clinical characteristics can highlight individuals who may benefit from future personalized interventions targeting pathological sympathetic activation.

Posture-related changes in sympathetic baroreflex sensitivity during normal pregnancy

Normal pregnancy is associated with vast adjustments in cardiovascular autonomic control. Sympathetic baroreflex sensitivity has been reported to be attenuated during pregnancy in animal models, but most studies in humans are cross-sectional and findings from longitudinal case studies are inconclusive. It remains unclear how sympathetic baroreflex sensitivity is altered longitudinally during pregnancy within an individual in different body postures. Therefore, this study examined the impact of posture on sympathetic baroreflex sensitivity in 24 normal-weight normotensive pregnant women. Spontaneous sympathetic baroreflex sensitivity was assessed during early (6-11 weeks) and late (32-36 weeks) pregnancy and 6-10 weeks postpartum in the supine posture and graded head-up tilt (30° and 60°). In addition, data from the postpartum period were compared with (and no different to) 18 age-matched non-pregnant women to confirm that the postpartum period was reflective of a non-pregnant condition (online supplement). When compared with postpartum (-3.8 ± 0.4 bursts/100 heartbeats/mmHg), supine sympathetic baroreflex sensitivity was augmented during early pregnancy (-5.9 ± 0.4 bursts/100 heartbeats/mmHg, P < 0.001). However, sympathetic baroreflex sensitivity at 30° or 60° head-up tilt was not different between any phase of gestation (P > 0.05). When compared to supine, sympathetic baroreflex sensitivity at 60° head-up tilt was significantly blunted during early (Δ2.0 ± 0.7 bursts/100 heartbeats/mmHg, P = 0.024) and late (Δ1.5 ± 0.6 bursts/100 heartbeats/mmHg, P = 0.049) pregnancy but did not change postpartum (Δ0.4 ± 0.6 bursts/100 heartbeats/mmHg, P = 1.0). These data show that time-course changes in sympathetic baroreflex sensitivity are dependent on the posture it is examined in and provides a foundation of normal blood pressure regulation during pregnancy for future studies in women at risk for adverse pregnancy outcomes.

Upper and lower limb muscle sympathetic responses to contralateral exercise in healthy humans: A pilot study

Muscle sympathetic nerve activity (MSNA) is similar between limbs at rest, although a subset of MSNA bursts do demonstrate limb-specific discharge. Whether limb differences in MSNA synchronicity are present during exercise remains controversial. We concurrently measured MSNA from the radial and fibular nerves at rest and during rhythmic handgrip (RHG), static handgrip (SHG), and post-exercise circulatory occlusion (PECO). MSNA burst frequency and incidence were similar between nerve sites during all conditions. Synchronous bursts resulted in larger increases in sympathetic-blood pressure transduction compared to isolated bursts (∆ + 3.6 ± 2.1 vs. +2.3 ± 2.4 mmHg, P = 0.01). The proportion of bursts firing synchronously between nerves at rest was slightly increased during RHG ([rest vs. exercise; mean ± SD] 45.3 ± 7.1 vs. 61.6 ± 7.2 %) and similar during SHG (56.2 ± 7.2 vs. 54 ± 10.6 %). In contrast, burst firing synchronicity increased during PECO (83.8 ± 12.4 %) alongside larger burst amplitudes. Inter-limb differences in resting MSNA are preserved during handgrip exercise, whereas isolated metaboreflex activation results in greater burst synchronization between limbs.

Comparison of signal-averaging and regression approaches to analyzing sympathetic transduction

PURPOSE

Spontaneous sympathetic transduction reflects the vascular and/or pressor responses to bursts of muscle sympathetic nerve activity (MSNA). Separately, signal-averaging and regression-based approaches have been implemented to quantify resting sympathetic transduction. It is unknown whether the outcomes of these analytical approaches provide (dis)similar information, which is imperative for between-study comparisons and the amalgamation of results for synthesis of multiple studies (i.e., meta-analyses). We explored the diastolic blood pressure (DBP) responses to spontaneous bursts of MSNA between these two methods of analysis.

METHODS

Resting beat-by-beat DBP (via finger photoplethysmography) and common peroneal nerve MSNA (via microneurography) were recorded in 52 healthy, normotensive adults (age 38 ± 20 years; 19 females). For the signal-averaged method, transduction was quantified as the mean peak increase in DBP (ΔDBP) during the 12 cardiac cycles following each MSNA burst. In addition, DBP was regressed to a moving two-cardiac-cycle window of normalized relative burst height (mmHg/relative %) to provide the regression-based transduction outcome.

RESULTS

The signal-averaged (1.2 ± 0.7 mmHg) and regression-based approaches (0.009 ± 0.016 mmHg/%) were unrelated (ρ = 0.03, p = 0.86). Adding to the discrepancy, only the signal-averaging approach demonstrated a lower transduction in middle-aged-older males versus younger males.

CONCLUSIONS

The decision of which method to use when calculating sympathetic transduction influences study outcomes, with the two most common methods of determining transduction being unrelated. There are challenges of making sweeping conclusions across studies if different analysis strategies are implemented. An understanding of when to use each method is needed to adopt a harmonized approach to quantifying sympathetic transduction.

Autoencoded deep features for semi-automatic, weakly supervised physiological signal labelling

BACKGROUND AND AIMS

Machine Learning is transforming data processing in medical research and clinical practice. Missing data labels are a common limitation to training Machine Learning models. To overcome missing labels in a large dataset of microneurography recordings, a novel autoencoder based semi-supervised, iterative group-labelling methodology was developed.

METHODS

Autoencoders were systematically optimised to extract features from a dataset of 478621 signal excerpts from human microneurography recordings. Selected features were clusters with k-means and randomly selected representations of the corresponding original signals labelled as valid or non-valid muscle sympathetic nerve activity (MSNA) bursts in an iterative, purifying procedure by an expert rater. A deep neural network was trained based on the fully labelled dataset.

RESULTS

Three autoencoders, two based on fully connected neural networks and one based on convolutional neural network, were chosen for feature learning. Iterative clustering followed by labelling of complete clusters resulted in all 478621 signal peak excerpts being labelled as valid or non-valid within 13 iterations. Neural networks trained with the labelled dataset achieved, in a cross validation step with a testing dataset not included in training, on average 93.13% accuracy and 91% area under the receiver operating curve (AUC ROC).

DISCUSSION

The described labelling procedure enabled efficient labelling of a large dataset of physiological signal based on expert ratings. The procedure based on autoencoders may be broadly applicable to a wide range of datasets without labels that require expert input and may be utilised for Machine Learning applications if weak-labels were available.

