Clusters Of Action Potentials Research Articles

The effects of biological sex and oral contraception on the sympathetic neurocirculatory adjustments to static handgrip exercise in humans.

Biological sex affects the neurocirculatory adjustments to fatiguing exercise. However, the effects of sex and oral contraceptive pills (OCPs) on sympathetic action potential (AP) emission patterns and the transduction of AP discharge into vasoconstriction during static handgrip (SHG) exercise remains unknown. We tested the hypothesis that males, and females using OCPs, would demonstrate larger increases in sympathetic activation and sympathetic vascular transduction compared to naturally menstruating females during SHG and post-exercise circulatory occlusion (PECO). Young males (n=14, 25[5] yr), females using OCPs (n=16, 24[6] yr), and naturally menstruating females (n=18, 26[4] yr) underwent assessments of multi-unit muscle sympathetic nerve activity (MSNA)/AP discharge patterns (microneurography), and femoral artery blood flow (ultrasound) during fatiguing SHG at 40% maximum voluntary contraction and 2-min PECO. Sympathetic vascular transduction was determined as the quotient of the change in leg vascular conductance (LVC) and MSNA/AP discharge. Males demonstrated greater increases in APs/burst (P=0.028) and total AP clusters (P=0.008) compared to naturally menstruating females only, but not those using OCPs during exercise. Sympathetic vascular transduction of MSNA burst amplitude, APs/burst, and total AP clusters was greater in males and females using OCPs compared to naturally menstruating females (range: P=0.004-0.044). In contrast, no group differences in AP discharge were observed during PECO (range: P=0.510-0.872), and AP discharge was not related to LVC during PECO (range: P=0.08-0.949). These data indicate that biological sex and OCP use impact the central generation of efferent AP discharge, as well as the transduction of these neuronal messages into peripheral vasoconstriction during static exercise.

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.

The interactive effects of age and sex on the neuro-cardiovascular responses during fatiguing rhythmic handgrip exercise.

The impact of age on exercise pressor responses is equivocal, likely because of sex-specific neuro-cardiovascular changes with age. However, assessments of the interactive effects of age and sex on muscle sympathetic nerve activity (MSNA) responses to exercise are lacking. We tested the hypothesis that older females would exhibit exaggerated increases in blood pressure (BP) and MSNA discharge patterns during handgrip exercise compared with similarly aged males and young adults. Twenty-five young (25 (2) years; mean (SD)) males (YM; n=12) and females (YF; n=13) and 23 older (71 (5) years) males (OM; n=11) and females (OF; n=12) underwent assessments of BP, total peripheral resistance (TPR; Modelflow) and MSNA action potential (AP) discharge patterns (microneurography) during incremental rhythmic handgrip exercise and post-exercise circulatory occlusion (PECO). OM demonstrated larger ∆BP and ∆TPR from baseline than YM (both P < 0.001) despite smaller increases in ∆APs/burst (OM: 0.4 (3) vs. YM: 5 (3) spikes/burst, P < 0.001) and ∆AP clusters/burst (OM: 0.1 (1) vs. YM: 1.8 (1) clusters/burst, P < 0.001) during exercise. Testosterone was lower in OM than YM (P < 0.001) and was inversely related to ∆BP but positively related to ∆AP clusters/burst in males (both P=0.03). Conversely, YF and OF demonstrated similar ∆BP and ∆AP discharge during exercise (range: P=0.75-0.96). Age and sex did not impact haemodynamics or AP discharge during PECO (range: P=0.08-0.94). Altogether, age-related changes in neuro-cardiovascular reactivity exist in males but not females during fatiguing exercise and seem to be related to testosterone. This sex-specific impact of age underscores the importance of considering biological sex when assessing age-related changes in neuro-cardiovascular control during exercise. KEY POINTS: Older males have the largest increase in blood pressure despite having the smallest increases in sympathetic vasomotor outflow during rhythmic handgrip exercise. Young males demonstrate greater increases in sympathetic action potential (AP) discharge compared with young females during rhythmic handgrip exercise. Older adults (regardless of sex) demonstrate smaller increases in muscle sympathetic nerve activity (MSNA) burst amplitude and total AP clusters compared with young adults during exercise, as well as smaller increases in integrated MSNA burst frequency, incidence and total MSNA activity during post-exercise circulatory occlusion (i.e. independent effect of age). Males, but not females (regardless of age), reflexively modify AP conduction velocity during exercise. Our results indicate that age and sex independently and interactively impact the neural and cardiovascular homeostatic adjustments to fatiguing small muscle mass exercise.

