Iannarelli, Nathaniel

Our knowledge regarding baroreflex regulation of subpopulations of differently sized sympathetic action potentials (APs) remains incomplete. This study tested the hypothesis that baroreflex sigmoid morphology varies across AP subpopulations. In 12 healthy individuals (6 females; 32 ± 5 years), we measured diastolic blood pressure (DBP; brachial artery catheter) and muscle sympathetic APs (peroneal microneurography; continuous wavelet transform) during a modified Oxford protocol (100 μg nitroprusside bolus, 150 μg phenylephrine bolus). Baroreflex sigmoids (four-parameter logistic functions) modelled the relationship between DBP and sympathetic AP discharge probability for 10 normalized AP clusters. For each AP cluster, baroreflex sigmoid morphology was quantified by max gain, threshold, centering, and saturation pressures, and operating and response ranges. One-way analyses of variance and Dunnett post-hoc tests were performed. Data are mean ± SD. At the centering pressure of the baroreflex sigmoid, max gain was greater for medium APs (e.g., Cluster 3: -5.6 ± 2.9%/mmHg) compared to small (e.g., Cluster 1: -1.4 ± 1.1%/mmHg; P < 0.01) and large APs (e.g., Cluster 9: -0.6 ± 0.4%/mmHg; P < 0.01). Discharge probability at the centering pressure was also greater for medium APs (e.g., Cluster 3: 61 ± 22%) compared to small (e.g., Cluster 1: 12 ± 10%; P < 0.01) and large APs (e.g., Cluster 9: 3 ± 3%; P < 0.01). There were no differences in the saturation pressure (i.e., DBP where any further reduction does not elicit an increase in AP discharge probability) between AP clusters (main effect P = 0.22). The threshold pressure (i.e., DBP where any further reductions elicit an increase in AP discharge probability) was greater for medium (e.g., Cluster 3: 78 ± 8mmHg) compared to large APs (e.g., Cluster 9: 74 ± 11mmHg; P = 0.03). The operating range (i.e., Threshold – Saturation DBP) was greater for medium (e.g., Cluster 3: 14 ± 3mmHg) compared to large APs (e.g., Cluster 9: 8 ± 5mmHg; P < 0.01). There were no differences in the threshold pressure or operating range of medium compared to small APs (both P ≥ 0.82). The response range (i.e., Saturation – Threshold discharge probability) was greater for medium (e.g., Cluster 3: 26 ± 13%) compared to small (e.g., Cluster 1: 5 ± 4%; P < 0.01) and large APs (e.g., Cluster 9: 1 ± 1%; P < 0.01). These data confirm prior work using linear modeling and spontaneous approaches to quantify sympathetic AP baroreflex gain and extend to demonstrate variation in the morphology of the baroreflex sigmoid. During large changes in blood pressures, we identify differences in broader features of baroreflex control (e.g., threshold pressure, operating range, and response range) between AP subpopulations that support strong baroreflex regulation over medium-sized APs. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.