Sympathetic nervous system and hypertension: New evidences

Evidences collected in the past few years have strengthened the concept that the sympathetic nervous system plays a primary role in the development and progression of the hypertensive state, starting from the early stage, and in the hypertension-related cardiovascular diseases. Several pathophysiological mechanisms are involved. Among them the genetic background, the immune system in conjunction with sympathetic activation. The present review will briefly discuss the importance of the above mentioned mechanisms in the development of hypertension. The paper will also examine the sympathetic mechanisms underlying attended vs unattended blood pressure measurements as well as their role in resistant vs pseudo-resistant hypertension. Finally evidence from recent meta-analysis on the relevance of sympathetic nerve traffic activation in the pathogenesis of hypertension will be briefly discussed.

Impaired sympathetic neural recruitment during exercise pressor reflex activation in women with post-traumatic stress disorder

Muscle sympathetic nerve activity (MSNA) increases during isometric exercise via increased firing of low-threshold action potentials (AP), recruitment of larger, higher-threshold APs, and synaptic delay modifications. Recent work found that women with post-traumatic stress disorder (PTSD) demonstrate exaggerated early-onset MSNA responses to exercise; however, it is unclear how PTSD affects AP recruitment patterns during fatiguing exercise. We hypothesized that women with PTSD (n = 11, 43 [11] [SD] years) would exhibit exaggerated sympathetic neural recruitment compared to women without PTSD (controls; n = 13, 40 [8] years). MSNA and AP discharge patterns (via microneurography and a continuous wavelet transform) were measured during 1 min of baseline, isometric handgrip exercise (IHG) to fatigue, 2 min of post-exercise circulatory occlusion (PECO), and 3 min of recovery. Women with PTSD were unable to increase AP content per burst compared to controls throughout IHG and PECO (main effect of group: P = 0.026). Furthermore, relative to controls, women with PTSD recruited fewer AP clusters per burst during the first (controls: ∆1.3 [1.2] vs. PTSD: ∆-0.2 [0.8]; P = 0.016) and second minute (controls: ∆1.2 [1.1] vs. PTSD: ∆-0.1 [0.8]; P = 0.022) of PECO, and fewer subpopulations of larger, previously silent axons during the first (controls: ∆5 [4] vs. PTSD: ∆1 [2]; P = 0.020) and second minute (controls: ∆4 [2] vs. PTSD: ∆1 [2]; P = 0.021) of PECO. Conversely, PTSD did not modify the AP cluster size-latency relationship during baseline, the end of IHG, or PECO (all P = 0.658-0.745). Collectively, these data indicate that women with PTSD demonstrate inherent impairments in the fundamental neural coding patterns elicited by the sympathetic nervous system during IHG and exercise pressor reflex activation.

Autonomic nervous system function in women with anorexia nervosa

PURPOSE

Abnormalities in autonomic function have been observed in people with anorexia nervosa. However, the majority of investigations have utilised heart rate variability as the sole assessment of autonomic activity. The current study utilised a variety of methodologies to assess autonomic nervous system function in women with a current diagnosis of anorexia, a past diagnosis of anorexia who were weight-restored, and healthy controls.

METHODS

The sample included 37 participants: 10 participants with anorexia, 17 weight-restored participants (minimum body mass index > 18.5 for minimum of 12 months) and 10 controls. Assessments of autonomic function included muscle sympathetic nerve activity (MSNA) using microneurography, heart rate variability, baroreflex sensitivity, blood pressure variability, head-up tilt table test, sudomotor function and assessment of plasma catecholamines.

RESULTS

MSNA (bursts/min) was significantly decreased in both anorexia (10.22 ± 6.24) and weight-restored (17.58 ± 1.68) groups, as compared to controls (23.62 ± 1.01, p < 0.001 and p = 0.033, respectively). Participants with anorexia had a significantly lower standard deviation in heart rate, lower blood pressure variability and decreased sudomotor function as compared to controls. Weight-restored participants demonstrated decreased baroreflex sensitivity in response to head-up tilt as compared to controls.

CONCLUSION

Women with a current or previous diagnosis of anorexia have significantly decreased sympathetic activity, which may reflect a physiological response to decreased energy intake. During the state of starvation, women with anorexia also displayed decreased sudomotor function. The consequences of a sustained decrease in MSNA are unknown, and future studies should investigate autonomic function in long-term weight-restored participants to determine whether activity returns to normal.

K-means panning - Developing a new standard in automated MSNA signal recognition with a weakly supervised learning approach

BACKGROUND

Accessibility of labelled datasets is often a key limitation for the application of Machine Learning in clinical research. A novel semi-automated weak-labelling approach based on unsupervised clustering was developed to classify a large dataset of microneurography signals and subsequently used to train a Neural Network to reproduce the labelling process.

METHODS

Clusters of microneurography signals were created with k-means and then labelled in terms of the validity of the signals contained in each cluster. Only purely positive or negative clusters were labelled, whereas clusters with mixed content were passed on to the next iteration of the algorithm to undergo another cycle of unsupervised clustering and labelling of the clusters. After several iterations of this process, only pure labelled clusters remained which were used to train a Deep Neural Network.

RESULTS

Overall, 334,548 individual signal peaks form the integrated data were extracted and more than 99.99% of the data was labelled in six iterations of this novel application of weak labelling with the help of a domain expert. A Deep Neural Network trained based on this dataset achieved consistent accuracies above 95%.

DISCUSSION

Data extraction and the novel iterative approach of labelling unsupervised clusters enabled creation of a large, labelled dataset combining unsupervised learning and expert ratings of signal-peaks on cluster basis in a time effective manner. Further research is needed to validate the methodology and employ it on other types of physiologic data for which it may enable efficient generation of large labelled datasets.

openMNGlab: Data Analysis Framework for Microneurography - A Technical Report

openMNGlab is an open-source software framework for data analysis, tailored for the specific needs of microneurography - a type of electrophysiological technique particularly important for research on peripheral neural fibers coding. Currently, openMNGlab loads data from Spike2 and Dapsys, which are two major data acquisition solutions. By building on top of the Neo software, openMNGlab can be easily extended to handle the most common electrophysiological data formats. Furthermore, it provides methods for data visualization, fiber tracking, and a modular feature database to extract features for data analysis and machine learning.

Sympathetic vasoconstrictor activity before and after left ventricular assist device implantation in patients with end-stage heart failure

AIMS

Sympathetic overactivity, which predicts poor outcome in patients with heart failure, normalizes following cardiac transplantation. We tested the hypothesis that haemodynamic improvement following left ventricular assist device (LVAD) implantation is also associated with reductions in centrally generated sympathetic activity.