Central alpha-2 adrenergic mechanisms affect human sympathetic neuronal discharge strategies

The central neural mechanisms affecting sympathetic neuronal discharge strategies supporting homeostatic adjustments in human muscle sympathetic nerve activity (MSNA) remain unclear. This study investigated the hypothesis that central alpha-2 adrenergic mechanisms affect sympathetic action potential (AP) discharge, recruitment, and latency patterns within human MSNA (peroneal nerve microneurography and continuous wavelet transform). An alpha-2 adrenergic agonist (dexmedetomidine) was infused in eight healthy male and female participants (5 females; 28 ± 7years). Data were recorded during a 5-minute resting control condition, a 10-minute intravenous dexmedetomidine loading dose (0.225 μg/kg), and a dexmedetomidine maintenance dose (~0.1 μg/kg/hr). Data are reported as mean (SD) for the 5-minute control condition and 2-minutes of the dexmedetomidine infusion when MSNA bursting activity was demonstrably reduced. Two-tailed paired t-tests were performed. Dexmedetomidine reduced mean pressure (92 ± 7 to 80 ± 8 mmHg; P &lt; 0.01) but did not affect heart rate (61 ± 13 to 60 ± 14 bpm; P = 0.75). Dexmedetomidine reduced integrated MSNA burst frequency (14 ± 6 to 4 ± 3 bursts/min; P &lt; 0.01), sympathetic AP frequency (75 ± 46 to 16 ± 14 AP/min; P &lt; 0.01), APs per burst (5 ± 1 to 4 ± 1 AP/burst; P = 0.01), AP clusters per burst (3.5 ± 0.7 to 3.0 ± 0.7 clusters/burst; P = 0.03), and de-recruited the largest AP clusters (12 ± 3 to 7 ± 2 AP clusters; P &lt; 0.01). Despite de-recruiting large AP clusters with short conduction times, dexmedetomidine elicited shorter sympathetic AP conduction times (1.18 ± 0.12 to 1.14 ± 0.13 s; P = 0.03). Six participants performed a Valsalva Maneuver (20 s, 40 mmHg mouth pressure) during the control condition and during the dexmedetomidine maintenance dose. Data are reported as mean (SD) change (Δ) from the 2-minute period preceding the Valsalva Maneuver to the Valsalva Maneuver for each condition. During the control condition, Valsalva Maneuver increased AP frequency (Δ 311 ± 317 AP/min; P = 0.048), recruited larger previously silent AP clusters (Δ 2 ± 1 AP clusters; P &lt; 0.01), and elicited shorter AP conduction times (Δ ‑0.09 ± 0.07 s; P = 0.02). Compared to the control condition, Valsalva Maneuver performed during the dexmedetomidine maintenance dose condition elicited lesser increases in AP frequency (Δ 116 ± 189 AP/min; P vs. control = 0.02) and lesser recruitment of larger previously silent AP clusters (Δ 0 ± 1 AP clusters; P vs. control = 0.02). Dexmedetomidine did not affect the latency reduction elicited by Valsalva Maneuver (Δ ‑0.07 ± 0.14 s; P vs. control = 0.61). These data suggest that human central alpha-2 adrenergic mechanisms affect: i) sympathetic postganglionic neuronal discharge and recruitment patterns, and ii) synaptic delays within the sympathetic neurocircuitry influencing AP latency. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the National Institutes of Health (NIH). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Sex-specific impact of aging on muscle sympathetic neural discharge patterns during incremental rhythmic handgrip exercise

