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

Central α2-adrenergic mechanisms regulate human sympathetic neuronal discharge strategies

The present study investigated the impact of central α2-adrenergic mechanisms on sympathetic action potential (AP) discharge, recruitment and latency strategies. We used the microneurographic technique to record muscle sympathetic nerve activity and a continuous wavelet transform to investigate postganglionic sympathetic AP firing during a baseline condition and an infusion of a α2-adrenergic receptor agonist, dexmedetomidine (10 min loading infusion of 0.225 µg kg-1; maintenance infusion of 0.1-0.5 µg kg h-1) in eight healthy individuals (28 ± 7 years, five females). Dexmedetomidine reduced mean pressure (92 ± 7 to 80 ± 8 mmHg, P < 0.001) but did not alter heart rate (61 ± 13 to 60 ± 14 bpm; P = 0.748). Dexmedetomidine reduced sympathetic AP discharge (126 ± 73 to 27 ± 24 AP 100 beats-1, P = 0.003) most strongly for medium-sized APs (normalized cluster 2: 21 ± 10 to 5 ± 5 AP 100 beats-1; P < 0.001). Dexmedetomidine progressively de-recruited sympathetic APs beginning with the largest AP clusters (12 ± 3 to 7 ± 2 clusters, P = 0.002). Despite de-recruiting large AP clusters with shorter latencies, dexmedetomidine reduced AP latency across remaining clusters (1.18 ± 0.12 to 1.13 ± 0.13 s, P = 0.002). A subset of six participants performed a Valsalva manoeuvre (20 s, 40 mmHg) during baseline and the dexmedetomidine infusion. Compared to baseline, AP discharge (Δ 361 ± 292 to Δ 113 ± 155 AP 100 beats-1, P = 0.011) and AP cluster recruitment elicited by the Valsalva manoeuvre were lower during dexmedetomidine (Δ 2 ± 1 to Δ 0 ± 2 AP clusters, P = 0.041). The reduction in sympathetic AP latency elicited by the Valsalva manoeuvre was not affected by dexmedetomidine (Δ -0.09 ± 0.07 to Δ -0.07 ± 0.14 s, P = 0.606). Dexmedetomidine reduced baroreflex gain, most strongly for medium-sized APs (normalized cluster 2: -6.0 ± 5 to -1.6 ± 2 % mmHg-1; P = 0.008). These data suggest that α2-adrenergic mechanisms within the central nervous system modulate sympathetic postganglionic neuronal discharge, recruitment and latency strategies in humans. KEY POINTS: Sympathetic postganglionic neuronal subpopulations innervating the human circulation exhibit complex patterns of discharge, recruitment and latency. However, the central neural mechanisms governing sympathetic postganglionic discharge remain unclear. This microneurographic study investigated the impact of a dexmedetomidine infusion (α2-adrenergic receptor agonist) on muscle sympathetic postganglionic action potential (AP) discharge, recruitment and latency patterns. Dexmedetomidine infusion inhibited the recruitment of large and fast conducting sympathetic APs and attenuated the discharge of medium sized sympathetic APs that fired during resting conditions and the Valsalva manoeuvre. Dexmedetomidine infusion elicited shorter sympathetic AP latencies during resting conditions but did not affect the reductions in latency that occurred during the Valsalva manoeuvre. These data suggest that α2-adrenergic mechanisms within the central nervous system modulate sympathetic postganglionic neuronal discharge, recruitment and latency strategies in humans.

Advancements in the neurocirculatory reflex response to hypoxia

Hypoxia is a pivotal factor in the pathophysiology of various clinical conditions, including obstructive sleep apnea, which has a strong association with cardiovascular diseases like hypertension, posing significant health risks. Although the precise mechanisms linking hypoxemia-associated clinical conditions with hypertension remains incompletely understood, compelling evidence suggests that hypoxia induces plasticity of the neurocirculatory control system. Despite variations in experimental designs and the severity, frequency, and duration of hypoxia exposure, evidence from animal and human models consistently demonstrates the robust effects of hypoxemia in triggering reflex-mediated sympathetic activation. Both acute and chronic hypoxia alters neurocirculatory regulation and, in some circumstances, leads to sympathetic outflow and elevated blood pressures that persist beyond the hypoxic stimulus. Dysregulation of autonomic control could lead to adverse cardiovascular outcomes and increase the risk of developing hypertension.

