Character of sympathetic neural rhythms

Like virtually all other physiological control systems, the sympathetic nervous system that controls cardiovascular function is characterized by the presence of rhythmic activity. The hallmark of sympathetic nerve activity is the presence of rhythms synchronized to the respiratory and cardiac cycles. But sympathetic networks are far more complex and generate a mixture of periodicities that range between ~0.04 and 10 Hz, depending on the physiological conditions, type of nerve being analyzed, and the species. Despite the ubiquity of these rhythms, their function is often not obvious and thus many investigators fail to consider these rhythms when recording from sympathetic nerves. I will show why frequency-domain (power density spectral) analysis is more suitable than time-domain (autocorrelation) analysis to quantify a complex signal (i.e., one with multiple frequency components) such as sympathetic nerve activity. I will present data that support the view that rhythmic activity leads to more effective activation of sympathetic neurons than randomly occurring activity and that rhythmicity is important for coordinating the discharges in sympathetic nerves supplying different cardiovascular target organs (e.g., heart and vasculature). I will also provide data showing that one can misinterpret the effects of some manipulations on sympathetic nerve activity by assessing only the “tonic” level of activity and ignoring its rhythmicity.