Sex-disparities exist in the risk of developing hypertension throughout the lifespan, with a greater prevalence of hypertension amongst postmenopausal females compared to similarly aged males. Though the underlying mechanisms are multifactorial, exaggerated sympathetic neuro-cardiovascular reactivity may be an important contributor. Indeed, postmenopausal females exhibit exaggerated exercise pressor responses compared to young adults, and older males. However, the interactive effects of age and sex on muscle sympathetic nerve activity (MSNA) and action potential (AP) coding patterns during exercise remains unclear. We hypothesized that older females would exhibit the greatest increase in MSNA and AP recruitment during exercise and post-exercise circulatory occlusion (PECO) relative to young males and females, as well as older males. MSNA and AP discharge patterns (microneurography and continuous wavelet transform) were assessed in 12 young males (YM (mean±SD); 26±4, years), 11 young females (YF; 25±4 years), 11 older males (OM; 71±11 years), and 12 older females (OF; 71±4 years) during incremental rhythmic handgrip exercise to fatigue followed by 2 minutes of PECO. At peak exercise, OM demonstrated a smaller change from baseline (Δ) in MSNA burst incidence (BI) compared to all other groups (YM: Δ8±9, YF: Δ9±8, OM: Δ-6±8, OF: Δ6±7 bursts/100heartbeats; all ANOVA post-hoc P&lt;0.05), whereas ΔMSNA burst frequency (BF), Δburst amplitude (BA), and Δtotal activity were not different between groups (all P&gt;0.05). Conversely, YM demonstrated greater ΔAPs/burst (YM: Δ5±3, OM: Δ0.4±3, OF: Δ2±2 APs/burst; post-hoc P&lt;0.05) and ΔAP clusters/burst (YM: Δ2±1, OM: Δ0.1±1, OF: Δ0.7±0.7 Clusters/burst; post-hoc P&lt;0.05) compared to OM and OF, but not compared to YF (Δ2±3 APs/burst and Δ0.9±0.8 clusters/burst; both P≥0.07). However, no group differences were observed in the recruitment of larger axons (YM: Δ6±2, YF: Δ4±2, OM: Δ2±5, OF: Δ4±3 clusters; P=0.33). Contrary to exercise, group-by-time interactions existed during PECO for ΔMSNA BF ( P&lt;0.01), ΔBI ( P&lt;0.01), and Δtotal activity ( P&lt;0.01) where greater increases occurred during the first minute of PECO in young compared to older adults (all post-hoc P&lt;0.05), whereas in the final minute of PECO, MSNA ΔBF and ΔBI were only greater in YM compared to OM and OF (all P&lt;0.05), and total activity was greater in YM and YF compared to OF only (all P&lt;0.05). No group-by-time interactions were observed for ΔMSNA BA ( P=0.37), ΔAPs/burst ( P=0.94), ΔAP clusters/burst ( P=0.95) or Δtotal AP clusters ( P=0.41) during PECO. Altogether, MSNA and AP reactivity during exercise was not exaggerated in OF, but age-related reductions in AP recruitment were observed in males. During PECO, MSNA responses were lower in older relative to young adults but, AP recruitment was unaltered by age. Thus, exaggerated MSNA or AP reactivity may not explain the greater prevalence of hypertension in OF. Supported by the Natural Sciences and Engineering Council of Canada, and IEEM Indirect Funds. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Baroreflex resetting of human sympathetic action potential subpopulations during exercise.

This study tested the hypothesis that during fatiguing volitional exercise in humans, descending cortical signals and ascending skeletal muscle metaboreflex signals exert divergent control over baroreflex resetting of sympathetic action potential (AP) discharge. We quantified the baroreflex gain for sympathetic AP clusters within the muscle sympathetic nerve activity neurogram (peroneal microneurography and continuous wavelet transform) during baseline (BSL), the first 2-minutes of a 5-minute isometric handgrip (20% of maximal effort; IHG1), the last 2-minutes of IHG (IHG2), and during post-exercise circulatory occlusion (PECO) in seven healthy participants. AP baroreflex threshold gain was measured as the slope of the linear relationship between AP probability (%) versus diastolic blood pressure (DBP; mmHg) for 10 normalized AP clusters. Compared to BSL, during IHG1, AP baroreflex threshold functions were only reset to greater DBP and baroreflex gain was unaffected. Compared to BSL, during IHG2 and PECO, baroreflex functions were reset to greater DBP and to greater AP firing probabilities, with medium-sized APs demonstrating the largest upward resetting (e.g., cluster 3 BSL: 26±7%, cluster 3 IHG2: 78±22%, cluster 3 PECO: 88±46%). Compared to BSL, AP baroreflex threshold gain was not different during IHG2 but was increased during PECO, with medium-sized APs demonstrating the largest increase in baroreflex gain (e.g., cluster 3 BSL: -6.31±3.1 %/mmHg, cluster 3 IHG2: ‑6.18±5.4 %/mmHg, cluster 3 PECO: ‑12.13±6.5 %/mmHg). These findings indicate that during IHG exercise, descending cortical signaling and ascending skeletal muscle metaboreceptor signals differentially affect baroreflex resetting of subpopulations of human muscle sympathetic postganglionic neurons.