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

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

Action potential amplitude and baroreflex resetting of action potential clusters mediate hypoxia-induced sympathetic long-term facilitation

KEY POINTS

Acute isocapnic hypoxia resets the arterial baroreflex and permits long-lasting sympathoexcitation called sympathetic long-term facilitation. Our understanding of sympathetic long-term facilitation following hypoxia in humans is based on multiunit muscle sympathetic nerve activity and does not fully characterize the underlying baroreflex control of sympathetic neuronal subpopulations or their discharge/recruitment strategies. We show that sympathetic long-term facilitation is mediated by baroreflex resetting of sympathetic action potential clusters to higher arterial pressure operating points, a reduction in the percentage of action potentials firing asynchronously, and a shift toward larger amplitude action potential activity. The results advance our fundamental understanding of how the sympathetic nervous system mediates sympathetic long-term facilitation following exposure to acute isocapnic hypoxia in humans.

ABSTRACT

Baroreflex resetting permits sympathetic long-term facilitation (sLTF) following hypoxia; however, baroreflex control of action potential (AP) clusters and AP recruitment patterns facilitating sLTF is unknown. We hypothesized that baroreflex resetting of arterial pressure operating points (OPs) of AP clusters and recruitment of large-amplitude APs would mediate sLTF following hypoxia. Eight men (age: 24 (3) yrs; BMI: 24 (3) kg/m² ) underwent 20-min isocapnic hypoxia (P_ET O₂ : 47 (2) mmHg) and 30-min recovery. Multi-unit microneurography (muscle sympathetic nerve activity; MSNA) and a continuous wavelet transform with matched mother wavelet was used to detect sympathetic APs during baseline, hypoxia, early (first 5-min), and late recovery (last 5-min). AP amplitude (normalized to largest baseline AP amplitude), percent APs occurring outside a MSNA burst (% asynchronous APs), and proportion of APs firing in small (1-3), medium (4-6), and large (7-10) normalized cluster sizes was calculated. Normalized clusters were used to assess baroreflex OPs and sensitivity. Hypoxia increased total MSNA activity, which remained elevated during recovery (P<0.0001). Baroreflex OPs were shifted rightward for all clusters in recovery, with no effect on slope. Compared to baseline, AP amplitude was elevated by 3 (2) % and 4 (2) % while asynchronous APs were reduced by 9 (5) % and 7 (6) % in early and late recovery, respectively. In early recovery, the proportion of APs firing in large clusters was increased compared to baseline. Hypoxia-induced sLTF is mediated by baroreflex resetting of AP clusters to higher OPs, reduced asynchronous AP firing, and increased contribution from large-amplitude APs. Abstract figure legend Eight healthy men underwent 20-min isocapnic hypoxia and 30-min recovery. The study tested the hypothesis that baroreflex resetting of arterial pressure operating points (OPs) of action potential (AP) clusters and recruitment of large-amplitude APs would mediate sympathetic long-term facilitation (sLTF) following acute hypoxic exposure. Hypoxia increased multi-unit muscle sympathetic nerve activity (MSNA; measured via microneurography), which remained elevated throughout recovery. Sympathetic APs were detected in the filtered MSNA neurogram using a continuous wavelet transform with matched mother wavelet. An effect of condition revealed that compared to baseline, AP amplitude was elevated while asynchronous APs were reduced in early and late recovery, respectively. Our findings show that AP amplitude distributions are shifting towards larger AP amplitudes in all subjects following hypoxia. Our findings indicate that hypoxia-induced sLTF is mediated by baroreflex resetting of AP clusters to higher OPs, reduced asynchronous AP firing, and increased contribution from large-amplitude APs. This article is protected by copyright. All rights reserved.

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.

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.

Muscle sympathetic single-unit response patterns during progressive muscle metaboreflex activation in young healthy adults

