Baroreflex mechanisms regulating the occurrence of neural spikes in human muscle sympathetic nerve activity

Abstract

Salmanpour A, Shoemaker JK. Baroreflex mechanisms regulating the occurrence of neural spikes in human muscle sympathetic nerve activity. J Neurophysiol 107: 3409 -3416, 2012. First published March 21, 2012 doi:10.1152/jn.00925.2011.-This study tested the hypothesis that the discharge patterns of action potentials (APs) within bursts of postganglionic muscle sympathetic nerve activity (MSNA) are subject to arterial baroreflex control but in a manner that varies inversely with AP size. MSNA data were collected over 5 min of supine rest in 15 young and healthy individuals (8 males; 24 Ϯ 4 yr of age; means Ϯ SD). The baroreflex threshold and sensitivity diagrams were constructed for both the integrated sympathetic bursts and for the AP clusters. For the integrated bursts, a strong linear relationship between burst probability and diastolic blood pressure (DBP) was observed (P Ͻ 0.05). There was little relationship between integrated burst strength (amplitude) and DBP. On average, 12 AP clusters were observed across individuals. Larger APs tended to appear in the larger bursts. Linear regression analysis was used to study the baroreflex threshold (probability of AP cluster occurrence vs. DBP) as well as the baroreflex sensitivity (AP cluster size vs. DBP). A significant reflex threshold relationship was observed in 75-100% of AP clusters across all individuals. In contrast, significant reflex sensitivity relationships were observed in only 9 of 15 individuals and for limited APs. Overall, the slope of the AP baroreflex threshold relationship was greater for the small-medium sized AP clusters than that of the larger APs. Therefore, within each burst, the small-medium sized APs are governed by the baroreflex mechanism. However, the large APs, which tend to appear in the large integrated bursts, are weakly associated with a baroreflex control feature. The variable impact of baroreflex control over AP occurrence provides a plausible explanation for the overall weak baroreflex control over integrated burst strength, a feature that is determined by both the number and size of the AP complement. microneurography; spike detection and classification; firing probability THE RECRUITMENT STRATEGIES used by the central nervous system to coordinate the discharge patterns in postganglionic sympathetic outflow have been a focus of research since the multi unit discharge behavior was first observed in mammals almost a century ago (Adrian et al. 1932). The discharge of sympathetic outflow occurs as volleys of synchronized action potentials (APs). Even under conditions of supine rest, these volleys, or so-called "bursts," vary in discharge rate as well as in size. The variations in burst size are believed to be determined by spontaneous fluctuations in the number of axons being recruited per burst (Ninomya et al. 1993). Sundlof and Wallin (1978) demonstrated that such bursts in the muscle sympathetic nerve activity (MSNA) neurogram are synchronized to the cardiac cycle with an occurrence probability that is inversely related to blood ...

Recruitment pattern of sympathetic muscle neurons during premature ventricular contractions in heart failure patients and controls

“…Therefore, both a greater frequency of integrated bursts and an increased AP content within each burst, contribute to the heightened baseline sympathetic outflow in CHF. Previous observations that the high-probability APs in sinus rhythm bursts of healthy individuals are related strongly to a baroreflex mechanism (27) supports the conjecture outlined above that lower diastolic blood pressure during the sinus rhythm may contribute to the larger number of APs per burst and active clusters in CHF patients than in controls. Inasmuch as the number of APs determine the size of an integrated burst and different central mechanisms are proposed to affect burst frequency (gating) vs. size (15), these data indicate that CHF produces aberrations in both features of control, such that more efferent APs/burst and bursts/min are emitted.…”

Spike rate of multi-unit muscle sympathetic nerve fibers after catheter-based renal nerve ablation

“…[6, 10] The methodology counts the number of single fiber discharges (spikes) in MSNA raw signals independently of synchronized burst activity. The methodology was shown to reliably detect changes in sympathetic activity elicited by baroreflex and chemoreflex mechanisms [4, 25] or diseases, such as heart failure[34]. In the present study, we observed no consistent reduction in wavelet-based MSNA measurements following catheter-based renal nerve ablation.…”

Fifty years of microneurography: learning the language of the peripheral sympathetic nervous system in humans

“…Furthermore, normalizing the burst size in the integrated neurogram overcomes this problem to some extent (Sverrisdóttir et al 1998) and when this is done, the distribu-tion of burst size exhibits strong within-individual reproducibility in studies separated by about 4 weeks (Kimmerly et al 2004). As an aside, baroreflex mechanisms appear to apply to the appearance of small APs only, but not the larger AP clusters (Salmanpour and Shoemaker 2012), providing a mechanistic link between earlier conjecture that larger bursts are regulated by different central features (Kienbaum et al 2001;Malpas and Ninomiya 1992).…”