Fractal activity generated independently by medullary sympathetic premotor and preganglionic sympathetic neurons

In anesthetized cats with cervical spinal cord transection, Fano factor analysis was used to test for time-scale invariant (fractal) fluctuations in spike counts of single preganglionic cervical sympathetic neurons (PSNs) and putative sympathetic premotor neurons located in the rostral ventrolateral medulla (RVLM) and caudal medullary raphe. The medullary neurons exhibited cardiac-related activity, and their axons projected to the spinal cord, as demonstrated by antidromic activation. The variance-to-mean spike count ratio (Fano factor) was plotted as a function of the window size used to count spikes. The Fano factor curves for seven PSNs, eight RVLM neurons, and eight raphe neurons contained a power law relationship extending over more than one time scale. In these cases, random shuffling of the interspike intervals in the original time series eliminated the power law relationship. Thus the power law relationships can be attributed to long-range correlations among interspike intervals rather than simply to the distribution of the intervals that is not changed by shuffling the data. It is concluded that PSNs and sympathetic premotor neurons in the medulla can independently generate fractal firing patterns.