Recruitment strategies in efferent sympathetic nerve activity

Aging in females is associated with changes in respiratory modulation of sympathetic nerve activity and blood pressure

Sympathetic nerve activity (SNA) is tightly coupled with the respiratory cycle. In healthy human males, respiratory modulation of SNA does not change with age. However, it is unclear how this modulation is affected by age in females. We investigated whether respiratory sympathetic modulation is altered in healthy postmenopausal (PMF) versus premenopausal female (YF), and younger male (YM) adults, and determined its relationship to resting blood pressure. Muscle SNA (MSNA; microneurography), respiration (transducer belt), ECG, and continuous blood pressure were measured in 12 YF, 13 PMF, and 12 YM healthy volunteers. Respiratory modulation of MSNA was quantified during two phases of the respiratory cycle: mid-late expiration and inspiration/postinspiration. All groups showed respiratory modulation of MSNA (P < 0.0005). There was an interaction between the respiratory phase and group for MSNA [bursts/100 heartbeats (HB) (P = 0.004) and bursts/min (P = 0.029)], with smaller reductions in MSNA during inspiration observed in PMF versus the other groups. Respiratory modulation of blood pressure was also reduced in PMF versus YF (6 [2] vs. 12 [9] mmHg, P = 0.008) and YM (13 [13] mmHg, P = 0.001, median [interquartile range]). The magnitude of respiratory sympathetic modulation was related to resting blood pressure in PMF only, such that individuals with less modulation had greater resting blood pressure. The data indicate that aging in postmenopausal females is associated with less inspiratory inhibition of MSNA. This correlated with a higher resting blood pressure in PMF only. Thus, the reduced modulation of MSNA could contribute to the age-related rise in blood pressure that occurs in females.NEW & NOTEWORTHY The current study demonstrates that respiratory modulation of sympathetic nerve activity (SNA) is reduced in healthy postmenopausal (PMF) versus premenopausal females (YF). Furthermore, respiratory sympathetic modulation was negatively related to resting blood pressure in postmenopausal females, such that blood pressure was greater in individual with less modulation. Reduced respiratory sympathetic modulation may have implications for the autonomic control of blood pressure in aging postmenopausal females, by contributing to age-related sympathetic activation and reducing acute, respiratory-linked blood pressure variation.

Greater resting muscle sympathetic nerve activity reduces cold pressor autonomic reactivity in older women, but not older men

Previous work demonstrates augmented muscle sympathetic nerve activity (MSNA) responses to the cold pressor test (CPT) in older women. Given its interindividual variability, however, the influence of baseline MSNA on CPT reactivity in older adults remains unknown. Sixty volunteers (60-83y; 30 women) completed testing where MSNA (microneurography), blood pressure (BP), and heart rate (HR) were recorded during baseline and a 2-min CPT (~4°C). Participant data were terciled by baseline MSNA (n=10/group); comparisons were made between the high baseline men (HM) and women (HW), and low baseline men (LM) and women (LW). By design, HM and HW, vs. LM and LW, had greater baseline MSNA burst frequency (37±5 and 38±3 vs. 9±4 and 15±5 bursts/min) and burst incidence (59±14 and 60±8 vs. 16±10 and 23±7 bursts/100hbs; both P<0.001). However, baseline BP and HR were not different between the groups (all P>0.05). During the CPT, there were no differences in the increase in BP and HR (all P>0.05). Conversely, ΔMSNA burst frequency was lower in HW vs. LW (8±9 vs. 22±12 bursts/min; P=0.012) yet was similar in HM vs. LM (17±12 vs. 19±10 bursts/min, P=0.994). Further, ΔMSNA burst incidence was lower in HW vs. LW (9±13 vs. 28±16 bursts/100hbs; P=0.020), with no differences between HM vs. LM (21±17 vs. 31±17 bursts/100hbs; P=0.455). Our findings suggest that heightened baseline activity in older women attenuates the typical CPT-mediated increase in MSNA without changing cardiovascular reactivity. While the underlying mechanisms remain unknown, altered sympathetic recruitment or neurovascular transduction may contribute to these disparate responses.

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

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

Relationships between muscle sympathetic nerve activity and novel indices of arterial stiffness using single oscillometric cuff in patients with hypertension

The arterial velocity pulse index (AVI) and arterial pressure-volume index (API) have been proposed as new arterial stiffness indices that can be measured using an oscillometric cuff. Sympathetic nerve activity (SNA) contributes to arterial stiffness via increasing vascular smooth muscle tone. However, the associations between SNA and the AVI or API are not understood. The purpose of this study was to evaluate the relationships between muscle sympathetic nerve activity (MSNA) and the AVI or API in healthy individuals and patients with hypertension (HT). Forty healthy individuals (40.1 ± 15.2 years, 8 females) (healthy group) and 40 patients with HT (60.2 ± 13.6, 18 females) (HT group) were included in this study. The AVI, API, MSNA, beat-by-beat blood pressure, and heart rate were recorded simultaneously. The AVI and API were higher in the HT group than in the healthy group (AVI, 26.1 ± 7.6 vs. 16.5 ± 4.0, p < 0.001; API, 31.2 ± 8.6 vs. 25.5 ± 7.2, p = 0.002). MSNA in the HT group was also higher than in the healthy group (p < 0.001). MSNA was correlated with the AVI, but not with the API, in both the healthy group (R = 0.52, p = 0.001) and HT group (R = 0.57, p < 0.001). MSNA was independently correlated with the AVI in multivariate analysis (ß = 0.34, p = 0.001). In conclusion, AVI, obtained by a simple and less user-dependent method, was related to the MSNA in healthy individuals and patients with HT.

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.

