Sympathetic neural burst amplitude distribution: A more specific indicator of sympathoexcitation in human heart failure

The interactive effects of posture and biological sex on the control of muscle sympathetic nerve activity during rhythmic handgrip exercise

Body posture and biological sex exhibit independent effects on the sympathetic neural responses to dynamic exercise. However, the neural mechanisms (e.g., baroreflex) by which posture impacts sympathetic outflow during rhythmic muscular contractions, and whether biological sex affects posture-mediated changes in efferent sympathetic nerve traffic during exercise, remain unknown. Thus, we tested the hypotheses that increases in muscle sympathetic nerve activity (MSNA) would be greater during upright compared with supine rhythmic handgrip (RHG) exercise, and that females would demonstrate smaller increases in MSNA during upright RHG exercise than males. Twenty young (30 [6] yr; means [SD]) individuals (9 males, 11 females) underwent 6 min of supine and upright (head-up tilt 45°) RHG exercise at 40% maximal voluntary contraction with continuous measurements of MSNA (microneurography), blood pressure (photoplethysmography), and heart rate (electrocardiogram). In the pooled group, absolute MSNA burst frequency (P < 0.001), amplitude (P = 0.009), and total MSNA (P < 0.001) were higher during upright compared with supine RHG exercise. However, body posture did not impact the peak change in MSNA during RHG exercise (range: P = 0.063-0.495). Spontaneous sympathetic baroreflex gain decreased from rest to RHG exercise (P = 0.006) and was not impacted by posture (P = 0.347). During upright RHG exercise, males demonstrated larger increases in MSNA burst amplitude (P = 0.002) and total MSNA (P = 0.001) compared with females, which coincided with greater reductions in sympathetic baroreflex gain among males (P = 0.004). Collectively, these data indicate that acute attenuation of baroreflex-mediated sympathoinhibition permits increases in MSNA during RHG exercise and that males exhibit a greater reserve for efferent sympathetic neural recruitment during orthostasis than females.NEW & NOTEWORTHY The impact of posture and sex on cardiovascular control during rhythmic handgrip (RHG) exercise is unknown. We show that increases in muscle sympathetic nerve activity (MSNA) during RHG are partly mediated by a reduction in sympathetic baroreflex gain. In addition, males demonstrate larger increases in total MSNA during upright RHG than females. These data indicate that the baroreflex partly mediates increases in MSNA during RHG and that males have a greater sympathetic vasoconstrictor reserve than females.

The effects of sex and menstrual cycle phase on sympathetic action potential recruitment patterns during hypercapnic-hypoxic apnea

Previously, we demonstrated that integrated muscle sympathetic nerve activity (MSNA) responses to acute chemoreflex stress were augmented during the early follicular (EF) phase of the menstrual cycle relative to both the midluteal (ML) phase and males. These differences were most pronounced in the amplitude component of MSNA, suggesting EF-driven increases in action potential (AP) recruitment in females. Therefore, we tested the hypothesis that neural recruitment, quantified as MSNA AP discharge patterns during acute chemoreflex stress, is potentiated during EF. We retrospectively analyzed MSNA data from 9 young males and 7 young females tested during the EF and ML phases at rest and during a voluntary end-inspiratory hypercapnic-hypoxic apnea. Sympathetic AP discharge patterns were analyzed using wavelet-based methodology. Apnea-driven increases in AP frequency and AP content per integrated burst were greater in EF relative to ML (APs/min: P = 0.02; APs/burst: P = 0.03) and to males (APs/min: P = 0.04; APs/burst: P = 0.02). The recruitment of new larger AP clusters was greater in EF than ML (P < 0.01) but not different from males (P = 0.50). Interestingly, we observed a positive association between the magnitude of change in the estrogen/progesterone ratio from EF to ML and the change in AP cluster recruitment, as both decreased from EF to ML (R² = 0.82; P < 0.01). This suggests that the enhanced progesterone dominance over estrogen during ML may blunt the recruitment of new larger APs. Overall, these data indicate that both sex and the menstrual cycle impact AP recruitment patterns in a manner which may be mediated, at least in part, by gonadal hormones.

Differential regulation of sympathetic neural burst frequency and amplitude throughout normal pregnancy: a longitudinal study

Sympathetic activation is a hallmark of pregnancy. However, longitudinal assessments of muscle sympathetic nerve activity (MSNA) in pregnancy are scarce and have primarily focused on burst occurrence (frequency) at rest, despite burst strength (amplitude) representing distinct characteristics of sympathetic outflow. Thus, we assessed MSNA burst amplitude distributions in healthy women to determine the impact of normal pregnancy on neural discharge patterns in response to orthostatic stress. Twenty-six women were studied longitudinally during pre-, early- (4-8 wk of gestation), and late (32-36 wk) pregnancy, as well as postpartum (6-10 wk after delivery). MSNA, blood pressure (BP), and heart rate (HR) were measured in the supine posture and during graded head-up tilt (30° and 60° HUT). Mean and median MSNA burst amplitudes were used to characterize burst amplitude distribution. In late pregnancy, women demonstrated smaller increases in HR (P < 0.001) during 60° HUT and larger increases in systolic BP (P = 0.043) throughout orthostasis, compared with prepregnancy. The increase in MSNA burst frequency during late- relative to prepregnancy (Late: Δ14[10] vs. Pre: Δ21[9] bursts/min; P = 0.001) was smaller during 60° HUT, whereas increases in burst incidence were smaller in late- relative to prepregnancy throughout orthostasis (P = 0.009). Nonetheless, median burst amplitude was smaller throughout orthostasis in late compared with prepregnancy (P = 0.038). Thus, while supine MSNA burst frequency was greater in late pregnancy, increases in burst frequency and strength during orthostasis were attenuated. These smaller, orthostatically induced MSNA increases may reflect natural adaptions of pregnancy serving to prevent sympathetic hyper-reactivity that is common in pathological states.

Interactive effects of apneic and baroreflex stress on neural coding strategies in human muscle sympathetic nerve activity

The sympathetic nervous system exhibits patterns of action potential (AP) discharge in human muscle sympathetic nerve activity that suggest coding strategies express reflex specificity. This study explored the interactive effects of baroreceptor unloading using lower body negative pressure (LBNP) and volitional end-expiratory apnea (APN) on sympathetic postganglionic neuronal discharge patterns inferred from the firing patterns of differently sized sympathetic AP clusters. Seven individuals were studied using multi-unit microneurography (fibular) and a continuous wavelet approach to quantify AP discharge probability, recruitment, and latency during APN performed under ambient conditions, -10 and -40 mmHg LBNP. Compared to the ambient condition, LBNP increased AP discharge rate at -10 and -40 mmHg and recruited larger previously-silent sympathetic neurons at -40 mmHg. Compared to spontaneous breathing, APN increased AP discharge when performed during the ambient condition (∆351±132 AP/min), -10 mmHg (∆423±184 AP/min), and -40 mmHg (∆355±278 AP/min; main effect APN: P<0.01; LBNP-by-APN interaction: P=0.55). APN recruited larger previously-silent AP clusters during the ambient condition (∆4±3; P<0.02) and -10 mmHg (∆4±3; P<0.01), but not -40 mmHg (∆0±2; P=0.53; LBNP-by-APN: P<0.01). LBNP did not affect AP latency. However, APN reduced AP latency similarly during all conditions (ambient pressure: ∆-0.04±0.04s, -10 mmHg: ∆-0.03±0.03s, -40 mmHg: ∆-0.03±0.04s; main effect APN: P<0.01; LBNP-by-APN: P=0.48). These data indicate that apneic and baroreflex mechanisms appear to additively modify the axonal discharge rate of previously active sympathetic postganglionic neurons and interact to affect recruitment of previously-silent sympathetic neurons. Reductions in AP latency due to apneic stress were not impacted by baroreflex unloading.