METHODS AND RESULTS

In eight patients with heart failure (two women, six men, age 44-66 years), we continuously recorded electrocardiogram, beat-to-beat finger blood pressure, respiration, and muscle sympathetic nerve activity (MSNA) before and after implantation of the continuous-flow LVAD devices HeartWare HVAD (n = 4) and HeartMate II (n = 2), and the non-continuous-flow device HeartMate 3 (n = 2). LVAD implantation increased cardiac output by 1.29 ± 0.88 L/min (P = 0.060) and mean arterial pressure by 16.2 ± 7.9 mmHg (P < 0.001), while reducing pulse pressure by 25.3 ± 9.8 mmHg (P < 0.001). LVAD implantation did not change MSNA burst frequency (-1.3 ± 7.5 bursts/min, P = 0.636), total activity (+0.62 ± 1.83 au, P = 0.369), or normalized activity (+0.63 ± 4.23, P = 0.685). MSNA burst incidence was decreased (-7.8 ± 9.3 bursts/100 heart beats, P = 0.049). However, cardiac ectopy altered MSNA bursting patterns that could be mistaken for sympatholysis.

CONCLUSION

Implantation of current design LVAD does not consistently normalize sympathetic activity in patients with end-stage heart failure despite haemodynamic improvement.

Aerobic fitness is inversely associated with neurohemodynamic transduction and blood pressure variability in older adults

Higher aerobic fitness is independently associated with better cardiovascular health in older adults. The transduction of muscle sympathetic nerve activity (MSNA) into mean arterial pressure (MAP) responses provides important insight regarding beat-by-beat neural circulatory control. Aerobic fitness is negatively associated with peak MAP responses to spontaneous MSNA in young males. Whether this relationship exists in older adults is known. We tested the hypothesis that aerobic fitness was inversely related to sympathetic neurohemodynamic transduction and blood pressure variability (BPV) in older adults. Relative peak oxygen consumption (V̇O₂peak, indirect calorimetry) was assessed in 22 older adults (13 males, 65 ± 5 years, 36.3 ± 11.5 ml/kg/min). Peroneal MSNA (microneurography) and arterial pressure (finger photoplethysmography) were recorded during ≥ 10-min of rest. BPV was assessed using the average real variability index. MAP was tracked for 12 cardiac cycles following heartbeats associated with MSNA bursts (i.e., peak ΔMAP). Peak ΔMAP responses (0.9 ± 0.6 mmHg) were negatively associated (all, P < 0.04) with resting burst frequency (30 ± 11 bursts/min; R = -0.47) and burst incidence (54 ± 22 bursts/100 heartbeats; R = -0.51), but positively associated with BPV (ρ = 0.47). V̇O₂peak was inversely related to the pressor responses to spontaneous bursts (R = -0.47, P = 0.03) and BPV (ρ = -0.54, P = 0.01), positively related to burst incidence (R = 0.42, P = 0.05), but unrelated to MSNA burst frequency (P = 0.20). The V̇O₂peak-BPV relationship remained after controlling for burst frequency, peak ΔMAP, age, and sex. Lower V̇O₂peak was associated with augmented neurohemodynamic transduction and BPV in older adults. These negative hemodynamic outcomes highlight the importance of higher aerobic fitness with ageing for optimal cardiovascular health.

Modeling of Activity-Induced Changes in Signal Propagation Speed of Mechano-Electrically Stimulated Nerve Fiber

One of the important questions in the research on neural coding is how the preceding axonal activity affects the signal propagation speed of the following one. We present an approach to solving this problem by introducing a multi-level spike count for activity quantification and fitting a family of linear regression models to the data. The best-achieved score is R2=0.89 and the comparison of different models indicates the importance of long and very short nerve fiber memory. Further studies are required to understand the complex axonal mechanisms responsible for the discovered phenomena.

Autonomic cardiovascular alterations as therapeutic targets in chronic kidney disease

Purpose

The present paper will review the impact of different therapeutic interventions on the autonomic dysfunction characterizing chronic renal failure.

Methods

We reviewed the results of the studies carried out in the last few years examining the effects of standard pharmacologic treatment, hemodialysis, kidney transplantation, renal nerve ablation and carotid baroreceptor stimulation on parasympathetic and sympathetic control of the cardiovascular system in patients with renal failure.

Results

Drugs acting on the renin–angiotensin system as well as central sympatholytic agents have been documented to improve autonomic cardiovascular control. This has also been shown for hemodialysis, although with more heterogeneous results related to the type of dialytic procedure adopted. Kidney transplantation, in contrast, particularly when performed together with the surgical removal of the native diseased kidneys, has been shown to cause profound sympathoinhibitory effects. Finally, a small amount of promising data are available on the potential favorable autonomic effects (particularly the sympathetic ones) of renal nerve ablation and carotid baroreceptor stimulation in chronic kidney disease.

Conclusions

Further studies are needed to clarify several aspects of the autonomic responses to therapeutic interventions in chronic renal disease. These include (1) the potential to normalize sympathetic activity in uremic patients by the various therapeutic approaches and (2) the definition of the degree of sympathetic deactivation to be achieved during treatment.

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.

The impact of ageing and sex on sympathetic neurocirculatory regulation

The sympathetic nervous system represents a critical mechanism for homoeostatic blood pressure regulation in humans. This review focuses on age-related alterations in neurocirculatory regulation in men and women by highlighting human studies that examined the relationship between muscle sympathetic nerve activity (MSNA) acquired by microneurography and circulatory variables (e.g., blood pressure, vascular resistance). We frame this review with epidemiological evidence highlighting sex-specific patterns in age-related blood pressure increases in developed nations. Indeed, young women exhibit lower blood pressure than men, but women demonstrate larger blood pressure increases with age, such that by about age 60 years, blood pressure is greater in women. Sympathetic neurocirculatory mechanisms contribute to sex differences in blood pressure rises with age. Muscle sympathetic nerve activity increases with age in both sexes, but women demonstrate greater age-related increases. The circulatory adjustments imposed by MSNA - referred to as neurovascular transduction or autonomic (sympathetic) support of blood pressure - differ in men and women. For example, whereas young men demonstrate a positive relationship between resting MSNA and vascular resistance, this relationship is absent in young women due to beta-2 adrenergic vasodilation, which offsets alpha-adrenergic vasoconstriction. However, post-menopausal women demonstrate a positive relationship between MSNA and vascular resistance due to a decline in beta-2 adrenergic vasodilatory mechanisms. Emerging data suggest that greater aerobic fitness appears to modulate neurocirculatory regulation, at least in young, healthy men and women. This review also highlights recent advances in microneurographic recordings of sympathetic action potential discharge, which may nuance our understanding of age-related alterations in sympathetic neurocirculatory regulation in humans.