Muscle sympathetic single units can respond differentially to stress, but whether these responses are linked to the degree of sympathoexcitation is unclear. Fifty-three muscle sympathetic single units (microneurography) were recorded in 17 participants (8 women; 24 ± 3 yr). Five 40-s bouts of 10% static handgrip were performed during a 10-min forearm ischemia to progressively increase metabolite accumulation. Each static handgrip was separated by a 75-s ischemic rest [postexercise circulatory occlusion (PECO)] to assess the isolated action of the muscle metaboreflex. During each set of PECO, individual single units were classified as activated, nonresponsive, or inhibited if the spike frequency was above, within, or below the baseline variability, respectively. From sets 1-5 of PECO, the proportion of single units with activated (34, 45, 68, 87, and 89%), nonresponsive (43, 44, 23, 7, and 9%), or inhibited (23, 11, 9, 6, and 2%) responses changed (P < 0.001) as total muscle sympathoexcitation increased. A total of 51/53 (96%) single units were activated in at least one set of PECO, 16 (31%) initially inhibited before activation. This response pattern delayed the activation onset compared with noninhibited units (set 3 ± 1 vs. 2 ± 1, P < 0.001). Once activated, the spike-frequency rate of rise was similar (8.5 ± 6.5 vs. 7.1 ± 6.0 spikes/min per set, P = 0.48). Muscle sympathetic single-unit firing demonstrated differential control during muscle metaboreflex activation. Single units that were initially inhibited during progressive metaboreflex activation were capable of being activated in later sets. These findings reveal that single-unit activity is influenced by convergent neural inputs (i.e., both inhibitory and excitatory), which yield heterogenous single-unit activation thresholds.NEW & NOTEWORTHY Muscle sympathetic single units respond differentially to sympathoexcitatory stress such that single units can increase firing to contribute to the sympathoexcitatory response or can be nonresponsive or even inhibited. We observed a subgroup of single units that can respond bidirectionally, being first inhibited before activated by progressive increases in forearm muscle metaboreflex activation. These results suggest convergent neural inputs (i.e., inhibitory and excitatory), which yield heterogenous muscle sympathetic single-unit activation thresholds.

Sympathetic neural recruitment strategies following acute intermittent hypoxia in humans

We examined the effect of acute intermittent hypoxia (IH) on sympathetic neural firing patterns and the role of the carotid chemoreceptors. We hypothesized exposure to acute IH would increase muscle sympathetic nerve activity (MSNA) via an increase in action potential (AP) discharge rates and within-burst firing. We further hypothesized any change in discharge patterns would be attenuated during acute chemoreceptor deactivation (hyperoxia). MSNA (microneurography) was assessed in 17 healthy adults (11 male/6 female; 31 ± 1 yr) during normoxic rest before and after 30 min of experimental IH. Prior to and following IH, participants were exposed to 2 min of 100% oxygen (hyperoxia). AP patterns were studied from the filtered raw MSNA signal using wavelet-based methodology. Compared with baseline, multiunit MSNA burst incidence (P < 0.01), AP incidence (P = 0.01), and AP content per burst (P = 0.01) were increased following IH. There was an increase in the probability of a particular AP cluster firing once (P < 0.01) and more than once (P = 0.03) per burst following IH. There was no effect of hyperoxia on multiunit MSNA at baseline or following IH (P > 0.05); however, hyperoxia following IH attenuated the probability of particular AP clusters firing more than once per burst (P < 0.01). Acute IH increases MSNA by increasing AP discharge rates and within-burst firing. A portion of the increase in within-burst firing following IH can be attributed to the carotid chemoreceptors. These data advance the mechanistic understanding of sympathetic activation following acute IH in humans.

Heterogeneous baroreflex control of sympathetic action potential subpopulations in humans

KEY POINTS

Emission patterns in muscle sympathetic nerve activity stem from differently sized action potential (AP) subpopulations that express varying discharge probabilities. The mechanisms governing these firing behaviours are unclear. This study investigated the hypothesis that the arterial baroreflex exerts varying control over the different AP subpopulations. During baseline, medium APs expressed the greatest baroreflex slopes, while small and large APs exhibited weaker slopes. On going from baseline to lower body negative pressure (LBNP; simulated orthostatic stress), baroreflex slopes for some clusters of medium APs expressed the greatest increase, while slopes for large APs also increased but to a lesser degree. A subpopulation of previously silent larger APs was recruited with LBNP but these APs expressed weak baroreflex slopes. The arterial baroreflex heterogeneously regulates sympathetic AP subpopulations, exerting its strongest effect over medium APs. Weak baroreflex mechanisms govern the recruitment of latent larger AP subpopulations during orthostatic stress.

ABSTRACT

Muscle sympathetic nerve activity (MSNA) occurs primarily in bursts of action potentials (AP) with subpopulations that differ in size and discharge probabilities. The mechanisms determining these discharge patterns remain unclear. This study investigated the hypothesis that variations in AP discharge are due to subpopulation-specific baroreflex control. We employed multi-unit microneurography and a continuous wavelet analysis approach to extract sympathetic APs in 12 healthy individuals during baseline (BSL) and lower body negative pressure (LBNP; -40, -60, -80 mmHg). For each AP cluster, the baroreflex threshold slope was measured from the linear regression between AP probability (%) and diastolic blood pressure (mmHg). During BSL, the baroreflex exerted non-uniform regulation over AP subpopulations: medium-sized AP clusters expressed the greatest slopes while clusters of small and large APs expressed weaker slopes. On going from BSL to LBNP, the baroreflex slopes for each AP subpopulation were modified differently. Baroreflex slopes (%/mmHg) for some medium APs (cluster 5: -4.4 ± 4 to -9.1 ± 5) expressed the greatest increase with LBNP, while slopes for large APs (cluster 9: -1.3 ± 1 to -2.6 ± 2) also increased, but to a lesser degree. Slopes for small APs present at BSL exhibited reductions with LBNP (cluster 2: -3.9 ± 3 to -2.2 ± 3). Larger previously silent AP clusters recruited with LBNP expressed weak baroreflex regulation (cluster 14: -0.9 ± 1%/mmHg). The baroreflex exerts the strongest control over medium-sized APs. Augmenting baroreflex gain and upward resetting of discrete AP subpopulations active at BSL, as well as recruiting larger previously silent APs with weak baroreflex control, facilitates elevated MSNA during orthostatic stress.