Sympathetic Neural Control in Humans with Anxiety-Related Disorders

Numerous conceptual models are used to describe the dynamic responsiveness of physiological systems to environmental pressures, originating with Claude Bernard's milieu intérieur and extending to more recent models such as allostasis. The impact of stress and anxiety upon these regulatory processes has both basic science and clinical relevance, extending from the pioneering work of Hans Selye who advanced the concept that stress can significantly impact physiological health and function. Of particular interest within the current article, anxiety is independently associated with cardiovascular risk, yet mechanisms underlying these associations remain equivocal. This link between anxiety and cardiovascular risk is relevant given the high prevalence of anxiety in the general population, as well as its early age of onset. Chronically anxious populations, such as those with anxiety disorders (i.e., generalized anxiety disorder, panic disorder, specific phobias, etc.) offer a human model that interrogates the deleterious effects that chronic stress and allostatic load can have on the nervous system and cardiovascular function. Further, while many of these disorders do not appear to exhibit baseline alterations in sympathetic neural activity, reactivity to mental stress offers insights into applicable, real-world scenarios in which heightened sympathetic reactivity may predispose those individuals to elevated cardiovascular risk. This article also assesses behavioral and lifestyle modifications that have been shown to concurrently improve anxiety symptoms, as well as sympathetic control. Lastly, future directions of research will be discussed, with a focus on better integration of psychological factors within physiological studies examining anxiety and neural cardiovascular health. © 2022 American Physiological Society. Compr Physiol 12:1-33, 2022.

Menstrual cycle effects on sympathetic neural burst amplitude distribution during orthostasis in young women

PURPOSE

Fluctuations in ovarian hormones during the menstrual cycle impact muscle sympathetic nerve activity burst frequency and burst incidence at rest. The purpose of this study was to investigate menstrual cycle effects on sympathetic neural burst amplitude distribution during an orthostatic challenge in young women.

METHODS

This study included 11 healthy women (33 ± 10 years [mean ± standard deviation]). Muscle sympathetic nerve activity was measured in the supine position as baseline measurement and during 5 min of 60° upright tilting, during the early follicular phase (low estrogen and progesterone) and mid-luteal phase (high estrogen and progesterone) of the menstrual cycle. Relative burst amplitude distribution of muscle sympathetic nerve activity was characterized by the mean, median, skewness, and kurtosis.

RESULTS

From the supine to upright position, mean and median values of relative burst amplitude increased (both P < 0.05), regardless of phases of the menstrual cycle (P = 0.5 and P = 0.7, respectively). In comparison, during the early follicular phase, skewness and kurtosis remained unchanged (P =  0.6 and P =   0.3, respectively) and kurtosis decreased (1.25 ± 1.11 supine vs. - 0.03 ± 0.73 upright; P = 0.02); there was no change in skewness during the mid-luteal phase (P = 0.4).

CONCLUSIONS

In response to orthostasis, while the symmetry and tailedness/peakness of burst amplitude distribution do not change during the early follicular phase, the distribution during the mid-luteal phase becomes flatter with a lower but broader peak. The latter result suggests that the firing probability of large axon action potentials in response to orthostatic challenge is higher when estrogen and progesterone levels are elevated. The role of changes in sympathetic neural burst amplitude distribution in orthostatic tolerance remains to be determined.

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.

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

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.

A Multi-Scale Computational Model of Excitotoxic Loss of Dopaminergic Cells in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder caused by loss of dopaminergic neurons in substantia nigra pars compacta (SNc). Although the exact cause of cell death is not clear, the hypothesis that metabolic deficiency is a key factor has been gaining attention in recent years. In the present study, we investigated this hypothesis using a multi-scale computational model of the subsystem of the basal ganglia comprising the subthalamic nucleus (STN), globus pallidus externa (GPe), and SNc. The proposed model is a multiscale model in that interaction among the three nuclei are simulated using more abstract Izhikevich neuron models, while the molecular pathways involved in cell death of SNc neurons are simulated in terms of detailed chemical kinetics. Simulation results obtained from the proposed model showed that energy deficiencies occurring at cellular and network levels could precipitate the excitotoxic loss of SNc neurons in PD. At the subcellular level, the models show how calcium elevation leads to apoptosis of SNc neurons. The therapeutic effects of several neuroprotective interventions are also simulated in the model. From neuroprotective studies, it was clear that glutamate inhibition and apoptotic signal blocker therapies were able to halt the progression of SNc cell loss when compared to other therapeutic interventions, which only slowed down the progression of SNc cell loss.

Human Dorsal Root Ganglion Stimulation Reduces Sympathetic Outflow and Long-Term Blood Pressure

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DRGS at upper lumbar levels significantly reduces sympathetic nerve firing

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Reduction in sympathetic activity appears to be independent to pain relief

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DRGS significantly reduced BP at 6 months and 2 years

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BP reduction was lateralized to DRGS on the left side

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Three refractory hypertensives became normotensive after chronic stimulation.

This study hypothesized that dorsal root ganglion (DRG) stimulation would reduce sympathetic nerve activity and would alter hemodynamic variables. This study directly recorded muscle sympathetic nerve activity during ON and OFF stimulation of the DRG while measuring hemodynamic parameters. DRG stimulation significantly reduced the firing frequency of sympathetic nerves, as well as significantly reducing blood pressure, with greater reductions evident when stimulation was left-sided. Left-sided DRG stimulation lowers sympathetic nerve activity, leading to long-term phenotypic changes. This raises the potential of DRG stimulation being used to treat de novo autonomic disorders such as hypertension or heart failure.

Does moxonidine reduce Achilles tendon or musculoskeletal pain in women with polycystic ovarian syndrome? A secondary analysis of a randomised controlled trial

BACKGROUND

Sympathetic activity and insulin resistance have recently been linked with chronic tendon and musculoskeletal pain. Polycystic ovarian syndrome is linked with insulin resistance and increased sympathetic drive and was therefore an appropriate condition to study the effects of modulating sympathetic activity on Achilles tendon and musculoskeletal symptoms.

METHODS

A secondary analysis of a double-blinded, randomised controlled trial on women with polycystic ovarian syndrome was conducted. Participants received 12 weeks of moxonidine (n = 14) or placebo (n = 18). Musculoskeletal symptom and Victorian Institute of Sport Assessment - Achilles (VISA-A) questionnaires were distributed, and ultrasound tissue characterisation quantified tendon structure at 0 and 12 weeks. 2-way ANOVA was used for multiple comparisons.

RESULTS

There was no difference in mean change in musculoskeletal symptoms (- 0.6 ± 1.7 vs - 0.4 ± 1.8, p = 0.69) or VISA-A (moxonidine - 0.2 ± 8.8 vs placebo + 4.2 ± 14.6, p = 0.24) attributable to the intervention. There was no difference in any measures of Achilles structure. Moxonidine did not reduce sympathetic drive when compared to placebo.

CONCLUSIONS

This was the first study to investigate the effects of blocking sympathetic drive on musculoskeletal and Achilles tendon symptoms in a metabolically diverse population. While the study was limited by small sample size and lack of sympathetic modulation, moxonidine did not change tendon pain/structure or musculoskeletal symptoms.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT01504321 . Registered 5 January 2012.

Impact of age and sex on neural cardiovascular responsiveness to cold pressor test in humans

Prior longitudinal work suggests that blood pressure (BP) reactivity to the cold pressor test (CPT) helps predict hypertension; yet the impact of age and sex on hemodynamic and neural responsiveness to CPT remains equivocal. Forty-three young (21 ± 1yr, means ± SE) men (YM, n = 20) and women (YW, n = 23) and 16 older (60 ± 1yr) men (OM, n = 9) and women (OW, n = 7) participated in an experimental visit where continuous BP (finger plethysmography) and muscle sympathetic nerve activity (MSNA; microneurography) were recorded during a 3- to 5-min baseline and 2-min CPT. Baseline mean arterial pressure (MAP) was greater in OM than in YM (92 ± 4 vs. 77 ± 1 mmHg, P < 0.01), but similar in women (P = 0.12). Baseline MSNA incidence was greater in OM [69 ± 6 bursts/100 heartbeats (hb)] than in OW (44 ± 7 bursts/100 hb, P = 0.02) and lower in young adults (YM: 17 ± 3 vs. YW: 16 ± 2 bursts/100 hb, P < 0.01), but similar across the sexes (P = 0.83). However, when exposed to the CPT, MSNA increased more rapidly in OW (Δ43 ± 6 bursts/100 hb; group × time, P = 0.01) compared with OM (Δ15 ± 3 bursts/100 hb) but was not different between YW (Δ30 ± 3 bursts/100 hb) and YM (Δ33 ± 4 bursts/100 hb, P = 1.0). There were no differences in MAP with CPT between groups (group × time, P = 0.33). These findings suggest that OW demonstrate a more rapid initial rise in MSNA responsiveness to a CPT compared with OM. This greater sympathetic reactivity in OW may be a contributing mechanism to the increased hypertension risk in postmenopausal women.