Action potential subpopulations within human muscle sympathetic nerve activity: Discharge properties and governing mechanisms

Sympathetic emissions directed towards the skeletal muscle circulation - muscle sympathetic nerve activity (MSNA) - represent a key mechanism for maintaining homeostasis and supporting human survival during physiological stress. Pulse-rhythmic bursts formed by the synchronous discharge of differently-sized sympathetic action potentials (APs) represent the primary characteristic of MSNA. Of the APs firing under baseline conditions (reflecting low-threshold c-fibre activity), a range of subpopulations exists, of which three general categories can be discussed based on their peak-to-peak amplitude in the filtered raw neurogram - small, medium, and large. These subpopulations express nonuniform discharge, recruitment, and synchronization patterns. The subpopulation of medium APs fires synchronously in most bursts, while the subpopulations of small and large APs fire less often. However, 30% of total AP discharge occurs asynchronously between sympathetic bursts, a pattern expressed most often by small APs. In response to physiological stress (e.g., baroreflex unloading), the subpopulation of medium APs exhibits the largest increase in firing probability and a subpopulation of previously-silent larger and faster-conducting APs (reflecting high-threshold c-fibre activity) becomes recruited. Heterogeneous discharge, synchronization, and recruitment thresholds among AP subpopulations stem from differential regulation within the sympathetic organization including the arterial baroreflex and paravertebral ganglia. Indeed, the arterial baroreflex strongly regulates medium APs at baseline and enhances its control over this subpopulation during periods of baroreflex unloading. Conversely, small and large APs express weak baroreflex control. Trimethaphan infusion has revealed that ganglionic processes including nicotinic and non-nicotinic mechanisms may contribute to heterogenous firing behaviours among low-threshold AP subpopulations. This review highlights recent work revealing new insight to the discharge properties expressed by, and mechanisms governing, AP subpopulations within human MSNA.

Identification of the human sympathetic connectome involved in blood pressure regulation

We review our recent data obtained on the cortical and subcortical components of the human sympathetic connectome - the network of regions involved in the sympathetic control of blood pressure. Specifically, we functionally identified the human homologue of the rostral ventrolateral medulla (RVLM), the primary premotor sympathetic nucleus in the medulla responsible for generating sympathetic vasoconstrictor drive. By performing functional magnetic resonance imaging (fMRI) of the brain at the same time as recording muscle sympathetic nerve activity (MSNA), via a microlectrode inserted into the common peroneal nerve, we are able to identify areas of the brain involved in the generation of sympathetic outflow to the muscle vascular bed, a major contributor to blood pressure regulation. Together with functional connectivity analysis of areas identified through MSNA-coupled fMRI, we have established key components of the human sympathetic connectome and their roles in the control of blood pressure. Whilst our studies confirm the role of lower brainstem regions such as the NTS, CVLM and RVLM in baroreflex control of MSNA, our findings indicate that the insula - hypothalamus - PAG - RVLM circuitry is tightly coupled to MSNA at rest. This fits with data obtained from experimental animals, but also emphasizes the role of areas above the brainstem in the regulation of blood pressure.

Microneurography as a tool to develop decoding algorithms for peripheral neuro-controlled hand prostheses

BACKGROUND

The usability of dexterous hand prostheses is still hampered by the lack of natural and effective control strategies. A decoding strategy based on the processing of descending efferent neural signals recorded using peripheral neural interfaces could be a solution to such limitation. Unfortunately, this choice is still restrained by the reduced knowledge of the dynamics of human efferent signals recorded from the nerves and associated to hand movements.

FINDINGS

To address this issue, in this work we acquired neural efferent activities from healthy subjects performing hand-related tasks using ultrasound-guided microneurography, a minimally invasive technique, which employs needles, inserted percutaneously, to record from nerve fibers. These signals allowed us to identify neural features correlated with force and velocity of finger movements that were used to decode motor intentions. We developed computational models, which confirmed the potential translatability of these results showing how these neural features hold in absence of feedback and when implantable intrafascicular recording, rather than microneurography, is performed.

CONCLUSIONS

Our results are a proof of principle that microneurography could be used as a useful tool to assist the development of more effective hand prostheses.

MR microneurography and quantitative T2 and DP measurements of the distal tibial nerve in CIDP

OBJECTIVE

In this study we investigated the potential of magnetic resonance (MR) micro-neurography to detect morphological and relaxometric changes in distal tibial nerves in patients affected with chronic inflammatory demyelinating polyneuropathy (CIDP), and their associations with clinical and electrophysiological features.

MATERIALS AND METHODS

10 subjects affected with CIDP and 10 healthy subjects were examined. Multiple MR parameters, including the number of fascicles (N), fascicles diameter (FD), total fascicles area (FA), epineurium area (EA), total nerve area (NA), fascicles to nerve ratio (FNR) and quantitative T2 and proton density (PD) were investigated on high resolution MR images of the distal tibial nerve. Those parameters were correlated with clinical scores, age of onset, disease duration and electrophysiologic data.

RESULTS

Median NA and FA were significantly increased in the CIDP population (median values for NA in cm² in CIDP: 0.185; controls: 0.135; p: 0.028; for FA in CIDP 0.136; controls 0.094; p: 0.021). There was no correlation between the parameters investigated and clinical or electrophysiologic features.

CONCLUSION

MR microneurography can detect increased total nerve and fascicle area in distal tibial nerves in CIDP and may be useful for diagnosing CIDP.