Asynchronous action potential discharge in human muscle sympathetic nerve activity

What strategies are employed by the sympathetic system to communicate with the circulation? Muscle sympathetic nerve activity (MSNA) occurs in bursts of synchronous action potential (AP) discharge, yet whether between-burst asynchronous AP firing exists remains unknown. Using multiunit microneurography and a continuous wavelet transform to isolate APs, we studied AP synchronicity within human MSNA. Asynchronous APs were defined as those which occurred between bursts. Experiment 1 quantified AP synchronicity in eight individuals at baseline (BSL), -10 mmHg lower body negative pressure (LBNP), -40 mmHg LBNP, and end-expiratory apnea (APN). At BSL, 33 ± 12% of total AP activity was asynchronous. Asynchronous discharge was unchanged from BSL (67 ± 37 AP/min) to -10 mmHg LBNP (69 ± 33 AP/min), -40 mmHg LBNP (83 ± 68 AP/min), or APN (62 ± 39 AP/min). Across all conditions, asynchronous AP probability and frequency decreased with increasing AP size. Experiment 2 examined the impact of the ganglia on AP synchronicity by using nicotinic blockade (trimethaphan). The largest asynchronous APs were derecruited from BSL (11 ± 4 asynchronous AP clusters) to the last minute of the trimethaphan infusion with visible bursts (7 ± 2 asynchronous AP clusters). However, the 6 ± 2 smallest asynchronous AP clusters could not be blocked by trimethaphan and persisted to fire 100 ± 0% asynchronously without forming bursts. Nonnicotinic ganglionic mechanisms affect some, but not all, asynchronous activity. The fundamental behavior of human MSNA contains between-burst asynchronous AP discharge, which accounts for a considerable amount of BSL activity.NEW & NOTEWORTHY Historically, sympathetic nerve activity destined for the blood vessels supplying skeletal muscle (MSNA) has been characterized by spontaneous bursts formed by synchronous action potential (AP) discharge. However, this study found a considerable amount (~30% during baseline) of sympathetic AP discharge to fire asynchronously between bursts of human MSNA. Trimethaphan infusion revealed that nonnicotinic ganglionic mechanisms contribute to some, but not all, asynchronous discharge. Asynchronous sympathetic AP discharge represents a fundamental behavior of MSNA.

Aging Alters the Relative Contributions of the Sympathetic and Parasympathetic Nervous System to Blood Pressure Control in Women

Autonomic support of blood pressure increases with age in humans. Large differences exist in the dose of trimethaphan (TMP) required for ganglionic blockade in young and older women. We asked whether differences in the dose of TMP required to achieve ganglionic blockade are because of differences in the relative contributions of the sympathetic and parasympathetic nervous system in control of blood pressure with age. Muscle sympathetic nerve activity (microneurography, peroneal nerve), heart rate (HR), and blood pressure were recorded before and during incremental doses of TMP camsylate until ganglionic blockade was achieved (absence of muscle sympathetic nerve activity and <5-bpm increase in HR during a valsalva maneuver; final TMP dose, 1-7 mg/min). HR variability was analyzed from the ECG waveform (WinCPRS). The dose of TMP required to achieve ganglionic blockade is positively related to basal HR variability, where women with high HR variability require a higher dose of TMP to achieve ganglionic blockade. In contrast, baseline muscle sympathetic nerve activity is inversely related with the dose of TMP required to achieve ganglionic blockade, such that women with high basal muscle sympathetic nerve activity required a lower dose of TMP. As such, the change in HR with ganglionic blockade was positively related, and the change in mean arterial pressure was inversely related, with the dose of TMP required to achieve ganglionic blockade. These data suggest loss of parasympathetic tone and increased sympathetic tone with aging contribute to the increase in blood pressure with age in women and dictate the dose of TMP that is necessary to achieve ganglionic blockade.