Using Deep Brain Stimulation to Unravel the Mysteries of Cardiorespiratory Control

This article charts the history of deep brain stimulation (DBS) as applied to alleviate a number of neurological disorders, while in parallel mapping the electrophysiological circuits involved in generating and integrating neural signals driving the cardiorespiratory system during exercise. With the advent of improved neuroimaging techniques, neurosurgeons can place small electrodes into deep brain structures with a high degree accuracy to treat a number of neurological disorders, such as movement impairment associated with Parkinson's disease and neuropathic pain. As well as stimulating discrete nuclei and monitoring autonomic outflow, local field potentials can also assess how the neurocircuitry responds to exercise. This technique has provided an opportunity to validate in humans putative circuits previously identified in animal models. The central autonomic network consists of multiple sites from the spinal cord to the cortex involved in autonomic control. Important areas exist at multiple evolutionary levels, which include the anterior cingulate cortex (telencephalon), hypothalamus (diencephalon), periaqueductal grey (midbrain), parabrachial nucleus and nucleus of the tractus solitaries (brainstem), and the intermediolateral column of the spinal cord. These areas receive afferent input from all over the body and provide a site for integration, resulting in a coordinated efferent autonomic (sympathetic and parasympathetic) response. In particular, emerging evidence from DBS studies have identified the basal ganglia as a major sub-cortical cognitive integrator of both higher center and peripheral afferent feedback. These circuits in the basal ganglia appear to be central in coupling movement to the cardiorespiratory motor program. © 2020 American Physiological Society. Compr Physiol 10:1085-1104, 2020.

Sympathetic neural overdrive in congestive heart failure and its correlates: systematic reviews and meta-analysis

BACKGROUND AND OBJECTIVES

Sympathetic neural activation occurs in congestive heart failure (CHF). However, the small sample size of the microneurographic studies, heterogeneity of the patients examined, presence of comorbidities as well as confounders (including treatment) represented major weaknesses not allowing to identify the major features of the phoenomenon, particularly in mild CHF. This meta-analysis evaluated 2530 heart failure (CHF) patients recruited in 106 microneurographic studies. It was based on muscle sympathetic nerve activity (MSNA) quantification in CHF of different clinical severity, but data from less widely addressed conditions, such as ischemic vs. idiopathic, were also considered.

METHODS

Assessment was extended to the relationships of MSNA with venous plasma norepinephrine, heart rate (HR) and echocardiographic parameters of cardiac morphology [left ventricular (LV) end-diastolic diameter] and function (LV ejection fraction) as well.

RESULTS

MSNA was significantly greater (1.9 times, P < 0.001) in CHF patients as compared with healthy controls, a progressive significant increase being observed from New York Heart Association classes I-IV in unadjusted and adjusted analyses. MSNA was significantly greater in both untreated and treated CHF (P < 0.001 for both), related to left ventricular (LV) end-diastolic diameter and to a lesser extent to LV ejection fraction (r = 0.24 and -0.05, P < 0.001 and <0.01, respectively), and closely associated with HR (r = 0.66, P < 0.001) and plasma norepinephrine (r = 0.68, P < 0.001).

CONCLUSION

CHF is characterized by sympathetic overactivity which mirrors the degree of LV dysfunction independently of the stage of CHF, its cause and presence of confounders or pharmacological treatment. plasma norepinephrine and HR represent potentially valuable surrogate markers of sympathetic activation in the clinical setting.

Broader adaptive range of sympathetic burst size in response to blood pressure change in older women with greater arterial stiffness

KEY POINTS

In this study, we focused on muscle sympathetic nerve activity (MSNA) burst size and occurrence separately as subcomponents of the sympathetic baroreflex in older adults, and we found that the distribution (variation) of burst size against burst occurrence was greater in women than men. Older women had greater carotid artery stiffness compared with older men, while blood pressure (BP) distribution (variation) was comparable between sexes. Sympathetic baroreflex sensitivity assessed with burst incidence was less sensitive as the carotid artery became stiffer in older men and women, while that assessed with burst area was more sensitive as the carotid artery became stiffer in older women but not in older men. These results help us understand the mechanisms underlying the compensation for the impaired response of MSNA burst occurrence in older women with greater carotid artery stiffness to regulate BP similar to that in older men.

ABSTRACT

There are sex differences in arterial stiffness and neural control of blood pressure (BP) among older adults. We examined whether the sympathetic response to BP is greater in older women than men in burst size but not burst occurrence. Burst occurrence and size were assessed with burst interval and area of muscle sympathetic nerve activity, respectively, and the distributions of these indices were evaluated by range during supine rest in 61 healthy older subjects (30 men (69 ± 6 years) and 31 women (68 ± 6 years); means ± SD). Also, we analysed sympathetic baroreflex sensitivity (BRS) with burst occurrence and area simultaneously. Carotid β-stiffness was measured with B-mode ultrasonic image and carotid BP. The range of burst interval was smaller in older women than men (P = 0.002), while there was no difference in the range of burst area. Carotid β-stiffness was greater in older women than men (6.7 ± 2.7 vs. 5.1 ± 2.7, P = 0.027). Sympathetic BRS assessed with burst incidence was lower in older women than men (-2.3 ± 1.4 vs. -3.3 ± 1.4 bursts·100 beats^-1  mmHg^-1 , P = 0.007), while this sex difference was observed when assessed with burst area after adjusting for carotid β-stiffness (-116.1 ± 135.0 vs. -185.9 ± 148.2 a.u. burst^-1  mmHg^-1 , P = 0.040), but not before. Sympathetic BRS assessed with burst area was negatively (more sensitive) correlated with carotid β-stiffness in older women (r = -0.53, P = 0.002) but not men. These data suggest that the response of burst size within each burst is augmented for the baroreflex BP control despite the impaired response of burst occurrence in older women with greater carotid stiffness.

Sympathetic activity in obesity: a brief review of methods and supportive data

The increase in the prevalence of obesity and the concomitant rise in obesity-related illness have led to substantial pressure on health care systems throughout the world. While the combination of reduced exercise, increased sedentary time, poor diet, and genetic predisposition is undoubtedly pivotal in generating obesity and increasing disease risk, a large body of work indicates that the sympathetic nervous system (SNS) contributes to obesity-related disease development and progression. In obesity, sympathetic nervous activity is regionalized, with activity in some outflows being particularly sensitive to the obese state, whereas other outflows, or responses to stimuli, may be blunted, thereby making the assessment of sympathetic nervous activation in the clinical setting difficult. Isotope dilution methods and direct nerve recording techniques have been developed and utilized in clinical research, demonstrating that in obesity there is preferential activation of the muscle vasoconstrictor and renal sympathetic outflows. With weight loss, sympathetic activity is reduced. Importantly, sympathetic nervous activity is associated with end-organ dysfunction and changes in sympathetic activation that accompany weight loss are often reflected in an improvement of end-organ function. Whether targeting the SNS directly improves obesity-related illness remains unknown, but merits further attention.

Relative burst amplitude of muscle sympathetic nerve activity is an indicator of altered sympathetic outflow in chronic anxiety

Relative burst amplitude of muscle sympathetic nerve activity (MSNA) is an indicator of augmented sympathetic outflow and contributes to greater vasoconstrictor responses. Evidence suggests anxiety-induced augmentation of relative MSNA burst amplitude in patients with panic disorder; thus we hypothesized that acute stress would result in augmented relative MSNA burst amplitude and vasoconstriction in individuals with chronic anxiety. Eighteen participants with chronic anxiety (ANX; 8 men, 10 women, 32 ± 2 yr) and 18 healthy control subjects with low or no anxiety (CON; 8 men, 10 women, 39 ± 3 yr) were studied. Baseline MSNA and 24-h blood pressure were similar between ANX and CON ( P > 0.05); however, nocturnal systolic blood pressure % dipping was blunted among ANX ( P = 0.02). Relative MSNA burst amplitude was significantly greater among ANX compared with CON immediately preceding (anticipation) and during physiological stress [2-min cold pressor test; ANX: 73 ± 5 vs. CON: 59 ± 3% arbitrary units (AU), P = 0.03] and mental stress (4-min mental arithmetic; ANX: 65 ± 3 vs. CON: 54 ± 3% AU, P = 0.02). Increases in MSNA burst frequency, incidence, and total activity in response to stress were not augmented among ANX compared with CON ( P > 0.05), and reduction in brachial artery conductance during cold stress was similar between ANX and CON ( P = 0.92). Relative MSNA burst amplitude during mental stress was strongly correlated with state ( P < 0.01) and trait ( P = 0.01) anxiety (State-Trait Anxiety Inventory), independent of age, sex, and body mass index. Thus in response to acute stress, both mental and physiological, individuals with chronic anxiety demonstrate selective augmentation in relative MSNA burst amplitude, indicating enhanced sympathetic drive in a population with higher risk for cardiovascular disease. NEW & NOTEWORTHY Relative burst amplitude of muscle sympathetic nerve activity in response to acute mental and physiological stress is selectively augmented in individuals with chronic anxiety, which is a prevalent condition that is associated with the development of cardiovascular disease. Augmented sympathetic burst amplitude occurs with chronic anxiety in the absence of common comorbidities. These findings provide important insight into the relation between anxiety, acute stress and sympathetic activation.

