Interactive effects of apneic and baroreflex stress on neuronal 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: < 0.01; LBNP-by-APN interaction: = 0.55). APN recruited larger previously silent AP clusters during the ambient condition (Δ4 ± 3; < 0.02) and -10 mmHg (Δ4 ± 3; < 0.01), but not -40 mmHg (Δ0 ± 2; = 0.53; LBNP-by-APN: < 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: < 0.01; LBNP-by-APN: = 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. 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: ) augmented sympathetic axonal firing probability, ) recruitment of larger and previously silent sympathetic postganglionic neurons at ambient and -10 mmHg, but not -40 mmHg LBNP, and ) APN reduced axonal discharge latency similarly across all conditions, independent of the level of baroreflex unloading.