Sixty Days of Head‐Down Tilt Bed Rest Augments Muscle Sympathetic Nerve Activity Responses to Maximal End‐Inspiratory Apnea: Evaluation of Sympathetic Neural Recruitment Strategies
Bibliographic record
Abstract
The objective of this study was to evaluate the impact of 60‐days head‐down tilt (−6°) bed rest (HDBR) on muscle sympathetic nerve activity (MSNA) responses to a maximal end‐inspiratory apnea (apnea) and the sympathetic neural recruitment strategies underlying the MSNA response. Thirteen male individuals ( n = 13, 30 ± 2 yr, 180.7 ± 1.5 cm, 77.7 ± 1.8 kg, 23.8 ± 0.4 kg/m 2 ) provided baseline MSNA data pre‐ and post‐HDBR, whereas eight participants provided complete apnea data as well. Heart rate (HR; ECG), blood pressure (Finometer) and MSNA (microneurography) were measured pre‐ and post‐HDBR, during ~3‐min of baseline and the last ~30‐s of a maximal end‐inspiratory apnea (apnea). Sympathetic action potentials (AP) were detected from the filtered raw MSNA signal using AP detection software. APs were binned based on peak‐to‐peak amplitude into clusters defined by Scott's rule. Values are mean ± SE. Apnea duration was 74 ± 5 s in the pre‐ and 85 ± 7 s in the post‐HDBR conditions ( P = 0.09). Baseline mean arterial pressure (MAP; 104 ± 5 to 94 ± 4 mmHg, P = 0.04) and pulse pressure (PP; 59 ± 3 to 50 ± 4 mmHg, P < 0.01) were reduced with HDBR. MAP during apnea (112 ± 5 to 103 ± 4 mmHg, P = 0.14) was similar pre‐ and post‐HDBR, while PP during apnea (55 ± 3 to 49 ± 4 mmHg, P = 0.06) trended towards being reduced in the post‐HDBR session. HR was similar across testing sessions at baseline (66 ± 2 to 68 ± 5 bpm, P = 0.24) and during apnea (66 ± 2 to 70 ± 2 bpm, P = 0.17). Baseline integrated MSNA (burst frequency: 13 ± 2 to 18 ± 2 bursts/min; burst incidence: 20 ± 2 to 27 ± 4 bursts/100beats, all P > 0.05) and baseline AP indices (AP frequency: 62 ± 22 to 106 ± 27 spikes/min; AP incidence: 92 ± 31 to 159 ± 40 spikes/100 beats; AP/burst: 4 ± 0.8 to 6 ± 0.9 spikes/burst; active clusters/burst: 3 ± 0.3 to 3 ± 0.4 clusters/burst; total clusters: 7 ± 0.9 to 9 ± 0.6 clusters, all P > 0.05) were similar pre‐ and post‐ HDBR. However, the change in integrated MSNA burst frequency, from baseline to apnea, was greater post‐HDBR (Δ 28 ± 3 bursts/min) than pre‐HDBR (Δ 15 ± 2, P < 0.01). This exaggerated response to apnea post‐HDBR was a function of increased AP firing frequency (pre‐HDBR, post‐HDBR: Δ 94 ± 25, Δ 317 ± 55 AP/min, P < 0.01). Although the change in integrated burst amplitude, from baseline to apnea, was similar pre‐ (Δ 0.040 ± 0.01) and post‐HDBR (Δ0.080 ± 0.02, P = 0.15), AP analysis suggests that the change in the number of active sympathetic clusters per burst, from baseline to apnea, was greater post‐HDBR (Δ 1 ± 0.2 clusters/burst) than pre‐HDBR (Δ 0 ± 0.2 clusters/burst, P = 0.02), while the change in total sympathetic clusters, from baseline to apnea, trended towards greater recruitment post‐HDBR (Δ 2 ± 0.7 clusters) than pre‐HDBR (Δ 0 ± 0.5 clusters, P = 0.07). Also, the change in the number of APs per burst, from baseline to apnea, was greater post‐HDBR (Δ 3 ± 0.8) than pre‐HDBR (Δ 1 ± 0.4, P = 0.02). These findings suggest that reflex MSNA‐arousal imposed by apnea is exaggerated by 60‐days HDBR, primarily by mechanisms involving augmented AP firing rate with minimal contributions from recruitment of latent sub‐populations of sympathetic axons. Support or Funding Information This work was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) Grant 217916‐2013 (awarded to J. K. Shoemaker).
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".