Sympathetic neural activation: an ordered affair

Abstract

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 (R 2 = 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.

Neural Control of Vascular Function in Skeletal Muscle

“…Also, the presence of faster conduction velocity axons with chemoreflex activation is consistent with earlier observations that conduction velocity of spinal descending pathways during chemoreflex stimulation (5.5 ± 0.9 m/s) were faster than for baroreflex activation (3.3 ± 0.7 m/s) (144). In view of these findings, the larger action potential in the faster conducting axons observed by Steinback et al (266) may form part of a specific pathway for chemoreflex neurovascular control. The neurovascular impact of these latent but recruitable subpopulations of sympathetic axons remains difficult to assess because the chemoreflex stress under which they were induced likely modifies the vascular response.…”

“…With this approach Steinback et al (266) reported higher firing rates of axons already active at baseline, as well as the recruitment of latent, higher threshold, and faster conducting sympathetic axons, during chemoreflex activation induced via prolonged breath holds (Fig. 6).…”

“…The sympathetic response to chemoreflex stimuli is robust (155,266) and represents a potent stimulus for postganglionic sympathetic axonal recruitment in anesthetized rodents (177)(178)(179) and awake humans (155,267). The MSNA responses to hypoxia and hypercapnia are determined by the excitatory chemoreflex stress but these undergo significant inhibition by the concurrent effects of ventilation.…”

Baroreflex mechanisms regulating the occurrence of neural spikes in human muscle sympathetic nerve activity

“…Evidence from this approach indicates that APs vary in size Steinback et al 2010) with larger APs demonstrating a faster conduction velocity . Thus these larger voltage signals appear to be from larger axons supporting evidence for the varied size of postganglionic C fibers (Fazan et al 2002).…”

“…Using this approach, we have observed that APs within the MSNA neurogram vary in size and conduction velocity Steinback et al 2010). Importantly, there is a range of AP sizes that are variously expressed over time.…”