Aging is associated with enhanced central but impaired peripheral arms of the sympathetic baroreflex arc.

We tested the hypotheses that spontaneous baroreflex control of integrated muscle sympathetic nerve activity (MSNA) burst occurrence and action potential (AP) subpopulations would be blunted in older compared with young adults and that sympathetic transduction will be blunted in older adults relative to young adults. Integrated muscle sympathetic nerve activity (MSNA) and the underlying sympathetic APs were obtained using microneurography and a continuous wavelet analysis approach, respectively, during 5 min of supine rest in 13 older (45-75 yr, 6 females) and 14 young (21-30 yr, 7 females) adults. Baroreflex threshold relationships were quantified as the slope of the linear regression between MSNA burst occurrence (%) and diastolic blood pressure (mmHg), or AP cluster firing probability (%) and diastolic blood pressure (mmHg). Integrated MSNA baroreflex threshold gain was greater in older compared with young adults (older: -5.7 ± 2.6%/mmHg vs. young: -2.7 ± 1.4%/mmHg, P < 0.001). Similarly, the baroreflex threshold gain of AP clusters was modified by aging (group-by-cluster effect: P < 0.001) such that older adults demonstrated greater baroreflex threshold gains of medium-sized AP clusters (e.g., Cluster 4, older: -8.2 ± 3.2%/mmHg vs. young: -3.6 ± 1.9%/mmHg, P = 0.003) but not for the smallest-sized (Cluster 1, older: -1.6 ± 1.9%/mmHg vs. young: -1.0 ± 1.7%/mmHg, P > 0.999) and largest-sized (Cluster 10, older: -0.5 ± 0.5%/mmHg vs. young: -0.2 ± 0.1%/mmHg, P = 0.819) AP clusters compared with young adults. In contrast, the peak change in mean arterial pressure (MAP) following a spontaneous MSNA burst (i.e., sympathetic transduction) was impaired with aging (older: -0.7 ± 0.3 mmHg vs. young: 1.8 ± 1.2 mmHg, P < 0.001). We conclude that aging is associated with elevated baroreflex control over high-probability AP content of sympathetic bursts that may compensate for impaired sympathetic neurovascular transduction.NEW & NOTEWORTHY The present study demonstrates for the first time that the spontaneous baroreflex threshold gains of integrated muscle sympathetic nerve activity burst occurrence and medium-sized action potential clusters are greater in older compared with young adults. Since sympathetic transduction was blunted in older compared with young adults, we interpret the data to indicate that the central arc of the baroreflex is enhanced in older adults to compensate for impairments in the peripheral arc.

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

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) years; body mass index: 24 (3) kg/m2 ) underwent 20min isocapnic hypoxia ( : 47 (2) mmHg) and 30min 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 5min), and late recovery (last 5min). AP amplitude (normalized to largest baseline AP amplitude), percentage APs occurring outside a MSNA burst (percentage 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. KEY POINTS: Acute isocapnic hypoxia resets the arterial baroreflex and permits long-lasting sympathoexcitation, termed 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.

The Effect of Menstrual Cycle Phase on Neural Recruitment Patterns during Acute Chemoreflex Stress

