Recruitment of larger, previously-silent sympathetic action potentials importantly contributes to neurovascular responses in humans

Our knowledge regarding the neurovascular effects of sympathetic action potential (AP) discharge versus sympathetic AP recruitment remains incomplete. This study tested the hypothesis that the recruitment of larger, previously-silent sympathetic APs elicits the strongest blood pressure and vasoconstrictor responses in humans. Muscle sympathetic APs (peroneal microneurography, continuous wavelet transform), diastolic blood pressure (DBP; Finometer), and leg vascular resistance (LVR; femoral artery Doppler ultrasound) were recorded in 11 healthy adults (23 ± 3 years; 3 females) during baseline rest (BSL) and a varying-length apnea protocol (APN; 25%, 50%, 75%, and 100% of maximal voluntary end-expiratory apnea). The varying-length apnea protocol was used as a model to elicit graded increases in sympathetic AP discharge and AP recruitment. Blood pressure and vasoconstrictor responses were quantified by the absolute change in DBP and LVR from baseline to apnea conditions, respectively. Statistical analyses included linear mixed-model regression analyses and mixed-effects analyses of variance (ANOVA). Data are presented as the mean ± SD. Mixed-model regression analyses indicated that DBP (BSL: 76 ± 7 mmHg, APN-50%: 81 ± 7 mmHg, APN-100%: 91 ± 10 mmHg; β = 0.847, p < 0.001) and LVR (BSL: 0.8 ± 0.4 mmHg/mL/min, APN-50%: 0.9 ± 0.5 mmHg/mL/min, APN-100%: 1.2 ± 0.7 mmHg/mL/min; β = 0.487, p < 0.001) increased progressively as the duration of apnea increased. Mixed-effects ANOVA indicated that, as the duration of apnea increased, there was a graded increase in sympathetic AP discharge frequency (BSL: 176 ± 94 APs/min, APN-50%: 337 ± 201 APs/min, APN-100%: 624 ± 352 APs/min; η2 = 0.651, p < 0.001). Further, larger, previously-silent APs were recruited but only during the longest apneas (BSL: 9 ± 3 total AP clusters, APN-50%: 10 ± 3 total AP clusters, APN-100%: 15 ± 4 total AP clusters; η2 = 0.661, p < 0.001). As indicated by mixed-model regression analyses, DBP increases were more strongly associated with AP recruitment (β = 0.433, p = 0.003) compared to increased AP discharge frequency (β = 0.256, p = 0.080). Similarly, LVR increases were more strongly associated with AP recruitment (β = 0.406, p = 0.005) compared to increased AP discharge frequency (β = 0.018, p = 0.905). This research provides new information regarding the neurovascular effects of sympathetic AP discharge versus sympathetic AP recruitment in humans. These data support the hypothesis that the recruitment of larger, previously-silent sympathetic APs elicits the strongest blood pressure and vasoconstrictor responses in humans. This work was supported by Queen Elizabeth II and Ontario Graduate Scholarships and the Natural Sciences and Engineering Research Council of Canada (NSERC). 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.