Journal of Physiology
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Is there an ordered pattern in the recruitment of postganglionic sympathetic neurones? Using new multi-unit action potential detection and analysis techniques we sought to determine whether the activation of sympathetic vasomotor neurones during stress is governed by the size principle of recruitment. Multi-unit postganglionic sympathetic activity (fibular nerve) was collected from five male subjects at rest and during periods of elevated sympathetic stress (end-inspiratory apnoeas; 178 ± 37 s(mean ± S.D.)). Compared to baseline (0.24 ± 0.04 V), periods of elevated stress resulted in augmented sympathetic burst size (1.34 ± 0.38 V, P < 0.05). Increased burst size was directly related to both the number of action potentials within a multi-unit burst of postganglionic sympathetic activity (r= 0.88 ± 0.04, P < 0.001 in all subjects), and the amplitude of detected action potentials (r= 0.88 ± 0.06, P < 0.001 in all subjects). The recruitment of larger, otherwise silent, neurons accounted for approximately 74% of the increase in detected action potentials across burst sizes. Further, action potential conduction velocities (inverse of latencies) were increased as a function of action potential size (R2= 0.936, P= 0.001). As axon diameter is positively correlated with action potential size and conduction velocity, these data suggest that the principle of ordered recruitment based on neuronal size applies to postganglionic sympathetic vasomotor neurones. This information may be pertinent to our understanding of reflex-specific recruitment strategies in postganglionic sympathetic nerves, patterns of vasomotor control during stress, and the malleability of sympathetic neuronal properties and recruitment in health and disease.The sympathetic nervous system is an important controller of blood pressure and blood flow to critical tissues and organs. In other neural systems (e.g. the skeletal motor system) there is a well understood pattern of neural recruitment during activation. Alternatively, our understanding of how sympathetic neurones are coordinated during stress is limited. We demonstrate that during stress otherwise silent sympathetic neurones are activated in an order based on neuronal size (from smallest to largest). This recruitment pattern is similar to what is known in other neural systems. This information has important implications for how blood pressure and blood flow are controlled, and the malleability of sympathetic activation in health and disease. © 2010 The Authors. Journal compilation © 2010 The Physiological Society.