We have shown previously that muscle sympathetic nerve activity (MSNA) responses to acute chemoreflex stress are augmented during the early follicular (EF) phase of the menstrual cycle relative to the midluteal (ML) phase in young healthy women. Interestingly, these differences were most pronounced in terms of MSNA burst amplitude, which may imply that neural recruitment strategies are affected by menstrual cycle phase. Therefore, the purpose of this retrospective analysis was to test the hypothesis that the recruitment of large action potential (AP) clusters occurs to a greater extent during the EF phase of the menstrual cycle compared with the ML phase. MSNA data (microneurography) from 7 young healthy women (aged 23±3 years; body mass index 23±3 kg/m2) were examined at rest and during a voluntary end‐inspiratory apnea which followed a rebreathing protocol to an end‐tidal PO2 of 65 Torr. These data were collected during the EF and ML phases. APs were identified from the filtered MSNA signal using a custom wavelet deconstruction approach which quantified AP firing frequencies (overall AP frequency and AP content per integrated burst). The identified APs were also binned in accordance to peak to peak amplitude to determine individual AP clusters (Scott’s rule). Resting AP frequency and AP content per MSNA burst were not different between menstrual cycle phases (APs/min: P=0.23; APs/burst: P=0.26). Similarly, the number of identified AP clusters at rest did not differ between the EF and ML phases (8±4 vs10±4 clusters; P=0.16). During apnea, changes in AP frequency and AP content per MSNA burst (i.e. expressed relative to baseline) were greater during the EF phase than the ML phase (ΔAPs/min: P=0.02; ΔAPs/burst: P=0.02). Similarly, apnea‐induced recruitment of new AP clusters (i.e. clusters which were not activated at baseline) was greater during the EF phase compared to the ML phase (ΔAP clusters: +6±8 vs+2±6 clusters; P=0.009). Taken together, these findings confirm that augmented integrated MSNA reactivity to chemoreflex stress during the EF phase of the menstrual cycle is mediated by a greater recruitment of latent subpopulations of larger sympathetic neurons relative to the ML phase. We speculate that the change in the gonadal hormone profile between EF (i.e. low estradiol, progesterone) and ML (i.e. elevated estradiol and progesterone) may be associated with centrally‐mediated alterations to neural outflow during chemoreflex stress, although further research is certainly required to address this hypothesis.

Hypoxia‐induced sympathetic long‐term facilitation is mediated by a rightward shift in sympathetic action potential amplitude distribution and baroreflex resetting of action potential clusters

IntroductionBaroreflex resetting permits sympathetic long‐term facilitation (sLTF) following hypoxia. Muscle sympathetic nerve activity (MSNA) bursts are generated by synchronous discharge of varying‐amplitude action potentials (APs), with medium APs under strong baroreflex control. AP discharge strategies and baroreflex control of AP clusters facilitating sLTF is unknown. We hypothesized that recruitment of previously latent, large‐amplitude APs and baroreflex resetting of AP cluster operating points (OPs) would mediate sLTF following acute hypoxia.MethodsEight men (age = 24±3 yrs; BMI = 24±3 kg/m2) were exposed to 20 min isocapnic hypoxia (PETO2: 47±2 mmHg) and 30 min recovery. Blood pressure (BP; photoplethysmography) and MSNA (fibular microneurography) were acquired during baseline, hypoxia, early (first 5‐min) and late recovery (last 5‐min). Multi‐unit MSNA burst frequency (BF) and total activity (TA) were quantified. A continuous wavelet transform with matched mother wavelet was used to extract sympathetic APs. AP frequency, AP amplitude (normalized % of largest baseline AP amplitude), percent APs occurring outside a MSNA burst (% asynchronous APs) and total AP clusters was calculated. The proportion of APs firing in small (1‐3), medium (4‐6) and large (7‐10) normalized cluster sizes was assessed. Baroreflex OP was measured by plotting the intersection point of mean cluster incidence and mean diastolic BP (DBP). Friedman repeated‐measures analysis of variance on ranks was used to determine the effect of condition (baseline, hypoxia, early, late). Data are means ± standard deviation or 95% confidence intervals.ResultsHypoxia increased BF (P&lt;0.01), TA (P&lt;0.01), AP frequency (Δ124{‐30, 279} AP/min, P&lt;0.05), AP amplitude (Δ3{1, 5} %, P&lt;0.05) and decreased asynchronous APs (Δ‐10{‐17, ‐4} %, P&lt;0.03). Compared to baseline, BF (P&lt;0.03), TA (P&lt;0.02) and AP amplitude (early: Δ3{0, 5} %, P&lt;0.05; late: Δ4{1, 6} %, P&lt;0.05) was elevated during recovery while asynchronous APs (early: Δ‐9{‐16, ‐3} %, P&lt;0.03; late: Δ‐7{‐14, ‐1} %, P&lt;0.03) were reduced. The total number of AP clusters was increased (P&lt;0.05) with no one condition different compared to baseline (hypoxia: Δ3{‐1, 7} clusters; early and late: Δ3{‐1, 6} clusters, P&gt;0.10). Proportion of APs in small clusters was reduced in hypoxia (hypoxia: 44±18 %, P&lt;0.05), early (46±21 %, P&lt;0.05) and late recovery (44±17 %, P&lt;0.05) compared with baseline (53±20 %) while the proportion of APs in large clusters was increased in early recovery (7±6 %, P&lt;0.05) compared with baseline (5±5 %). Baroreflex OPs were shifted rightward for all AP clusters in recovery (baseline DBP: 63±5; early DBP: 64±5; late DBP: 65±4, mmHg; P&lt;0.05) with no effect on slope (P&gt;0.20).ConclusionsHypoxia‐induced sLTF is mediated by reduced asynchronous AP firing, a proportional shift toward large‐amplitude AP activity, and baroreflex resetting of AP clusters to higher OPs.