Muscle sympathetic single-unit responses during rhythmic handgrip exercise and isocapnic hypoxia in males: the role of sympathoexcitation magnitude

A small proportion of postganglionic muscle sympathetic single units can be inhibited during sympathoexcitatory stressors in humans. However, whether these responses are dependent on the specific stressor or the level of sympathoexcitation remains unclear. We hypothesize that, when matched by sympathoexcitatory magnitude, different stressors can evoke similar proportions of inhibited single units. Multiunit and single-unit muscle sympathetic nerve activity (MSNA) were recorded in seven healthy young males at baseline and during 1) rhythmic handgrip exercise (40% of maximum voluntary contraction) and 2) acute isocapnic hypoxia (partial pressure of end-tidal O₂ 47 ± 3 mmHg). Single units were classified as activated, nonresponsive, or inhibited if the spike frequency was above, within, or below the baseline variability, respectively. By design, rhythmic handgrip and isocapnic hypoxia similarly increased multiunit total MSNA [Δ273 ± 208 vs. Δ254 ± 193 arbitrary units (AU), P = 0.84] and single-unit spike frequency (Δ8 ± 10 vs. Δ12 ± 13 spikes/min, P = 0.12). Among 19 identified single units, the proportions of activated (47% vs. 68%), nonresponsive (32% vs. 16%), and inhibited (21% vs. 16%) single units were not different between rhythmic handgrip and isocapnic hypoxia (P = 0.42). However, only 9 (47%) single units behaved with concordant response patterns across both stressors (7 activated, 1 nonresponsive, and 1 inhibited during both stressors). During the 1-min epoch with the highest increase in total MSNA during hypoxia (Δ595 ± 282 AU, P < 0.01) only one single unit was inhibited. These findings suggest that the proportions of muscle sympathetic single units inhibited during stress are associated with the level of sympathoexcitation and not the stressor per se in healthy young males.NEW & NOTEWORTHY Subpopulations of muscle sympathetic single units can be inhibited during mild sympathoexcitatory stress. We demonstrate that rhythmic handgrip exercise and isocapnic hypoxia, when matched by multiunit sympathoexcitation, induce similar proportions of single-unit inhibition, highlighting that heterogeneous single-unit response patterns are related to the level of sympathoexcitation independent of the stressor type. Interestingly, only 47% of single units behaved with concordant response patterns between stressors, suggesting the potential for functional specificity within the postganglionic neuronal pool.

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.

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.

Effect of varying chemoreflex stress on sympathetic neural recruitment strategies during apnea

We sought to examine the effect of varying chemoreflex stress on sympathetic neural recruitment strategies during end-expiratory apnea. We hypothesized that increases in the firing frequency and probability of low-threshold axons at the asphyxic "break point" would be exaggerated during hypoxia and attenuated during hyperoxia. Multiunit muscle sympathetic nervous system activity (MSNA) (peroneal nerve microneurography) was measured in 10 healthy male subjects (31 ± 2 yr, 25 ± 1 kg/m²). Individuals completed maximal voluntary end-expiratory apnea under normoxic, hypoxic (inspired O₂ fraction: 0.17 ± 0.01), and hyperoxic (inspired O₂ fraction: 0.92 ± 0.03) conditions. Action potential (AP) patterns were examined from the filtered raw signal with wavelet-based methodology. Multiunit MSNA was increased (P ≤ 0.05) during normoxic apnea, because of an increase in the frequency and incidence of AP spikes (243 ± 75 to 519 ± 134 APs/min, P = 0.048; 412 ± 133 to 733 ± 185 APs/100 heartbeats, P = 0.02). Multiunit MSNA increased from baseline (P < 0.01) during hypoxic apnea, which was due to an increase in the frequency and incidence of APs (192 ± 59 to 952 ± 266 APs/min, P < 0.01; 326 ± 89 to 1,212 ± 327 APs/100 heartbeats, P < 0.01). Hypoxic apnea also resulted in an increase in the probability of a particular AP cluster firing more than once per burst (P < 0.01). Hyperoxia attenuated any increase in MSNA with apnea, such that no changes in multiunit MSNA or frequency or incidence of AP spikes were observed (P > 0.05). We conclude that increases in frequency and incidence of APs during apnea are potentiated during hypoxia and suppressed when individuals are hyperoxic, highlighting the important impact of chemoreflex stress in AP discharge patterns. The results may have implications for neural control of the circulation in recreational activities and/or clinical conditions prone to apnea.NEW & NOTEWORTHY Our results demonstrate that, compared with normoxic end-expiratory apnea, hypoxic apnea increases the frequency and incidence of action potential spikes as well as the probability of multiple firing. We further show that this response is suppressed when individuals are hyperoxic. These data highlight the potentially important role of chemoreflex stress in neural firing and recruitment and may have implications for neural control of the circulation in recreational and/or clinical conditions prone to apnea.

Microneurography and sympathetic nerve activity: a decade-by-decade journey across 50 years

The technique of microneurography has advanced the field of neuroscience for the past 50 years. While there have been a number of reviews on microneurography, this paper takes an objective approach to exploring the impact of microneurography studies. Briefly, Web of Science (Thomson Reuters) was used to identify the highest citation articles over the past 50 years, and key findings are presented in a decade-by-decade highlight. This includes the establishment of microneurography in the 1960s, the acceleration of the technique by Gunnar Wallin in the 1970s, the international collaborations of the 1980s and 1990s, and finally the highest impact studies from 2000 to present. This journey through 50 years of microneurographic research related to peripheral sympathetic nerve activity includes a historical context for several of the laboratory interventions commonly used today (e.g., cold pressor test, mental stress, lower body negative pressure, isometric handgrip, etc.) and how these interventions and experimental approaches have advanced our knowledge of cardiovascular, cardiometabolic, and other human diseases and conditions.

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.