Recruitment strategies in efferent sympathetic nerve activity

In 1968, the first reported microneurographic recordings of muscle sympathetic nerve activity (MSNA) in humans revealed the bursty behavior of efferent sympathetic nerve activity. The timing of bursts could be explained by baroreflex physiology, but the variability in size of each burst was left unexplained. On the basis of shorter latencies of larger bursts, Wallin's group [53] proposed the existence of variable supraspinal synaptic delays and/or options for recruitment of faster-conducting sympathetic neurons when bursts become stronger. These options represent features of recruitable neural systems. Based on the highly variable latencies of single axons whose firing patterns could not explain reflexive increases in burst size, the concept of a latent subpopulation of sympathetic axons was speculated to exist. Using evidence from experimental preparations in anesthetized smaller animals, to recent signal processing of multi-fiber recordings in humans, this brief review will discuss the attempts to discover and understand recruitment strategies within the peripheral sympathetic nervous system. The review focuses on new information from human recordings supporting the idea that rate coding, population coding (recruitment), and temporal coding are options available within the peripheral sympathetic nervous system to adjust efferent sympathetic outflow. Although data are limited, possible clinical applications are discussed.