Long-duration bed rest modifies sympathetic neural recruitment strategies in male and female participants

To understand the impact of physical deconditioning with head-down tilt bed rest (HDBR) on the malleability of sympathetic discharge patterns, we studied 1) baseline integrated muscle sympathetic nerve activity (MSNA; microneurography) from 13 female participants in the WISE-2005 60-day HDBR study (retrospective analysis), 2) integrated MSNA and multiunit action potential (AP) analysis in 13 male participants performed on data collected at baseline and during physiological stress imposed by end-inspiratory apnea in a new 60-day HDBR study, and 3) a repeatability study (control; n = 6, retrospective analysis, 4 wk between tests). Neither baseline integrated burst frequency nor incidence were altered with HDBR (both P > 0.35). However, baseline integrated burst latency increased in both HDBR studies (male: 1.35 ± 0.02 to 1.39 ± 0.02 s, P < 0.01; female: 1.23 ± 0.02 to 1.29 ± 0.02 s, P < 0.01), whereas controls exhibited no change across two visits (1.25 ± 0.02 to 1.25 ± 0.02 s, group-by-time interaction, P = 0.02). With the exception of increased AP latency ( P = 0.03), male baseline AP data did not change with HDBR (all P > 0.19). The change in AP frequency on going from baseline to apnea (∆94 ± 25 to ∆317 ± 55 AP/min, P < 0.01) and the number of active sympathetic clusters per burst (∆0 ± 0.2 to ∆1 ± 0.2 clusters/burst, P = 0.02) were greater post- compared with pre-HDBR. The change in total clusters with apnea was ∆0 ± 0.5 clusters pre- and ∆2 ± 0.7 clusters post-HDBR ( P = 0.07). These data indicate that 60-day HDBR modified discharge characteristics in baseline burst latency and sympathetic neural recruitment during apneic stress. NEW & NOTEWORTHY Long-duration bed rest did not modify baseline sympathetic burst frequency in male and female participants, but examination of additional features of the multiunit signal provided novel evidence to suggest augmented synaptic delays or processing times at baseline for all sympathetic action potentials. Furthermore, long-duration bed rest increased reflex-sympathetic arousal to apneic stress in male participants primarily by mechanisms involving an augmented firing rate of action potential clusters active at baseline.

Physiological and pathophysiological firing properties of single postganglionic sympathetic neurons in humans

It has long been known from microneurographic recordings in human subjects that the activity of postganglionic sympathetic axons occurs as spontaneous bursts, with muscle sympathetic nerve activity (MSNA) exhibiting strong cardiac rhythmicity via the baroreflex and skin sympathetic nerve activity showing much weaker cardiac modulation. Here we review the firing properties of single sympathetic neurons, obtained using highly selective microelectrodes. Individual vasoconstrictor neurons supplying muscle or skin, or sudomotor neurons supplying sweat glands, always discharge with a low firing probability (~30%) and at very low frequencies (~0.5 Hz). Moreover, they usually fire only once per cardiac interval but can fire greater than four times within a burst. Modeling has shown that this pattern can best be explained by individual neurons being driven by, on average, two preganglionic inputs. Unitary recordings of muscle vasoconstrictor neurons have been made in several pathophysiological states, including heart failure, hypertension, obstructive sleep apnea, bronchiectasis, chronic obstructive pulmonary disease, depression, and panic disorder. The augmented MSNA in each of these diseases features an increase in firing probability and discharge frequency of individual muscle vasoconstrictor neurons above that seen in healthy subjects, yet firing rates rarely exceed 1 Hz. However, unlike patients with heart failure, all patients with respiratory disease or panic disorder, and patients with hyperhidrosis, exhibited an increase in multiple within-burst firing, which emphasizes the different modes by which the sympathetic nervous system grades its output in pathophysiological states of high sympathetic nerve activity.

Influence of age on respiratory modulation of muscle sympathetic nerve activity, blood pressure and baroreflex function in humans

New Findings

What is the central question of this study?

Does ageing influence the respiratory‐related bursting of muscle sympathetic nerve activity (MSNA) and the association between the rhythmic fluctuations in MSNA and blood pressure (Traube–Hering waves) that occur with respiration?

What is the main finding and its importance?

Despite the age‐related elevation in MSNA, the cyclical inhibition of MSNA during respiration is similar between young and older individuals. Furthermore, central respiratory–sympathetic coupling plays a role in the generation of Traube–Hering waves in both young and older humans.

Healthy ageing and alterations in respiratory–sympathetic coupling have been independently linked with heightened sympathetic neural vasoconstrictor activity. We investigated how age influences the respiratory‐related modulation of muscle sympathetic nerve activity (MSNA) and the association between the rhythmic fluctuations in MSNA and blood pressure that occur with respiration (Traube–Hering waves; THW). Ten young (22 ± 2 years; mean ± SD) and 10 older healthy men (58 ± 6 years) were studied while resting supine and breathing spontaneously. MSNA, blood pressure and respiration were recorded simultaneously. Resting values were ascertained and respiratory cycle‐triggered averaging of MSNA and blood pressure measurements performed. The MSNA burst incidence was higher in older individuals [22.7 ± 9.2 versus 42.2 ± 13.7 bursts (100 heart beats)⁻¹, P < 0.05], and was reduced to a similar extent in the inspiratory to postinspiratory period in young and older subjects (by ∼25% compared with mid‐ to late expiration). A similar attenuation of MSNA burst frequency (in bursts per minute), amplitude and total activity (burst frequency × mean burst amplitude) was also observed in the inspiratory to postinspiratory period in both groups. A significant positive correlation between respiratory‐related MSNA and the magnitude of Traube–Hering waves was observed in all young (100%) and most older subjects (80%). These data suggest that the strength of the cyclical inhibition of MSNA during respiration is similar between young and older individuals; thus, alterations in respiratory–sympathetic coupling appear not to contribute to the age‐related elevation in MSNA. Furthermore, central respiratory–sympathetic coupling plays a role in the generation of Traube–Hering waves in both healthy young and older humans.

Modelling the vascular response to sympathetic postganglionic nerve activity

This paper explores the influence of burst properties of the sympathetic nervous system on arterial contractility. Specifically, a mathematical model is constructed of the pathway from action potential generation in a sympathetic postganglionic neurone to contraction of an arterial smooth muscle cell. The differential equation model is a synthesis of models of the individual physiological processes, and is shown to be consistent with physiological data.

The model is found to be unresponsive to tonic (regular) stimulation at typical frequencies recorded in sympathetic efferents. However, when stimulated at the same average frequency, but with repetitive respiratory-modulated burst patterns, it produces marked contractions. Moreover, the contractile force produced is found to be highly dependent on the number of spikes in each burst. In particular, when the model is driven by preganglionic spike trains recorded from wild-type and spontaneously hypertensive rats (which have increased spiking during each burst) the contractile force was found to be 10-fold greater in the hypertensive case. An explanation is provided in terms of the summative increased release of noradrenaline. Furthermore, the results suggest the marked effect that hypertensive spike trains had on smooth muscle cell tone can provide a significant contribution to the pathology of hypertension.

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We model the sympathetic-driven contraction of a vascular smooth muscle cell.

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The cell is unresponsive to tonic stimulation at typical sympathetic frequencies.

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We quantify the force produced by the cell in response to sympathetic bursting.

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The response of the cell is strongly dependent on burst amplitude and duration.

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Recordings from hypertensive animals produce significant contractile forces.