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 multiunit 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 with the ambient condition, LBNP increased AP discharge rate at -10 and -40 mmHg and recruited larger previously silent sympathetic neurons at -40 mmHg. Compared with spontaneous breathing, APN increased AP discharge when performed during the ambient condition (Δ351 ± 132 AP/min), -10 mmHg (Δ423 ± 184 AP/min), and -40 mmHg (Δ355 ± 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 (Δ4 ± 3; P < 0.02) and -10 mmHg (Δ4 ± 3; P < 0.01), but not -40 mmHg (Δ0 ± 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: Δ-0.04 ± 0.04s, -10 mmHg: Δ-0.03 ± 0.03s, -40 mmHg: Δ-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.NEW & NOTEWORTHY Discrete physiological stressors differentially affect sympathetic postganglionic neuronal rate-, population-, and temporal-coding strategies. When performing end-expiratory apnea (APN) during graded baroreflex unloading via lower body negative pressure (LBNP), we found: 1) augmented sympathetic axonal firing probability, 2) recruitment of larger and previously silent sympathetic postganglionic neurons at ambient and -10 mmHg, but not -40 mmHg LBNP, and 3) APN reduced axonal discharge latency similarly across all conditions, independent of the level of baroreflex unloading.

The impact of 6 months of exercise-based cardiac rehabilitation on sympathetic neural recruitment during apneic stress.

The current study evaluated the hypothesis that 6 mo of exercise-based cardiac rehabilitation (CR) would improve sympathetic neural recruitment in patients with ischemic heart disease (IHD). Microneurography was used to evaluate action potential (AP) discharge patterns within bursts of muscle sympathetic nerve activity (MSNA), in 11 patients with IHD (1 female; 61 ± 9 yr) pre (pre-CR) and post (post-CR) 6 mo of aerobic and resistance training-based CR. Measures were made at baseline and during maximal voluntary end-inspiratory (EI-APN) and end-expiratory apneas (EE-APN). Data were analyzed during 1 min of baseline and the second half of apneas. At baseline, overall sympathetic activity was less post-CR (all P < 0.01). During EI-APN, AP recruitment was not observed pre-CR (all P > 0.05), but increases in both within-burst AP firing frequency (Δpre-CR: 2 ± 3 AP spikes/burst vs. Δpost-CR: 4 ± 3 AP spikes/burst; P = 0.02) and AP cluster recruitment (Δpre-CR: -1 ± 2 vs. Δpost-CR: 2 ± 2; P < 0.01) were observed in post-CR tests. In contrast, during EE-APN, AP firing frequency was not different post-CR compared with pre-CR tests (Δpre-CR: 269 ± 202 spikes/min vs. Δpost-CR: 232 ± 225 spikes/min; P = 0.54), and CR did not modify the recruitment of new AP clusters (Δpre-CR: -1 ± 3 vs. Δpost-CR: 0 ± 1; P = 0.39), or within-burst firing frequency (Δpre-CR: 3 ± 3 AP spikes/burst vs. Δpost-CR: 2 ± 2 AP spikes/burst; P = 0.21). These data indicate that CR improves some of the sympathetic nervous system dysregulation associated with cardiovascular disease, primarily via a reduction in resting sympathetic activation. However, the benefits of CR on sympathetic neural recruitment may depend upon the magnitude of initial impairment.

Responses of spinal trigeminal neurons to noxious stimulation of paranasal cavities - a rat model of rhinosinusitis headache.

