Asynchronous Action Potential Discharge in Human Muscle Sympathetic Nerve Activity

Abstract

Pulse‐rhythmic bursts formed by synchronous action potential (AP) discharge characterises the firing behaviour of muscle sympathetic nerve activity (MSNA). However, complex AP discharge patterns related to multiple sites of control warrant an investigation of synchronicity in the human sympathetic system. Therefore, this study quantified the synchronicity of muscle sympathetic AP discharge in eight healthy individuals (4 females, 23 – 31 years, 170 ± 7 cm, 69 ± 13 kg, 60 ± 7 bpm, 89 ± 6 mmHg). MSNA (microneurography) was measured during baseline (BSL), −10 mmHg lower body negative pressure (LBNP), −40 mmHg LBNP, and an end‐expiratory apnea (APN; 30 ± 7 s). MSNA APs were detected from the filtered raw signal using a wavelet deconstruction approach and binned in clusters based on peak‐to‐peak amplitude (normalized within‐participant). An AP was considered to fire synchronously: 1) if its occurrence corresponded with a burst in the integrated MSNA signal (i.e., within ± 0.4 s of the burst peak), or 2) if it was one of ≥ 2 visible APs in the filtered microneurographic signal firing within ± 0.4 s of an ECG R‐wave, after adjusting for the conduction delay. All other APs were considered to fire asynchronously. At BSL, 33 ± 12 % of total AP activity was asynchronous (99 ± 48 asynchronous APs/100beats) and a pattern emerged whereby the probability of asynchronous AP firing decreased logarithmically (R2 = 0.80, P < 0.01) as AP cluster size increased. Compared to BSL (20 ± 6 bursts/min, 42 ± 7 AU), MSNA burst frequency increased on going to −10 mmHg LBNP (24 ± 6 bursts/min), −40 mmHg LBNP (35 ± 8 bursts/min), and APN (35 ± 9 bursts/min), while burst amplitude increased only with −40 mmHg LBNP (57 ± 8 AU) and APN (64 ± 14 AU) (both repeated measures analysis of variance [RM ANOVA]: P < 0.05, all Ppost‐hoc < 0.05). Compared with BSL, within‐burst AP discharge increased across all conditions (BSL: 144 ± 69 AP/min, 7 ± 2 AP/burst; −10 mmHg LBNP: 211 ± 87 AP/min, 8 ± 3 AP/burst; −40 mmHg LBNP: 410 ± 207 AP/min, 11 ± 4 AP/burst; APN: 397 ± 160 AP/min, 11 ± 3 AP/burst) (both RM ANOVA: P < 0.05, all Ppost‐hoc < 0.05). Compared to BSL, increased burst amplitude with −40 mmHg LBNP and APN was attributed to an increase in the within‐burst firing of previously active APs (BSL: 4 ± 1 clusters/burst, −40 mmHg LBNP: 5 ± 2 clusters/burst, APN: 5 ± 1 clusters/burst; RM ANOVA: P < 0.05, both Ppost‐hoc < 0.05) more so than the recruitment of new AP clusters (BSL: 14 ± 4 total clusters; −40 mmHg LBNP: 18 ± 7 total clusters; APN: 16 ± 6 total clusters; RM ANOVA: P > 0.05). However, compared to BSL (33 ± 12 %), the probability of asynchronous AP firing was reduced with −10 mmHg LBNP (26 ± 10 %), −40 mmHg LBNP (16 ± 7 %), and APN (14 ± 7 %; RM ANOVA: P < 0.05, all Ppost‐hoc < 0.05). The probability of asynchronous AP discharge was inversely and linearly related to burst frequency (β = −0.98, r2 = 0.67) and amplitude (β = −0.41, r2 = 0.20; both P < 0.01). This study suggests that: 1) integrated bursts of MSNA formed by synchronous AP firing are not separated by ‘neural silence’ but rather, by asynchronous AP discharge, and 2) smaller sympathetic APs are more likely to exhibit asynchronous discharge.Support or Funding InformationThis work was funded by a Natural Sciences and Engineering Research Council of Canada Discovery Grant.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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

“…4E). APs exhibiting asynchronous discharge activity between pulse-synchronous bursts is in agreement with the results from Klassen et al, who suggested that reduced percentage of asynchronous APs during orthostatic and apnoeic stress may be reflective of one and/or two phenomena (Klassen et al, 2019). The first theory is the existence of an active AP synchronization mechanism.…”

“…1C. Previous findings from Klassen et al (2019) suggest that a considerable number of APs discharge asynchronously between sympathetic bursts. Therefore, any detected AP during baseline, hypoxia and recovery was extracted regardless of whether it occurred within an identified MSNA burst or not.…”

“…occurring outside of ±0.4 s of the burst peak) and asynchronous APs are reported as a percentage of total AP activity. This criterion is a less conservative approach than previously published methods (Klassen et al, 2019) and was utilized, in part, due to differences in AP detection and analysis techniques (e.g. independently reproduced matched-mother wavelet analysis vs. closed source software).…”

“…For instance, the baroreflex exhibits the strongest influence over medium-sized AP activity at rest but weakly governs larger, low probability APs (Klassen et al, 2020;Salmanpour & Shoemaker, 2012). Smaller-sized APs are not only under weak baroreflex control (Klassen et al, 2020), but also express greater asynchronous firing patterns between bursts during periods previously considered 'neurally silent' (Klassen et al, 2019). Sympathetic activation during severe orthostatic (Badrov et al, 2015;Klassen et al, 2019Klassen et al, , 2020Salmanpour et al, 2011) and chemical (Badrov et al, 2015;Ott et al, 2019Ott et al, , 2020Steinback et al, 2010) stressors appears to be facilitated by both enhanced neural recruitment of previously latent, larger-diameter sympathetic axons and resetting of the arterial baroreflex to permit increased firing frequency of medium sized AP subpopulations (Klassen et al, 2020), in addition to reduced incidence of asynchronous AP activity (Klassen et al, 2019).…”

“…Smaller-sized APs are not only under weak baroreflex control (Klassen et al, 2020), but also express greater asynchronous firing patterns between bursts during periods previously considered 'neurally silent' (Klassen et al, 2019). Sympathetic activation during severe orthostatic (Badrov et al, 2015;Klassen et al, 2019Klassen et al, , 2020Salmanpour et al, 2011) and chemical (Badrov et al, 2015;Ott et al, 2019Ott et al, , 2020Steinback et al, 2010) stressors appears to be facilitated by both enhanced neural recruitment of previously latent, larger-diameter sympathetic axons and resetting of the arterial baroreflex to permit increased firing frequency of medium sized AP subpopulations (Klassen et al, 2020), in addition to reduced incidence of asynchronous AP activity (Klassen et al, 2019). However, the baroreflex governing strategies over varying-sized sympathetic axons that permit sLTF following hypoxia-induced chemoreflex activation is unknown.…”