Menstrual cycle and sex effects on sympathetic responses to acute chemoreflex stress

This study aimed to examine the effects of sex (males vs. females) and sex hormones (menstrual cycle phases in women) on sympathetic responsiveness to severe chemoreflex activation in young, healthy individuals. Muscle sympathetic nerve activity (MSNA) was measured at baseline and during rebreathing followed by a maximal end-inspiratory apnea. In women, baseline MSNA was greater in the midluteal (ML) than early-follicular (EF) phase of the menstrual cycle. Baseline MSNA burst incidence was greater in men than women, while burst frequency and total MSNA were similar between men and women only in the ML phase. Chemoreflex activation evoked graded increases in MSNA burst frequency, amplitude, and total activity in all participants. In women, this sympathoexcitation was greater in the EF than ML phase. The sympathoexcitatory response to chemoreflex stimulation of the EF phase in women was also greater than in men. Nonetheless, changes in total peripheral resistance were similar between sexes and menstrual cycle phases. This indicates that neurovascular transduction was attenuated during the EF phase during chemoreflex activation, thereby offsetting the exaggerated sympathoexcitation. Chemoreflex-induced increases in mean arterial pressure were similar across sexes and menstrual cycle phases. During acute chemoreflex stimulation, reduced neurovascular transduction could provide a mechanism by which apnea-associated morbidity might be attenuated in women relative to men.

Use of metformin in diseases of aging

Metformin is the most commonly prescribed medication for type 2 diabetes (T2DM) in the world. It has primacy in the treatment of this disease because of its safety record and also because of evidence for reduction in the risk of cardiovascular events. Evidence has accumulated indicating that metformin is safe in people with stage 3 chronic kidney disease (CKD-3). It is estimated that roughly one-quarter of people with CKD-3 and T2DM in the United States (well over 1 million) are ineligible for metformin treatment because of elevated serum creatinine levels. This could be overcome if a scheme, perhaps based on pharmacokinetic studies, could be developed to prescribe reduced doses of metformin in these individuals. There is also substantial evidence from epidemiologic studies to indicate that metformin may not only be safe, but may actually benefit people with heart failure (HF). Prospective, randomized trials of the use of metformin in HF are needed to investigate this possibility.

Hormone phase dependency of neural responses to chemoreflex-driven sympathoexcitation in young women using hormonal contraceptives

Hormone fluctuations in women may influence muscle sympathetic nerve activity (MSNA) in a manner dependent on the severity of the sympathoexcitatory stimulus. This study examined MSNA patterns at rest and during chemoreflex stimulation in low- (LH) vs. high-hormone (HH) phases of contraceptive use in healthy young women (n = 7). We tested the hypothesis that MSNA would be greater in the HH phase at baseline and in response to chemoreflex stimulation. MSNA recordings were obtained through microneurography in LH and HH at baseline, during rebreathing causing progressive hypoxia and hypercapnia, and during a hypercapnic-hypoxic end-inspiratory apnea. Baseline MSNA burst incidence (P = 0.03) and burst frequency (P = 0.02) were greater in the HH phase, while MSNA burst amplitude distributions and hemodynamic measures were similar between phases. Rebreathing elicited increases in all MSNA characteristics from baseline (P < 0.05), but was not associated with hormone phase-dependent changes to MSNA patterns. Apnea data were considered in two halves, both of which caused large increases in all MSNA variables from baseline in each hormone phase (P < 0.01). Increases in burst incidence and frequency were greater in LH during the first half of the apnea (P = 0.03 and P = 0.02, respectively), while increases in burst amplitude and total MSNA were greater in LH during the second half of the apnea (P < 0.05). These results indicate that change in hormone phase brought on through use of hormonal contraceptives influences MSNA patterns such that baseline MSNA is greater in the HH phase, but responses to severe chemoreflex stimulation are greater in the LH phase.

The neural interaction between the arterial baroreflex and muscle metaboreflex is preserved in older men

Sympathetic baroreflex sensitivity is increased during selective activation of the skeletal muscle metaboreflex with postexercise ischaemia (PEI) in young adults. However, to date, there are no data demonstrating this neural interaction between the arterial baroreflex and the muscle metaboreflex in healthy older adults. Therefore, the goal of the present study was to examine the influence of healthy ageing on the metabolic component of the exercise pressor reflex and its interaction with the arterial baroreflex in the control of sympathetic outflow. Postexercise ischaemia following static hand grip performed at 30% maximal voluntary contraction was used to isolate muscle metaboreflex activation in young [n = 10; 24 ± 1 years old; resting blood pressure (BP) 116 ± 3/64 ± 3 mmHg] and older men (n = 9; 59 ± 2 years old; resting BP 120 ± 2/77 ± 2 mmHg). Arterial BP (Finometer) and muscle sympathetic nerve activity (MSNA) were measured continuously. Weighted linear regression analysis between MSNA and diastolic BP was used to estimate arterial baroreflex MSNA gain. There were no age-related differences in the increase in mean BP (young, Δ14 ± 3 mmHg versus older, Δ15 ± 2 mmHg; P > 0.05) or MSNA burst frequency (young, Δ11 ± 2 bursts min(-1) versus older, Δ9 ± 1 bursts min(-1); P > 0.05) during PEI. Likewise, the gain of arterial baroreflex control of total MSNA increased to a similar extent in both groups during PEI (young, -4.2 ± 0.9 baseline versus -6.3 ± 1.1 PEI a.u. beat(-1) mmHg(-1); and older, -3.7 ± 1.1 baseline versus -6.7 ± 1.4 PEI a.u. beat(-1) mmHg(-1); P < 0.05 for both). Collectively, these findings indicate that the neural interaction between the arterial baroreflex and the skeletal muscle metaboreflex in the regulation of MSNA is preserved in healthy ageing.

Recruitment pattern of sympathetic muscle neurons during premature ventricular contractions in heart failure patients and controls

Premature ventricular contractions (PVC) elicit larger bursts of multiunit muscle sympathetic nerve activity (MSNA), reflecting the ability to increase postganglionic axonal recruitment. We tested the hypothesis that chronic heart failure (CHF) limits the ability to recruit postganglionic sympathetic neurons as a response to PVC due to the excessive sympathetic activation in these patients. Sympathetic neurograms of sufficient signal-to-noise ratio were obtained from six CHF patients and from six similarly aged control individuals. Action potentials (APs) were extracted from the multiunit sympathetic neurograms during sinus rhythm bursts and during the post-PVC bursts. These APs were classified on the basis of the frequency per second, the content per burst, and the peak-to-peak amplitude, which formed the basis of binning the APs into active clusters. Compared with controls, CHF had higher APs per burst and higher number of active clusters per sinus rhythm burst (P < 0.05). Compared with sinus rhythm bursts, both groups increased AP frequency and the number of active clusters in the post-PVC burst (P < 0.05). However, compared with controls, the increase in burst integral, AP frequency, and APs per burst during the post-PVC burst was less in CHF patients. Nonetheless, the PVC-induced increase in active clusters per burst was similar between the groups. Thus, these CHF patients retained the ability to recruit larger APs but had a diminished ability to increase overall AP content.

Differential regulation of sympathetic burst frequency and amplitude following acute hypoxia in humans

Current evidence suggests that the persistent sympathetic nerve activity (SNA), commonly observed after exposure to hypoxia (HX), is mediated by chemoreceptor sensitization and or baroreflex resetting. Evidence in humans and animals suggests that these reflexes may independently regulate the frequency (gating) and amplitude (neuronal recruitment) of SNA bursts. In humans ( n = 7), we examined the regulation of SNA following acute isocapnic HX (5 min; end-tidal Po2 = 45 Torr) and euoxic hypercapnia (HC; 5 min; end-tidal Pco2 = +10 from baseline). HX increased SNA burst frequency (21 ± 7 to 28 ± 8 bursts/min, P < 0.05) and amplitude (99 ± 10 to 125 ± 19 au, P < 0.05) as did HC (14 ± 6 to 22 ± 10 bursts/min, P < 0.05 and 100 ± 12 to 133 ± 29 au, P < 0.05, respectively). Burst frequency (26 ± 7 bursts/min, P < 0.05), but not amplitude (97 ± 12 au), remained elevated 10 min post-HX. The change in burst amplitude (but not frequency) was significantly related to the measured change in ventilation ( r2 = 0.527, P < 0.001). Both frequency and amplitude decreased during recovery following HC. These data indicate the differential regulation of pattern and magnitude of sympathetic outflow in humans with sympathetic persistence following HX being specific to burst frequency and not amplitude.

Sympathetic nerve activity in women 40 years after a hypertensive pregnancy

OBJECTIVES

Epidemiological studies show that women with pregnancies complicated by hypertension have an increased risk of cardiovascular morbidity later in life. The underlying mechanisms to the risk increase remain largely unknown. This study evaluated sympathetic nerve activity in women with hypertensive pregnancies 40 years earlier compared to women with normotensive pregnancies. We hypothesized that sympathetic outflow would be increased in women with previous hypertensive pregnancies and that this partly may explain the increased cardiovascular risk.

