Baroreflex resetting of human sympathetic action potential subpopulations during exercise

This study tested the hypothesis that during fatiguing volitional exercise in humans, descending cortical signals and ascending skeletal muscle metaboreflex signals exert divergent control over baroreflex resetting of sympathetic action potential (AP) dis-charge. We quantified the baroreflex gain for sympathetic AP clusters within the muscle sympathetic nerve activity neurogram (peroneal microneurography and continuous wavelet transform) during baseline (BSL), the first 2-min of a 5-min isometric hand -grip (20% of maximal effort; IHG1), the last 2-min of IHG (IHG2), and during postexercise circulatory occlusion (PECO) in seven healthy participants. AP baroreflex threshold gain was measured as the slope of the linear relationship between AP probability (%) versus diastolic blood pressure (DBP; mmHg) for 10 normalized AP clusters. Compared with BSL, during IHG1, AP baroreflex threshold functions were only reset to greater DBP and baroreflex gain was unaffected. Compared with BSL, during IHG2 and PECO, baroreflex functions were reset to greater DBP and to greater AP firing probabilities, with medium-sized APs demonstrat-ing the largest upward resetting (e.g., cluster 3 BSL: 26 +/- 7%, cluster 3 IHG2: 78 +/- 22%, cluster 3 PECO: 88 +/- 46%). Compared with BSL, AP baroreflex threshold gain was not different during IHG2 but was increased during PECO, with medium-sized APs dem-onstrating the largest increase in baroreflex gain (e.g., cluster 3 BSL:-6.31 +/- 3.1%/mmHg, cluster 3 IHG2:-6.18 +/- 5.4%/mmHg, cluster 3 PECO:-12.13 +/- 6.5%/mmHg). These findings indicate that during IHG exercise, descending cortical signaling and ascending skeletal muscle metaboreceptor signals differentially affect baroreflex resetting of subpopulations of human muscle sympathetic postganglionic neurons.NEW & NOTEWORTHY This study provides new insight to baroreflex resetting of MSNA during exercise in humans. Both fatigu-ing IHG and PECO reset baroreflex control of sympathetic APs to higher blood pressures and greater MSNA. However, only PECO increased baroreflex threshold gain of medium-sized sympathetic APs, an effect that was concealed when focusing on the integrated MSNA neurogram to quantify baroreflex gain. These data suggest that descending central versus ascending mus-cle metaboreflex mechanisms differentially affect baroreflex resetting of sympathetic APs.

Automated summary

This study found that during fatiguing volitional exercise in humans, descending cortical signals and ascending skeletal muscle metaboreflex signals have divergent control over baroreflex resetting of sympathetic action potential discharge. Both isometric hand-grip exercise and postexercise circulatory occlusion reset baroreflex control of sympathetic action potentials to higher blood pressures. However, only postexercise circulatory occlusion increased the baroreflex gain of medium-sized sympathetic action potentials.