Sympathetic neural recruitment strategies: responses to severe chemoreflex and baroreflex stress

This study tested the hypothesis that neural coding patterns exist within the autonomic nervous system. We investigated sympathetic axonal recruitment strategies in humans during chemoreflex- and baroreflex-mediated sympathoexcitation using a novel action potential (AP) analysis technique. Muscle sympathetic nerve activity (microneurography) was collected in 11 young individuals (6 females) during baseline and two subsequent protocols: 1) severe chemoreflex stimulation (maximal end-inspiratory apnea following rebreathe), and 2) severe baroreceptor unloading (-80 mmHg lower body negative pressure; LBNP). When compared with each respective baseline, apnea and LBNP increased AP frequency and mean AP content per sympathetic burst (all P < 0.01). When APs were binned according to peak-to-peak amplitude (i.e., into clusters), total clusters detected increased during both apnea (Delta 7 +/- 5; P = 0.0009) and LBNP (Delta 11 +/- 8; P = 0.0012) compared with baseline. This was concomitant to an increased number of active clusters per burst during apnea (Delta 3 +/- 1; P < 0.0001) and LBNP (Delta 3 +/- 3; P = 0.0076). At baseline and during apnea (R-2 = 0.98; P < 0.0001) and LBNP (R-2 = 0.95; P < 0.0001), a pattern emerged whereby AP cluster latency decreased as cluster size increased. Furthermore, the AP cluster latency profile was shifted downward during apnea (similar to 53 ms) and upward during LBNP (similar to 31 ms). The data indicate that variations in synaptic delays and latent subpopulations of larger axons exist as recruitment strategies for sympathetic outflow. The synaptic delay component appears to express reflex specificity, whereas latent subpopulation recruitment demonstrates sensitivity to stress severity.