Clinical neurophysiology of pain

Clinical neurophysiologic investigation of pain pathways in humans is based on specific techniques and approaches, since conventional methods of nerve conduction studies and somatosensory evoked potentials do not explore these pathways. The proposed techniques use various types of painful stimuli (thermal, laser, mechanical, or electrical) and various types of assessments (measurement of sensory thresholds, study of nerve fiber excitability, or recording of electromyographic reflexes or cortical potentials). The two main tests used in clinical practice are quantitative sensory testing and pain-related evoked potentials (PREPs). In particular, PREPs offer the possibility of an objective assessment of nociceptive pathways. Three types of PREPs can be distinguished depending on the type of stimulation used to evoke pain: laser-evoked potentials, contact heat evoked potentials, and intraepidermal electrical stimulation evoked potentials (IEEPs). These three techniques investigate both small-diameter peripheral nociceptive afferents (mainly Aδ nerve fibers) and spinothalamic tracts without theoretically being able to differentiate the level of lesion in the case of abnormal results. In routine clinical practice, PREP recording is a reliable method of investigation for objectifying the existence of a peripheral or central lesion or loss of function concerning the nociceptive pathways, but not the existence of pain. Other methods, such as nerve fiber excitability studies using microneurography, more directly reflect the activities of nociceptive axons in response to provoked pain, but without detecting or quantifying the presence of spontaneous pain. These methods are more often used in research or experimental study design. Thus, it should be kept in mind that most of the results of neurophysiologic investigation performed in clinical practice assess small fiber or spinothalamic tract lesions rather than the neuronal mechanisms directly at the origin of pain and they do not provide objective quantification of pain.

Pain relief in a neuropathy patient by lacosamide: Proof of principle of clinical translation from patient-specific iPS cell-derived nociceptors

BACKGROUND

Small fiber neuropathy (SFN) is a severe and disabling chronic pain syndrome with no causal and limited symptomatic treatment options. Mechanistically based individual treatment is not available. We report an in-vitro predicted individualized treatment success in one therapy-refractory Caucasian patient suffering from SFN for over ten years.

METHODS

Intrinsic excitability of human induced pluripotent stem cell (iPSC) derived nociceptors from this patient and respective controls were recorded on multi-electrode (MEA) arrays, in the presence and absence of lacosamide. The patient's pain ratings were assessed by a visual analogue scale (10: worst pain, 0: no pain) and treatment effect was objectified by microneurography recordings of the patient's single nerve C-fibers.

FINDINGS

We identified patient-specific changes in iPSC-derived nociceptor excitability in MEA recordings, which were reverted by the FDA-approved compound lacosamide in vitro. Using this drug for individualized treatment of this patient, the patient's pain ratings decreased from 7.5 to 1.5. Consistent with the pain relief reported by the patient, microneurography recordings of the patient's single nerve fibers mirrored a reduced spontaneous nociceptor (C-fiber) activity in the patient during lacosamide treatment. Microneurography recordings yielded an objective measurement of altered peripheral nociceptor activity following treatment.

INTERPRETATION

Thus, we are here presenting one example of successful patient specific precision medicine using iPSC technology and individualized therapeutic treatment based on patient-derived sensory neurons.

The role of the paravertebral ganglia in human sympathetic neural discharge patterns

KEY POINTS

The mechanisms affecting recruitment patterns of postganglionic sympathetic nerves remain unclear. The divergent and convergent preganglionic innervation patterns of postganglionic neurons and the presence of differently sized postganglionic nerves suggest that the ganglia may participate in modifying the discharge patterns of single sympathetic postganglionic neurons innervating the skeletal muscle circulation. Whether the ganglia affect the ordered behaviour of varying sized postganglionic sympathetic neurons in humans has not been studied. Trimethaphan infusion produced an ordered pattern of action potential (AP) de-recruitment whereby the firing of larger, low probability APs present at baseline was abolished first, followed by progressive decreased probability of smaller APs. Although integrated sympathetic bursts were no longer detected after several minutes of trimethaphan, firing of the smallest APs was detected. These data suggest the ganglia affect the distribution of firing probabilities exhibited by differently sized sympathetic neurons. The ganglia may contribute to sympathetic neural emission patterns involved in homeostatic regulation.

ABSTRACT

Do the ganglia contribute to the ordered behaviour of postganglionic neuronal discharge within the sympathetic nervous system? To further understand the functional organization of the sympathetic nervous system we employed the microneurographic approach to record muscle sympathetic nerve activity (MSNA) and a continuous wavelet transform to study postganglionic action potential (AP) behaviour during nicotinic blockade at the ganglia (trimethaphan camsylate, 1-7 mg min-1 ) in seven females (37 ± 5 years). Trimethaphan elicited a progressive reduction in sympathetic outflow characterized by fewer integrated bursts with decaying amplitude. Underlying trimethaphan-mediated attenuations in integrated MSNA were reductions in AP incidence (186 ± 101 to 29 ± 31 AP (100 beats)-1 ) and AP content per integrated burst (7 ± 2 to 3 ± 1 APs burst-1 ) (both P < 0.01) in the final minute of detectable bursting activity in the trimethaphan condition, compared to baseline. We observed an ordered de-recruitment of larger to smaller AP clusters active at baseline (14 ± 3 to 8 ± 2 active AP clusters, P < 0.01). Following cessation of integrated bursts in the trimethaphan condition, the smallest 6 ± 2 sympathetic AP clusters persisted to fire in an asynchronous pattern (49 ± 41 AP (100 beats)-1 ) in all participants. Valsalva's manoeuvre did not increase the incidence of these persistent APs (60 ± 42 AP (100 beats)-1 , P = 0.52), or recruit any larger APs in six of seven participants (6 ± 1 total AP clusters, P = 0.30). These data suggest that the ganglia participate in the ordered recruitment of differently sized postganglionic sympathetic nerves.

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.

Effects of acupuncture at the ST-36 point on muscle sympathetic nerve activity and blood pressure in normal adults

The aim of the present study was to determine the effects of acupuncture on post-ganglionic muscle sympathetic nerve activity (MSNA) in humans. MSNA was measured in 8 healthy adult males by microneurography evaluation of the left peroneal nerve. Blood pressure (BP) and heart rate (HR) were simultaneously recorded. MSNA was evaluated as the burst rate, with total MSNA, BP and HR normalized to their respective baseline values. After 10min of rest in the supine position, acupuncture was applied to the right ST-36 point in the tibialis anterior muscle for 15min, with recovery then monitored over a 20-min period. While the burst rate and total MSNA remained constant throughout the study, there was a significant decrease in BP during the real but not sham acupuncture procedure (p<0.05). HR did not significantly change throughout the study. The results rule out the role of MSNA in the BP fall during acupuncture at the ST-36 point, and suggest possible involvement of other factors in the fall of BP.

An intra-neural microstimulation system for ultra-high field magnetic resonance imaging and magnetoencephalography

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We propose an intra-neural microstimulation system for 7 T fMRI and MEG.

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This custom-built system removes issues with existing equipment.

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It provides efficient work-flow and improved participant comfort and safety.

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Stimulating single mechanoreceptors evokes activity in 7 T fMRI and MEG.

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Responses to unitary stimulation are shown for the first time in MEG.