The pathophysiology of headaches associated with rhinosinusitis is poorly known. Since the generation of headaches is thought to be linked to the activation of intracranial afferents, we used an animal model to characterise spinal trigeminal neurons with nociceptive input from the dura mater and paranasal sinuses. In isoflurane anaesthetised rats, extracellular recordings were made from neurons in the spinal trigeminal nucleus with afferent input from the exposed frontal dura mater. Dural and facial receptive fields were mapped and the paranasal cavities below the thinned nasal bone were stimulated by sequential application of synthetic interstitial fluid, 40 mM potassium chloride, 100 µM bradykinin, 1% ethanol (vehicle) and 100 µm capsaicin. Twenty-five neurons with input from the frontal dura mater and responses to chemical stimulation of the paranasal cavities were identified. Some of these neurons had additional receptive fields in the parietal dura, most of them in the face. The administration of synthetic interstitial fluid, potassium chloride and ethanol was not followed by significant changes in activity, but bradykinin provoked a cluster of action potentials in 20 and capsaicin in 23 neurons. Specific spinal trigeminal neurons with afferent input from the cranial dura mater respond to stimulation of paranasal cavities with noxious agents like bradykinin and capsaicin. This pattern of activation may be due to convergent input of trigeminal afferents that innervate dura mater and nasal cavities and project to spinal trigeminal neurons, which could explain the genesis of headaches due to disorders of paranasal sinuses.

Abstract P103: Aberrant Sympathetic Neural Recruitment Strategies During Exercise Pressor Reflex Activation In Postmenopausal Women

Age-related elevations in sympathetic nerve activity contribute to numerous deleterious consequences for cardiovascular health, particularly in postmenopausal women (PMW). Aging adversely affects aspects of sympathetic nervous system (SNS) coding to control blood pressure at rest and during physiological stress. Although PMW exhibit greater neural activity at rest, the underlying sympathetic discharge patterns, particularly during sympathoexcitation, remain unknown. This study aimed to test the hypothesis that PMW display aberrant neural coding strategies during physiological stress compared to young women (YW). Efferent muscle sympathetic nerve activity (microneurography) was measured at baseline and during the final minute of both 30% isometric handgrip (HG) and post-exercise ischemia (PEI) in five healthy YW (22 ± 2 y, 20 ± 4 kg/m 2 , mean blood pressure: 82 ± 7 mmHg) and five healthy PMW (60 ± 5 y, 23 ± 3 kg/m 2 , 87 ± 8 mmHg). Sympathetic action potential (AP) discharge patterns were examined using wavelet-based methodology. At baseline, PMW exhibited elevated integrated burst frequency (PMW 30 ± 10 vs YW 9 ± 3 bursts/min; p &lt; 0.01) and increased AP firing (PMW 240 ± 117 vs YW 90 ± 64 APs/min, p = 0.04). Both groups had similar increases in integrated burst frequency during HG and PEI (p &lt; 0.01 vs baseline). AP firing was elevated to a similar extent in both groups during HG (PMW Δ76 ± 60; YW Δ129 ± 146 APs/min; p &lt; 0.01 vs baseline) and PEI (PMW Δ72 ± 51; YW Δ93 ± 64 APs/min; p &lt; 0.01 vs baseline), with no change in the mean AP content per burst (p = 0.86). PMW had reduced recruitment of latent and larger AP subpopulations that were previously silent at baseline (group-by-condition interaction: p &lt; 0.01), as evidenced by an increase in total AP clusters (binned according to peak-to-peak amplitude) during HG and PEI in YW (HG: Δ2 ± 1; PEI: Δ2 ± 2; both p ≤ 0.03 vs baseline) but not PMW (HG: Δ0 ± 1; PEI: Δ0 ± 1; both p ≥ 0.75 vs baseline). These preliminary data suggest that PMW display elevated resting sympathetic AP firing and aberrant reflex AP recruitment, such that the ability of the SNS to recruit subpopulations of previously silent axons during a sympathetic stressor is reduced in PWM compared to YW. These findings extend our knowledge of age-related neural coding strategies in women.

Case Studies in Physiology: Sympathetic neural discharge patterns in a healthy young male during end-expiratory breath hold-induced sinus pause.