METHODS

Sympathetic nerve activity to the muscle vascular bed [muscle sympathetic nerve activity (MSNA)] was recorded in 28 women, 18 with and 10 without a hypertensive manifestation during pregnancy. Women were also examined with ambulatory blood pressure measurements, pulse wave velocity, blood pressure response during Stroop test and laboratory analysis.

RESULTS

Women with previous hypertensive pregnancies did not show an increased sympathetic outflow compared to women with normotensive pregnancies. In eight women with treated hypertension sympathetic outflow was increased compared to normotensive women despite similar ambulatory blood pressure values (P < 0.05). During Stroop test the hypertensive women showed increased systolic blood pressure and also displayed the highest augmentation index compared to normotensive women (P < 0.05).

CONCLUSION

Hypertensive pregnancies per se were not associated with increased sympathetic outflow 40 years later. The increased cardiovascular risk in women with previous hypertensive pregnancies cannot be explained by chronic activation of the sympathetic nervous system.In women with previous hypertensive pregnancies, still hypertensive though well controlled, sympathetic outflow and arterial stiffness were, however, increased compared to normotensive counterparts.

Augmented single-unit muscle sympathetic nerve activity in heart failure with chronic atrial fibrillation

Atrial fibrillation (AF) is a common complication in heart failure (HF) patients. However, it remains unclear whether irregular ventricular response patterns induced by AF increase sympathetic nerve activity. We measured resting multi- and single-unit muscle sympathetic nerve activity (MSNA) in 21 age-matched HF patients with chronic AF (n = 11) rhythm or sinus rhythm (SR, n = 10). The multi-unit MSNA, which was expressed as total activity, was similar between HF + AF patients and HF + SR patients. However, the single-unit MSNA in HF + AF patients was significantly greater than that in HF + SR patients (62 ± 9 spikes min(-1) vs. 42 ± 4 spikes min(-1), P < 0.05). Moreover, the incidence of multiple firing of single-unit MSNA within a given burst was augmented in HF + AF patients as compared with HF + SR patients (48 ± 8% vs. 26 ± 3%, P < 0.01). A significant negative relationship was observed between the reduced diastolic pressure induced by a prolonged cardiac interval in AF subjects and single-unit MSNA frequency within one cardiac interval in each HF + AF subject. The firing characteristics of single-unit MSNA were different between HF patients with AF and HF patients with SR; particularly, those with a prolonged long RR interval showed multiple firings of single-unit MSNA. These findings suggest that AF per se leads to the instantaneous augmentation of single-unit MSNA induced by decreased diastolic pressure, which might partially contribute to disease progression in HF patients.

Respiratory modulation of muscle sympathetic nerve activity is not increased in essential hypertension or chronic obstructive pulmonary disease

We examined cardiac and respiratory modulation of muscle sympathetic nerve activity (MSNA) in 13 patients with essential hypertension (HT) and 15 with chronic obstructive pulmonary disease (COPD), and compared these with a group of young healthy controls (YHC) and older healthy controls (OHC). There were no significant differences in age of the OHC and HT subjects. MSNA was recorded via a tungsten microelectrode inserted percutaneously into the common peroneal nerve. Respiration was recorded by a strain-gauge transducer around the chest and ECG recorded by surface electrodes. Cardiac and respiratory modulation of MSNA was quantified by fitting polynomials to the cross-correlation histograms constructed between the sympathetic spikes and ECG or respiration. Cardiac modulation was high across all groups, but was significantly lower in COPD (75.9 ± 4.4%) than in the HT (92.4 ± 3.0%), OHC (93.7 ± 1.3%) or YHC (89.1 ± 1.6%) groups. Across all groups, respiratory modulation was significantly lower than cardiac modulation. Respiratory modulation in HT (45.2 ± 5.7%) and COPD (37.5 ± 6.3%) was not higher than in the OHC (47.2 ± 5.4%) or YHC (49.5 ± 6.0%) groups. We have shown that respiratory modulation of MSNA is present in all groups, is consistently lower than the magnitude of cardiac modulation, and is not increased in HT or COPD, arguing against an amplified respiratory-sympathetic coupling in hypertension. Moreover, given that patients with COPD are chronically asphyxic, these data indicate that an increased chemical drive does not increase respiratory modulation of MSNA.

Single-unit muscle sympathetic nervous activity and its relation to cardiac noradrenaline spillover

Recent work using single-unit sympathetic nerve recording techniques has demonstrated aberrations in the firing pattern of sympathetic nerves in a variety of patient groups. We sought to examine whether nerve firing pattern is associated with increased noradrenaline release. Using single-unit muscle sympathetic nerve recording techniques coupled with direct cardiac catheterisation and noradrenaline isotope dilution methodology we examined the relationship between single-unit firing patterns and cardiac and whole body noradrenaline spillover to plasma. Participants comprised patients with hypertension (n=6), depression (n=7) and panic disorder (n =9) who were drawn from our ongoing studies. The patient groups examined did not differ in their single-unit muscle sympathetic nerve firing characteristics nor in the rate of spillover of noradrenaline to plasma from the heart. The median incidence of multiple spikes per beat was 9%. Patients were stratified according to the firing pattern: low level of incidence (less than 9% incidence of multiple spikes per beat) and high level of incidence (greater than 9% incidence of multiple spikes per beat). High incidence of multiple spikes within a cardiac cycle was associated with higher firing rates (P <0.0001) and increased probability of firing (P <0.0001). Whole body noradrenaline spillover to plasma and (multi-unit) muscle sympathetic nerve activity in subjects with low incidence of multiple spikes was not different to that of those with high incidence of multiple spikes. In those with high incidence of multiple spikes there occurred a parallel activation of the sympathetic outflow to the heart, with cardiac noradrenaline spillover to plasma being two times that of subjects with low nerve firing rates (11.0 ± 1.5 vs. 22.0 ± 4.5 ng min⁻¹, P <0.05). This study indicates that multiple within-burst firing and increased single-unit firing rates of the sympathetic outflow to the skeletal muscle vasculature is associated with high cardiac noradrenaline spillover.

Differential Effects of Oral β Blockade on Cardiovascular and Sympathetic Regulation

In patients with hypertension, β blockade decreases muscle sympathetic nerve activity (MSNA; micrographic technique) expressed in burst frequency (burst/min) but does not affect MSNA expressed in burst incidence (burst/100 heart beats), because reductions in blood pressure (BP) upon each diastole continue to deactivate the arterial baroreceptors, but at a slower heart rate (HR). We studied the effects of oral β blockade on MSNA and baroreflex sensitivity (BRS) in normal participants. Bisoprolol (5 mg, 1 week) was administered in 10 healthy young adults, using a double-blind, placebo-controlled, randomized cross-over study design. The beat-to-beat mean RR interval (RR) and systolic blood pressure (SBP) series were analyzed by power spectral analysis and power computation over the very low frequency (VLF), low frequency, and high frequency (HF) bands. Baroreflex sensitivity was computed from SBP and RR cross-analysis, using time and frequency domain methods.

Bisoprolol increased RR (P < .0005), decreased mean SBP and diastolic blood pressure values (P < .01), did not change the SBP and RR powers, except for RR power in VLF (P < .02) and SBP power in HF (P < .03). The MSNA variability (P > .13) and respiratory pattern (P = .84) did not change from placebo to bisoprolol condition. The bisoprolol-induced bradycardia was associated with higher burst/100 heart beats (P < .05) and bisoprolol did not affect burst/min (P = .80). Time domain BRS estimates were increased after bisoprolol (P < .05), while frequency domain ones did not change (P > .1).

Oral bisoprolol induces differential effects on sympathetic burst frequency and incidence in normal participants. Peripheral sympathetic outflow over time is preserved as a result of an increased burst incidence, in the presence of a slower HR. Unchanged BP and HR and MSNA variability suggests that the larger burst incidence is not due to sympathetic activation.