Background

Intra-neural microstimulation (INMS) is a technique that allows the precise delivery of low-current electrical pulses into human peripheral nerves. Single unit INMS can be used to stimulate individual afferent nerve fibres during microneurography. Combining this with neuroimaging allows the unique monitoring of central nervous system activation in response to unitary, controlled tactile input, with functional magnetic resonance imaging (fMRI) providing exquisite spatial localisation of brain activity and magnetoencephalography (MEG) high temporal resolution.

New method

INMS systems suitable for use within electrophysiology laboratories have been available for many years. We describe an INMS system specifically designed to provide compatibility with both ultra-high field (7 T) fMRI and MEG. Numerous technical and safety issues are addressed. The system is fully analogue, allowing for arbitrary frequency and amplitude INMS stimulation.

Results

Unitary recordings obtained within both the MRI and MEG screened-room environments are comparable with those obtained in ‘clean’ electrophysiology recording environments. Single unit INMS (current <7 μA, 200 μs pulses) of individual mechanoreceptive afferents produces appropriate and robust responses during fMRI and MEG.

Comparison with existing method(s)

This custom-built MRI- and MEG-compatible stimulator overcomes issues with existing INMS approaches; it allows well-controlled switching between recording and stimulus mode, prevents electrical shocks because of long cable lengths, permits unlimited patterns of stimulation, and provides a system with improved work-flow and participant comfort.

Conclusions

We demonstrate that the requirements for an INMS-integrated system, which can be used with both fMRI and MEG imaging systems, have been fully met.

Non-invasive approach for the assessment of sympathetic baroreflex function: A feasibility study

BACKGROUND

Evaluation of sympathetic baroreflex (sBR) function in humans requires intra-neural recording of muscle sympathetic nerve activity (MSNA) by microneurography.

AIMS

We proposed noninvasive approach for the evaluation of sBR function by applying the threshold-analysis (traditionally, based on MSNA) to systemic vascular resistance (SVR) measurement by photoplethysmography.

METHODS & RESULTS

In nine healthy subjects (5M; age: 25±5y), the threshold-analysis was calculated twice: using MSNA and SVR. Both methods yield comparable results in men (T₅₀(burst-vs.-svr): CV=8.8%, r>0.9; Slope_(burst-svr): CV=30.1%; r>0.9), but not in women.

CONCLUSIONS

SVR-based threshold-analysis is feasible in healthy young subjects and provides a promising alternative to the traditional MSNA-based approach.

Simultaneous noninvasive recording of skin sympathetic nerve activity and electrocardiogram

BACKGROUND

Sympathetic nerve activity is important to cardiac arrhythmogenesis.

OBJECTIVE

The purpose of this study was to develop a method for simultaneous noninvasive recording of skin sympathetic nerve activity (SKNA) and electrocardiogram (ECG) using conventional ECG electrodes. This method (neuECG) can be used to adequately estimate sympathetic tone.

METHODS

We recorded neuECG signals from the skin of 56 human subjects. The signals were low-pass filtered to show the ECG and high-pass filtered to show nerve activity. Protocol 1 included 12 healthy volunteers who underwent cold water pressor test and Valsalva maneuver. Protocol 2 included 19 inpatients with epilepsy but without known heart diseases monitored for 24 hours. Protocol 3 included 22 patients admitted with electrical storm and monitored for 39.0 ± 28.2 hours. Protocol 4 included 3 patients who underwent bilateral stellate ganglion blockade with lidocaine injection.

RESULTS

In patients without heart diseases, spontaneous nerve discharges were frequently observed at baseline and were associated with heart rate acceleration. SKNA recorded from chest leads (V₁-V₆) during cold water pressor test and Valsalva maneuver (protocol 1) was invariably higher than during baseline and recovery periods (P < .001). In protocol 2, the average SKNA correlated with heart rate acceleration (r = 0.73 ± 0.14, P < .05) and shortening of QT interval (P < .001). Among 146 spontaneous ventricular tachycardia episodes recorded in 9 patients of protocol 3, 106 episodes (73%) were preceded by SKNA within 30 seconds of onset. Protocol 4 showed that bilateral stellate ganglia blockade by lidocaine inhibited SKNA.

CONCLUSION

SKNA is detectable using conventional ECG electrodes in humans and may be useful in estimating sympathetic tone.

Skin biopsy and microneurography disclose selective noradrenergic dysfunction due to dopamine-β-hydroxylase deficiency

Skin biopsy and microneurography are autonomic tests directly evaluating adrenergic and cholinergic sympathetic fibers to identify selective deficiency of a specific peripheral sympathetic subdivision. We describe a patient with tomacular neuropathy due to a deletion of the PMP22 gene who complained of chronic orthostatic hypotension due to a dopamine-β-hydroxylase deficiency confirmed by genetic analysis demonstrating two novel mutations in the DβH gene. To further characterize autonomic dysfunctions the proband underwent skin biopsy and microneurography. These tests disclosed a selective peripheral adrenergic dysfunction demonstrating the possibility to ascertain DβH deficiency. In conclusion, skin biopsy and microneurography may help to increase the diagnosis of this peculiar disorder particularly when routine autonomic nervous system tests show uncertain results.

Automated detection of latency tracks in microneurography recordings using track correlation

BACKGROUND

The marking technique in microneurography uses stimulus-induced changes in neural conduction velocity to characterize human C-fibers. Changes in conduction velocity are manifested as variations in the temporal latency between periodic electrical stimuli and the resulting APs. When successive recorded sweeps are displayed vertically in a "waterfall" format, APs correlated with the stimulus form visible vertical tracks. Automated detection of these latency tracks is made difficult by sometimes poor signal-to-noise ratio in recordings, spontaneous neural firings uncorrelated with the stimuli, and multi-unit recordings with crossing or closely parallel tracks.

NEW METHOD

We developed an automated track-detection technique based on a local linearization of the latency tracks of stimulus-correlated APs. This technique enhances latency tracks, eliminates transient noise spikes and spontaneous neural activity not correlated with the stimulus, and automatically detects latency tracks across successive sweeps in a recording.

RESULTS

We evaluated our method on microneurography recordings showing varying signal quality, spontaneous firing, and multiple tracks that run closely parallel and cross. The method showed excellent detection of latency tracks in all of our recordings.

COMPARISON WITH EXISTING METHOD(S)

We compare our method to the commonly used track detection method of Hammarberg as implemented in the Drever program.

CONCLUSIONS

Our method is a robust means of automatically detecting latency tracks in typical microneurography recordings.

