The rhythmicity of sympathetic nerve activity.

This review focuses on that most engaging feature of the sympathetic nervous system, its rhythmicity. In particular examining the nature of sympathetic nerve activity (SNA), its characteristics, the frequencies of these rhythms and possible mechanisms responsible for their generation. Sympathetic activity can be thought of as a complex output of the central nervous system providing subtle control over end organ function. This control is exerted in a number of frequency bands including rhythms related to the cardiac and respiratory cycles, 10 Hz, and between 0.2 and 0.4 Hz. The generation and control over the occurrence of each of these rhythms is likely to be quite separate. Although afferent feedback from sources such as baroreceptors can explain some of the rhythmical properties in each case there is good evidence for inherent generation of aspects of these rhythms. A variety of brainstem cell groups are thought to be involved in their generation with the rostral ventrolateral medulla, although unlikely to be solely responsible for tone generation, an important regulator of overall activity. SNA also varies in the number of nerves recruited to fire in each synchronized discharge. Little is known about this control other than it appears to be quite separate from the control over the timing of discharges. Spinal cord mechanisms are possibly involved. SNA frequencies above 0.7 Hz do not appear to directly induce oscillations in innervated vasculature, however, are likely to contribute to setting the level of vasconstrictive tone. Slower frequencies appear to directly cause oscillations in blood flow.