This case study reports the efferent muscle sympathetic nerve activity (MSNA) discharge patterns during a sinus pause observed during a maximal end-expiratory apnea in a young healthy male (age = 26 yr). During a 15.3-s end-expiratory apnea following a bout of intermittent hypercapnic hypoxia, we observed a 5.2-s (R-R interval) sinus pause and integrated MSNA recording, demonstrating a square-wave discharge pattern atypical of sharp MSNA burst peaks entrained to cardiac cycles or during preventricular contractions. This abnormal MSNA discharge pattern was observed again during a follow-up experiment, where an end-expiratory apnea at baseline resulted in pronounced bradycardia (R-R intervals >2.5-s) but failed to reproduce the 5.2-s sinus pause. Action potential (AP) discharge patterns during MSNA bursts were detected using a continuous wavelet transform approach. AP discharge increased by 300% during the end-expiratory apnea with 5.2-s sinus pause compared with baseline and involved increased firing (i.e., rate-coding) of AP clusters (bins of AP with similar morphology) already present during baseline and pronounced recruitment of larger-amplitude AP clusters not present at baseline. Large-amplitude AP clusters continued to discharge during sinus pause. In summary, we show MSNA discharge during sinus pause and pronounced bradycardia during end-expiratory apnea, which demonstrates a square-wave discharge with recruitment of latent larger-amplitude AP clusters. The MSNA discharge was terminated before systole following sinus pause potentially through an inhibitory influence of inspiration, or cardiac mechanoreceptor feedback causing burst termination.NEW & NOTEWORTHY We characterize the occurrence of a square-wave discharge pattern of efferent muscle sympathetic nerve activity during a sinus pause in a young healthy male. This discharge pattern comprised large recruited action potential clusters undetected at baseline that continuously discharged during the sinus pause. Notably, this discharge pattern was still contained within a single cardiac cycle.

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.

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. 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, -80mmHg). 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.

Baroreflex Resetting of Sympathetic Action Potential Subpopulations During Fatiguing Isometric Handgrip and Post‐Exercise Circulatory Occlusion

The arterial baroreflex exerts heterogeneous control over the varying‐sized action potential (AP) subpopulations active within human muscle sympathetic nerve activity (MSNA) under baseline conditions. How baroreflex control of sympathetic AP discharge becomes reset to facilitate sympathoexcitation during physical exercise remains to be investigated. Therefore, we quantified the baroreflex gain for each sympathetic AP cluster (i.e., reflecting different c‐fibres) within the MSNA filtered neurogram (microneurography and continuous wavelet transform) during baseline conditions (BSL), the last 2‐min of a 5‐min fatiguing isometric handgrip (20% of max; IHG), and during post‐exercise circulatory occlusion (PECO) in six healthy young participants (3 females, 23–31 years). Baroreflex gain was measured as the slope of the linear function between AP probability (%) versus diastolic blood pressure (DBP; Finometer). We classified AP clusters based on peak‐to‐peak amplitude using the following allocations: small (clusters 1–2), medium (clusters 3–8) and large (clusters 9–15). During BSL, medium AP clusters (e.g., cluster 3: −9.4 ± 2.7 %/mmHg) expressed the greatest baroreflex gains while small (e.g., cluster 1: −1.9 ± 0.4 %/mmHg) and large (e.g., cluster 9: −2.4 ± 0.9 %/mmHg) clusters expressed lesser gains (both P &lt; 0.05 vs. cluster 3). Compared to BSL, baroreflex functions for most AP clusters during IHG and PECO were reset upwards to greater firing probabilities (both Condition‐by‐Cluster Interactions: P &lt; 0.05) and rightwards to higher DBP (BSL: 69 ± 4, IHG: 81 ± 3, PECO: 79 ± 2 mmHg; both P &lt; 0.05 vs. BSL). However, compared to BSL, the gains of the AP cluster baroreflex functions remained unchanged with IHG (e.g., cluster 1: −1.6 ± 0.4, cluster 3: −8.2 ± 2.9, cluster 9: −4.3 ± 1.8 %/mmHg, Main Effect: P &gt; 0.05). Nonetheless, AP cluster gains were augmented above BSL during PECO (e.g., cluster 1: −2.6 ± 1.0, cluster 3: −16.0 ± 4.4, cluster 9: −5.5 ± 1.8 %/mmHg; Main Effect: P &lt; 0.05). IHG (e.g., cluster 15: −1.6 ± 0.5 %/mmHg) and PECO (e.g., cluster 15: −0.6 ± 0.3 %/mmHg) recruited a subpopulation of previously silent larger and faster conducting APs that expressed weak baroreflex gains. These data suggest that during fatiguing isometric handgrip exercise the interaction between the central command feature and the skeletal muscle metaboreflex resets baroreflex control of discrete AP subpopulations to higher blood pressures and greater levels of discharge. In addition to resetting the baroreflex functions, independent activation of the muscle metaboreflex augments the strength of baroreflex control over subpopulations of previously active APs.Support or Funding InformationSupported by the Natural Sciences and Engineering Research Council (NSERC) of Canada.

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.

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.