Altered firing pattern of single-unit muscle sympathetic nerve activity during handgrip exercise in chronic heart failure

Sympathetic activation in chronic heart failure (CHF) is greatly augmented at rest but the response to exercise remains controversial. We previously demonstrated that single-unit muscle sympathetic nerve activity (MSNA) provides a more detailed description of the sympathetic response to physiological stress than multi-unit nerve recordings. The purpose of this study was to determine whether the reflex response and discharge properties of single-unit MSNA are altered during handgrip exercise (HG, 30% of maximum voluntary contraction for 3 min) in CHF patients (New York Heart Association functional class II or III, n = 16) compared with age-matched healthy control subjects (n = 13). At rest, both single-unit and multi-unit indices of sympathetic outflow were augmented in CHF compared with controls (P < 0.05). However, the percentage of cardiac intervals that contained one, two, three or four single-unit spikes were not different between the groups. Compared to the control group, HG elicited a larger increase in multi-unit total MSNA (Delta1002 +/- 50 compared with Delta636 +/- 76 units min(-1), P < 0.05) and single-unit MSNA spike incidence (Delta27 +/- 5 compared with Delta8 +/- 2 spikes (100 heart beats)(-1)), P < 0.01) in the CHF patients. More importantly, the percentage of cardiac intervals that contained two or three single-unit spikes was increased (P < 0.05) during exercise in the CHF group only (Delta8 +/- 2% and Delta5 +/- 1% for two and three spikes, respectively). These results suggest that the larger multi-unit total MSNA response observed during HG in CHF is brought about in part by an increase in the probability of multiple firing of single-unit sympathetic neurones.

Lateralization of expression of neural sympathetic activity to the vessels and effects of carotid baroreceptor stimulation

Human studies suggest that cardiovascular neural sympathetic control is predominantly modulated by the right cerebral hemisphere. It is unknown whether post-ganglionic sympathetic activity [muscle sympathetic nerve activity (MSNA)] shows any functional asymmetry. Eight right-handed volunteers (3 women and 5 men, 32 +/- 2 yr of age) underwent ECG, beat-by-beat blood pressure, respiratory activity, and simultaneous right and left MSNA recordings during spontaneous and controlled breathing (CB, 15 breaths/min, 0.25 Hz). Dynamic carotid baroreceptor stimulation was obtained by 0.1-Hz sinusoidal suction, from 0 to -50 mmHg, randomly applied to the right, left, and combined right and left sides of the neck during CB. Laterality was assessed by changes in the MSNA burst rate (in bursts/min, and bursts/100 beats), strength [amplitude (A) and area (AA)], and the oscillatory component at 0.1 Hz during baroreceptor stimulation. Amplitude parameters were normalized by CB burst mean amplitude and area of the same side. At rest, the right and left MSNA burst rate and total MSNA activity were similar. Conversely, the right MSNA normalized burst A(N) (1.36 +/- 0.18) and AA(N) (1.31 +/- 0.16) were larger than the left MSNA A(N) (1.04 +/- 0.09) and AA(N) (1.02 +/- 0.08). Unilateral and bilateral carotid baroreflex stimulation abolished the right prevalence of A(N) and AA(N). In conclusion, the right lateralization of sympathetic activity to the vessels is indicated by normalized burst strength parameters of bilateral MSNA recordings at rest during spontaneous breathing. Carotid baroreceptor stimulation disrupted such expression of MSNA lateralization possibly by disturbing the synchronizing action of right cerebral hemisphere.

Modulation of the control of muscle sympathetic nerve activity during incremental leg cycling

We tested the hypotheses that arterial baroreflex (ABR) control over muscle sympathetic nerve activity (MSNA) in humans does not remain constant throughout a bout of leg cycling ranging in intensity from very mild to exhausting. ABR control over MSNA (burst incidence, burst strength and total MSNA) was evaluated by analysing the relationship between beat-to-beat spontaneous variations in diastolic arterial pressure (DAP) and MSNA in 15 healthy subjects at rest and during leg cycling in a seated position at five workloads: very mild (10 W), mild (82 +/- 5.0 W), moderate (126 +/- 10.2 W), heavy (156 +/- 14.3 W), and exhausting (190 +/- 21.2 W). The workload was incremented every 6 min. The linear relationships between DAP and MSNA variables were significantly shifted downward during very mild exercise, but then shifted progressively upward as exercise intensity increased. During heavy and exhausting exercise, moreover, the DAP-MSNA relationships were also significantly shifted rightward from the resting relationship. The sensitivity of ABR control over burst incidence and total MSNA was significantly lower during very mild exercise than during rest, and the sensitivity of the burst incidence control remained lower than the resting level at all higher exercise intensities. By contrast, the sensitivity of the total MSNA control recovered to the resting level during mild and moderate exercise, and was significantly increased during heavy and exhausting exercise (versus rest). We conclude that, in humans, ABR control over MSNA is not uniform throughout a leg cycling exercise protocol in which intensity was varied from very mild to exhausting. We suggest that this non-uniformity of ABR function is one of the mechanisms by which sympathetic and cardiovascular responses are matched to the exercise intensity.

Sympathetic neural activity, sex dimorphism, and postural tachycardia syndrome

OBJECTIVE

To elucidate the mechanisms whereby postural tachycardia syndrome (POTS) patients maintain orthostatic blood pressure and explain the greater prevalence of female cases in POTS cohorts.

METHODS

We investigated muscle sympathetic nerve (MSN) discharge characteristics in 12 female POTS patients and in 9 male and 12 female control subjects using the burst amplitude distribution and the relative contribution of burst frequency and burst amplitude.

RESULTS

At rest, burst distribution was shifted toward larger amplitudes in POTS patients (p < 0.005), consistent with increased sympathetic activity. During hypotension, the distribution shifted toward larger amplitude bursts in control subjects (p < 0.001), whereas it did not change in POTS patients. Total MSN activity increase to hypotension did not differ between subject groups, but the relative contribution of burst frequency change to the total activity increase was greater in POTS patients than in female (p < 0.05) and male (p < 0.001) control subjects. In contrast, the relative contribution of burst amplitude change to total MSN activity increase was greater in male compared with female control subjects (p < 0.05) and POTS patients (p < 0.001).

INTERPRETATION

At rest, the burst amplitude distribution was consistent with increased sympathetic activity in POTS and did not change in response to hypotension. In response to hypotension, burst frequency makes a proportionally greater contribution to the increase in total MSN activity in POTS patients compared with female control subjects, and female compared with male control subjects. These physiological differences in MSN discharge characteristics, in the setting of sympathetic fiber loss associated with POTS, may contribute to the predisposition to and greater prevalence of POTS in female individuals.

Modulation of the control of muscle sympathetic nerve activity during severe orthostatic stress

We tested the hypothesis that arterial baroreflex (ABR)-mediated beat-to-beat control over muscle sympathetic nerve activity (MSNA) is progressively modulated as orthostatic stress increases in humans, but that this control becomes impaired just before the onset of orthostatic syncope. In 17 healthy subjects, the ABR control over MSNA (burst incidence, burst strength and total MSNA) was evaluated by analysing the relationship between beat-to-beat spontaneous variations in diastolic blood pressure (DAP) and MSNA during supine rest (control) and during progressive, stepwise increases in lower body negative pressure (LBNP) that were incremented by -10 mmHg every 5 min until presyncope (nine subjects) or -60 mmHg was reached. (1) The linear relationships between DAP and burst strength and between DAP and total MSNA were shifted progressively upward as LBNP increased until the level at which syncope occurred. The relationship between DAP and burst incidence, however, gradually shifted upward from control only to LBNP = -30 mmHg; there was no further upward shift at higher LBNPs. (2) Although the slope of the relationship between DAP and burst strength and between DAP and total MSNA remained constant at all LBNPs tested, except at the level where syncope occurred, the slope of the relationship between DAP and burst incidence was reduced at LBNPs of -40 mmHg and higher (versus control). (3) In syncopal subjects, the slopes of the relationships between DAP and burst incidence, burst strength, and total MSNA were all substantially reduced during the 1-2 min period prior to the onset of syncope. Taken together, these results suggest baroreflex control over MSNA is progressively modulated as orthostatic stress increases, so that its sensitivity is substantially reduced during the period immediately preceding the severe hypotension associated with orthostatic syncope.

The activity of a single muscle sympathetic vasoconstrictor nerve unit is affected by physiological stress in humans

Recording of neural firing from single-unit muscle sympathetic nerve activity (MSNA) is a new strategy offering information about the frequency of pure sympathetic firing. However, it is uncertain whether and when single-unit MSNA would be more useful than multiunit MSNA for analysis of various physiological stresses in humans. In 15 healthy subjects, we measured single-unit and multiunit MSNA before and during handgrip exercise at 30% of maximum voluntary contraction for 3 min and during the Valsalva maneuver at 40 mmHg expiratory pressure for 15 s. Shapes of individual single-unit MSNA were proved to be consistent and suitable for further evaluation. Single-unit and multiunit MSNA exhibited similar responses during handgrip exercise. However, acceleration of neural firing determined from single-unit MSNA became steeper than multiunit MSNA during the Valsalva maneuver. During the Valsalva maneuver, unlike handgrip exercise, the distribution of multiunit burst between 0, 1, 2, 3, and 4 spikes was significantly shifted toward multiple spikes within a given burst (P < 0.05). These results indicated that evaluation of single-unit MSNA could provide more detailed and accurate information concerning the role and responses of neuronal discharges induced by various physiological stresses in humans, especially amid intense sympathetic activity.

