Mechanism of cardiac-related synchronized cardiac sympathetic nerve activity in awake cats

To examine the mechanism of cardiac-related sympathetic discharge, the periodicity of synchronized cardiac sympathetic nerve activity (CSNA) was measured in awake cats before and after baroceptor denervation using an interval histogram. We found two major periodicities, i.e., Tc = 48-164 ms (20-6 cycles/s) and Tb = 164-512 ms (6-2 cycles/s). In the baroceptor-intact state, the modes of Tc (C) and Tb (B) were 96 ms (10 cycles/s) and 368 ms (2.7 cycles/s), respectively, whereas the cardiac interval (CI) was 366 ms (2.7 cycles/s). Probability of B mode was larger than that of C mode. During tachycardia by atropine, B mode decreased to 304 ms in proportion to CI of 308 ms, whereas C mode remained the same. Probability of B and C modes decreased reflexly by hypertension. Under baroceptor denervation, C mode remained unchanged but its probability increased significantly. A 2-6 cycles/s Tb rhythm could be found, but its probability decreased significantly. Tb periodicity consists of 2Tc, 3Tc, and 4Tc. These results suggest that C mode may reflect a fundamental periodicity of central origin and that B mode reflects a periodicity of cardiac-related synchronized CSNA, which is produced in a probabilistic manner by reflex inhibition of fundamental rhythm due to periodic baroceptor input.