Sympathetic regulation during thermal stress in human aging and disease

Humans control their core temperature within a narrow range via precise adjustments of the autonomic nervous system. In response to changing core and/or skin temperature, several critical thermoregulatory reflex effector responses are initiated and include shivering, sweating, and changes in cutaneous blood flow. Cutaneous vasomotor adjustments, mediated by modulations in sympathetic nerve activity (SNA), aid in the maintenance of thermal homeostasis during cold and heat stress since (1) they serve as the first line of defense of body temperature and are initiated before other thermoregulatory effectors, and (2) they are on the efferent arm of non-thermoregulatory reflex systems, aiding in the maintenance of blood pressure and organ perfusion. This review article highlights the sympathetic responses of humans to thermal stress, with a specific focus on primary aging as well as impairments that occur in both heart disease and type 2 diabetes mellitus. Age- and pathology-related changes in efferent muscle and skin SNA during cold and heat stress, measured directly in humans using microneurography, are discussed.

Methods and considerations for the analysis and standardization of assessing muscle sympathetic nerve activity in humans

The technique of microneurography and the assessment of muscle sympathetic nerve activity (MSNA) are used in laboratories throughout the world. The variables used to describe MSNA, and the criteria by which these variables are quantified from the integrated neurogram, vary among studies and laboratories and, therefore, can become confusing to those starting to learn the technique. Therefore, the purpose of this educational review is to discuss guidelines and standards for the assessment of sympathetic nervous activity through the collection and analysis of MSNA. This review will reiterate common practices in the collection of MSNA, but will also introduce considerations for the evaluation and physiological inference using MSNA.

Kinesthetic illusions attenuate experimental muscle pain, as do muscle and cutaneous stimulation

In the present study, muscle pain was induced experimentally in healthy subjects by administrating hypertonic saline injections into the tibialis anterior (TA) muscle. We first aimed at comparing the analgesic effects of mechanical vibration applied to either cutaneous or muscle receptors of the TA or to both types simultaneously. Secondly, pain alleviation was compared in subjects in whom muscle tendon vibration evoked kinesthetic illusions of the ankle joint. Muscle tendon vibration, which primarily activated muscle receptors, reduced pain intensity by 30% (p<0.01). In addition, tangential skin vibration reduced pain intensity by 33% (p<0.01), primarily by activating cutaneous receptors. Concurrently stimulating both sensory channels induced stronger analgesic effects (-51%, p<0.01), as shown by the lower levels of electrodermal activity. The strongest analgesic effects of the vibration-induced muscle inputs occurred when illusory movements were perceived (-38%, p=0.01). The results suggest that both cutaneous and muscle sensory feedback reduce muscle pain, most likely via segmental and supraspinal processes. Further clinical trials are needed to investigate these new methods of muscle pain relief.

Autonomic responses to exercise: cortical and subcortical responses during post-exercise ischaemia and muscle pain

Sustained isometric contraction of skeletal muscle causes an increase in blood pressure, due to an increase in cardiac output and an increase in total peripheral resistance-brought about by an increase in sympathetically-mediated vasoconstriction. Both central command and reflex inputs from metaboreceptors in the contracting muscles have been shown to contribute to this sympathetically mediated increase in blood pressure. Occluding the blood supply and trapping the metabolites in the contracted muscle (post-exercise ischaemia) has shown that, while heart rate returns to baseline following exercise, the increase in MSNA and blood pressure persists in the absence of central command-sustained by peripheral inputs. Post-exercise ischaemia activates group III and IV muscle afferents, which are also activated during noxious stimulation. Indeed, post-exercise ischaemia is painful, so what is the role of pain in the increase in blood pressure? Intramuscular injection of hypertonic saline causes a deep dull ache, not unlike that produced by post-exercise ischaemia, and we have shown that this can cause a sustained increase in MSNA and blood pressure. We have used functional Magnetic Resonance Imaging (fMRI) of the brain to identify the cortical and subcortical sites involved in the sensory processing of muscle pain, and in the generation of the autonomic responses to muscle pain, produced either by post-exercise ischaemia or intramuscular injection of hypertonic saline. During static hand-grip exercise there were parallel increases in signal intensity in the contralateral primary motor cortex, deep cerebellar nuclei and cerebellar cortex that ceased at the end of the exercise, reflecting the start and end of central command. Progressive increases during the contraction phase occurred in the contralateral insula, as well as the contralateral primary somatosensory cortex, and continued during the period of post-exercise ischaemia. Decreases in signal intensity occurred in the perigenual anterior cingulate cortex during the contraction phase; these too were sustained during post-exercise ischaemia. That similar changes occurred with intramuscular injection of hypertonic saline suggests that much of the cortical and subcortical changes seen during post-exercise ischaemia reflect the sensory and affective attributes of the muscle pain, rather than in furnishing the cardiovascular responses per se.

Functional and structural changes in the brain associated with the increase in muscle sympathetic nerve activity in obstructive sleep apnoea

Muscle sympathetic nerve activity (MSNA) is greatly elevated in patients with obstructive sleep apnoea (OSA) during daytime wakefulness, leading to hypertension, but the underlying mechanisms are poorly understood. By recording MSNA concurrently with functional Magnetic Resonance Imaging (fMRI) of the brain we aimed to identify the central processes responsible for the sympathoexcitation. Spontaneous fluctuations in MSNA were recorded via tungsten microelectrodes inserted percutaneously into the common peroneal nerve in 17 OSA patients and 15 healthy controls lying in a 3 T MRI scanner. Blood Oxygen Level Dependent (BOLD) contrast gradient echo, echo-planar images were continuously collected in a 4 s ON, 4 s OFF (200 volumes) sampling protocol. Fluctuations in BOLD signal intensity covaried with the intensity of the concurrently recorded bursts of MSNA. In both groups there was a positive correlation between MSNA and signal intensity in the left and right insulae, dorsolateral prefrontal cortex (dlPFC), dorsal precuneus, sensorimotor cortex and posterior temporal cortex, and the right mid-cingulate cortex and hypothalamus. In OSA the left and right dlPFC, medial PFC (mPFC), dorsal precuneus, anterior cingulate cortex, retrosplenial cortex and caudate nucleus showed augmented signal changes compared with controls, while the right hippocampus/parahippocampus signal intensity decreased in controls but did not change in the OSA subjects. In addition, there were significant increases in grey matter volume in the left mid-insula, the right insula, left and right primary motor cortices, left premotor cortex, left hippocampus and within the brainstem and cerebellum, and significant decreases in the mPFC, occipital lobe, right posterior cingulate cortex, left cerebellar cortex and the left and right amygdala in OSA, but there was no overlap between these structural changes and the functional changes in OSA. These data suggest that the elevated muscle vasoconstrictor drive in OSA may result from functional changes within these brain regions, which are known to be directly or indirectly involved in the modulation of sympathetic outflow via the brainstem. That there was no overlap in the structural and functional changes suggests that asphyxic damage due to repeated episodes of nocturnal obstructive apnoea is not the main cause of the sympathoexcitation.