Modulation of control of muscle sympathetic nerve activity during orthostatic stress in humans

We tested the hypothesis that orthostatic stress would modulate the arterial baroreflex (ABR)-mediated beat-by-beat control of muscle sympathetic nerve activity (MSNA) in humans. In 12 healthy subjects, ABR control of MSNA (burst incidence, burst strength, and total activity) was evaluated by analysis of the relation between beat-by-beat spontaneous variations in diastolic blood pressure (DAP) and MSNA during supine rest (CON) and at two levels of lower body negative pressure (LBNP: -15 and -35 mmHg). At -15 mmHg LBNP, the relation between burst incidence (bursts per 100 heartbeats) and DAP showed an upward shift from that observed during CON, but the further shift seen at -35 mmHg LBNP was only marginal. The relation between burst strength and DAP was shifted upward at -15 mmHg LBNP (vs. CON) and further shifted upward at -35 mmHg LBNP. At -15 mmHg LBNP, the relation between total activity and DAP was shifted upward from that obtained during CON and further shifted upward at -35 mmHg LBNP. These results suggest that ABR control of MSNA is modulated during orthostatic stress and that the modulation is different between a mild (nonhypotensive) and a moderate (hypotensive) level of orthostatic stress.

Pulse-synchronous sympathetic burst power as a new index of sympathoexcitation in patients with heart failure

The upper limit of incidence of muscle sympathetic neural bursts can lead to underestimation of sympathetic activity in patients with severe heart failure. This study aimed to evaluate the pulse-synchronous burst power of muscle sympathetic nerve activity (MSNA) as a more specific indicator that could discriminate sympathetic activity in patients with heart failure. In 54 patients with heart failure, the pulse-synchronous burst power at the mean heart rate was quantified by spectral analysis of MSNA. Thirteen patients received a central sympatholytic agent (guanfacine) for 5 days to validate the feasibility of this new index. Both burst incidence and plasma norepinephrine level showed no significant difference between patients in New York Heart Association functional class III (94 ± 6 per 100 heartbeats and 477 ± 219 pg/ml, respectively) and class II (79 ± 14 per 100 heartbeats and 424 ± 268 pg/ml, respectively). In contrast, the burst power was useful for discriminating patients in class III from those in class II (61 ± 8% vs. 39 ± 10%; P < 0.05). Inhibition of sympathetic nerve activity by guanfacine was more sensitively reflected by the change of burst power (−36 ± 25%) than by that of burst incidence (−12 ± 14%; P < 0.001). The sympathetic burst power reflects both burst frequency and amplitude independently of the absolute values and provides a sensitive new index for interindividual comparisons of sympathetic activity in patients with heart failure.

Test–retest repeatability of muscle sympathetic nerve activity: influence of data analysis and head-up tilt

Total integrated muscle sympathetic nerve activity (MSNA) is composed of bursts that vary in both frequency and amplitude. Various quantifiable indices are currently used to characterize MSNA and its reflex-mediated responses. However, a comprehensive and systematic analysis on the test-retest repeatability of these measures has not been conducted. Therefore, the purpose of this study was to compare the consistency of supine and passive head-up tilt-mediated sympathetic nerve activity using different descriptors of MSNA and a statistical paradigm that included Model II ordinary least products (OLP) regression, Bland-Altman method of differences, and analysis of variance. MSNA (microneurography), stroke volume (SV, Doppler), and arterial blood pressure (ABP, Finapres) were measured during repeated supine and 60 degrees head-up tilt (HUT) conditions separated by a minimum of 3 weeks. MSNA was quantified using; burst frequency (and incidence), burst amplitudes (and total integrated activity) normalized to the largest absolute amplitude within each posture, and calculated percent changes (from supine) in absolute burst amplitude and total integrated activity. Most indices of MSNA showed excellent test-retest repeatability during both postures with neither fixed nor proportional bias. However, MSNA expressed as burst incidence demonstrated both fixed and proportional bias in the supine position, but not during HUT. In addition, HUT-induced percent changes in absolute burst amplitude and total activity displayed a fixed bias with greater increases during the second test (P<0.05). The hemodynamic variables associated with the reflex responses were quite similar between tests (i.e., no bias). It was concluded that, with the exception of burst incidence, the majority of MSNA indices provided reliable markers of sympathetic activity on repeated tests. However, care must be taken when using percent changes in MSNA that incorporate absolute amplitudes.

Hypovolemia and MSNA discharge patterns: assessing and interpreting sympathetic responses

We previously demonstrated that diuretic-induced hypovolemia resulted in an enhanced baroreflex-mediated increase in integrated muscle sympathetic nerve activity (MSNA) and vasomotor tone during lower body negative pressure (LBNP) (Am J Physiol Heart Circ Physiol 282: H645-H655, 2002). The purpose of this study was to perform a retrospective analysis of these data and examine the ability of relative MSNA burst amplitude distributions to highlight differences in baseline sympathetic nerve discharge patterns. An additional purpose was to determine whether differential responses in MSNA burst frequency and burst amplitude affect conclusions regarding sympathetic reflex control. MSNA, stroke volume (SV, Doppler), and estimated central venous pressure (CVP, dependent arm technique) were measured during LBNP within the placebo (Normo) and diuretic (Hypo; 100 mg/day spironolactone for 3 days) conditions (n = 8). Compared with Normo, MSNA burst frequency at rest was elevated, and there was a rightward shift in the median of the relative burst amplitude distribution (P < 0.05) in Hypo. During LBNP, the larger rise in total MSNA during Hypo versus Normo was due to greater increases in relative burst amplitude with no difference in the burst frequency response. The MSNA burst frequency response to LBNP was shifted to a higher position on the same MSNA-CVP curve during Hypo compared with Normo. In contrast, the Hypo burst amplitude response was shifted to a new curve with a slope that was similar to the Normo relationship. These data support the use of probability distribution analysis to examine intraindividual differences in baseline and reflex-mediated increases in MSNA burst amplitude. Furthermore, the differential effect of hypovolemia on the responses of burst frequency and amplitude during graded LBNP suggests that burst frequency data alone may not adequately represent reflex control of sympathetic outflow.

Modulation of arterial baroreflex dynamic response during muscle metaboreflex activation in humans

We aimed to investigate the interaction between the arterial baroreflex and muscle metaboreflexes (as reflected by alterations in the dynamic responses shown by muscle sympathetic nerve activity (MSNA), mean arterial blood pressure (MAP) and heart rate (HR)) in humans. In nine healthy subjects (eight male, one female) who performed a sustained 1 min handgrip exercise at 50 % maximal voluntary contraction followed by forearm occlusion, a 5 s period of neck pressure (NP) (30 and 50 mmHg) or neck suction (NS)(-30 and -60 mmHg) was used to evaluate carotid baroreflex function at rest (CON) and during post-exercise muscle ischaemia (PEMI). In PEMI (as compared with CON): (a) the augmentations in MSNA and MAP elicited by 50 mmHg NP were both greater; (b) MSNA seemed to be suppressed by NS for a shorter period, (c) the decrease in MAP elicited by NS was smaller, and (d) MAP recovered to its initial level more quickly after NS. However, the HR responses to NS and NP were not different between PEMI and CON. These results suggest that during muscle metaboreflex activation, the dynamic arterial baroreflex response is modulated, as exemplified by the augmentation of the MSNA response to arterial baroreflex unloading (i.e. NP) and the reduction in the suppression of MSNA induced by baroreceptor stimulation (i.e. NS).

Firing properties of single postganglionic sympathetic neurones recorded in awake human subjects

For over three decades, the technique of microneurography has allowed us to record sympathetic neural outflow directly from postganglionic axons in awake human subjects. But because sympathetic axons are clustered within a nerve fascicle, such recordings have been limited to the analysis of multi-unit neural activity. To improve the information content of intraneural recordings, we developed the single-unit approach, in which focal recordings can be made from a single C-fibre via a high-impedance tungsten microelectrode. In this review, we describe our methodology for analyzing unitary sympathetic activity and discuss the similarities in the firing properties of individual muscle vasoconstrictor, cutaneous vasoconstrictor and sudomotor neurones.