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Obstructive sleep apnea increases muscle sympathetic nerve activity (MSNA).

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fMRI was used to identify brain sites temporally coupled to the increase in MSNA.

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Augmented BOLD signal intensity occurred in several cortical and subcortical sites.

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The elevated MSNA in OSA may result from functional changes within these sites.

Reduced spontaneous sympathetic nerve activity in multiple sclerosis patients

For the first time, we obtained direct intra-neural measurements of muscle sympathetic nerve activity (MSNA) in relapsing-remitting multiple sclerosis (MS) patients to test the hypothesis that spontaneous resting MSNA is reduced in MS patients compared to age, sex-matched healthy controls. Spontaneous MSNA (microneurography; peroneal nerve), plasma norepinephrine, arterial blood pressure (finger photoplethysmography), and heart rate were measured at rest in three groups: 1) relapsing-remitting MS patients on disease modifying therapy only (MS-DT; n=6); 2) relapsing-remitting MS patients on disease modifying therapy and medications for MS-related symptoms that are known to effect the central nervous system (MS-DT/ST; n=5), and 3) healthy age and sex-matched controls (CON; n=6). Compared to the CON group, MSNA burst frequency (bursts/min) was significantly lower in both MS-DT (P=0.027) and MS-DT/ST groups (P=0.003). Similarly, MSNA burst incidence (bursts/100 heartbeats) was significantly reduced in both MS-DT (P=0.049) and MS-DT/ST groups (P=0.004) compared to the CON group. Burst frequency and burst incidence were not different between MS-DT and MS-DT/ST groups. Resting plasma norepinephrine was also significantly lower in both MS-DT (P=0.039) and MS-DT/ST groups (P=0.021) compared to the CON group. Reduced MSNA may signify an important dysfunction in autonomic control of cardiovascular function in patients with MS.

Sympathetic and cardiovascular changes during sleep in narcolepsy with cataplexy patients

OBJECTIVE

Neural mechanisms underlying sleep-onset rapid eye movement (REM) periods (SOREMPs) in narcolepsy and the role of hypocretin in driving sympathetic changes during sleep are misunderstood. We aimed to characterize autonomic changes during sleep in narcolepsy with cataplexy (NC) patients to clarify the nature of SOREMP events and the effect of hypocretin deficiency on sympathetic activity during sleep.

METHODS

We observed 13 hypocretin-deficient NC patients and five healthy controls who underwent nocturnal video-polysomnography (v-PSG) with blood pressure (BP) recording, heart rate (HR), skin sympathetic activity (SSA), and muscle sympathetic nerve activity (MSNA) from the peroneal nerve by microneurography.

RESULTS

Compared to wake, control participants displayed a progressive significant decrease of BP and sympathetic activities during nonrapid eye movement (NREM) sleep and an increase of autonomic activity during REM sleep, as expected. NC patients showed: (1) a decrease of sympathetic activities during SOREMP comparable to NREM sleep stage 1 (N1) but in contrast to the increased activity typical of REM sleep; and (2) physiologic sympathetic change during the following sleep stages with a progressive decrease during NREM sleep stage 2 (N2) and NREM sleep stage 3 (N3) and a clear increase in REM sleep, though BP did not show the physiologic decrease during sleep (nondipper pattern).

CONCLUSIONS

SOREMPs in NC patients lack the sympathetic activation occurring during physiologic REM sleep, thus suggesting a dissociated REM sleep condition. In addition, our data indicated that hypocretin plays a limited role in the regulation of sympathetic changes during sleep.

Sympathetic nerve activity and simulated diving in healthy humans

The goal of our study was to develop a simple and practical method for simulating diving in humans using facial cold exposure and apnea stimuli to measure neural and circulatory responses during the stimulated diving reflex. We hypothesized that responses to simultaneous facial cold exposure and apnea (simulated diving) would be synergistic, exceeding the sum of responses to individual stimuli. We studied 56 volunteers (24 female and 32 male), average age of 39 years. All subjects were healthy, free of cardiovascular and other diseases, and on no medications. Although muscle sympathetic nerve activity (MSNA), blood pressure, and vascular resistance increased markedly during both early and late phases of simulated diving, significant reductions in heart rate were observed only during the late phase. Total MSNA during simulated diving was greater than combined MSNA responses to the individual stimuli. We found that simulated diving is a powerful stimulus to sympathetic nerve traffic with significant bradycardia evident in the late phase of diving and eliciting synergistic sympathetic and parasympathetic responses. Our data provide insight into autonomic triggers that could help explain catastrophic cardiovascular events that may occur during asphyxia or swimming, such as in patients with obstructive sleep apnea or congenital long QT syndrome.

Anodal transcranial direct current stimulation (tDCS) over the motor cortex increases sympathetic nerve activity

BACKGROUND

Transcranial direct current stimulation (tDCS) is currently being investigated as a non-invasive neuromodulation therapy for a range of conditions including stroke rehabilitation. tDCS affects not only the area underlying the electrodes but also other areas of the cortex and subcortical structures. This could lead to unintended alteration in brain functions such as autonomic control.

OBJECTIVE

We investigated the potential effects of tDCS on cardiovascular autonomic function in healthy volunteers.

METHODS

Anodal (n = 14) or cathodal (n = 8) tDCS at 1 mA was applied over the primary motor cortex with the second electrode placed on the contralateral supraorbital region. Subjects visited the department twice and received active or sham tDCS for 15 min. Heart rate, blood pressure and respiration were recorded at baseline, during tDCS and after stimulation. Heart rate variability (HRV) was calculated using spectral analysis of beat-to-beat intervals derived from ECG data. Microneurography was also used to record muscle sympathetic nerve activity (MSNA; n = 5).

RESULTS

Anodal tDCS caused a significant shift in HRV toward sympathetic predominance (P = 0.017), whereas there was no significant change in the cathodal or sham groups. Microneurography results also showed a significant increase in MSNA during anodal tDCS that continued post-stimulation.

CONCLUSIONS

Anodal tDCS of the motor cortex shifts autonomic nervous system balance toward sympathetic dominance due at least in part to an increase in sympathetic output. These results suggest further investigation is warranted on tDCS use in patient groups with potential autonomic dysfunction, such as stroke patients.