THE AMPLITUDE OF SYNCHRONIZED CARDIAC SYMPATHETIC-NERVE ACTIVITY REFLECTS THE NUMBER OF ACTIVATED PRE-GANGLIONIC AND POSTGANGLIONIC FIBERS IN ANESTHETIZED CATS

In order to obtain information regarding the number of pre- and postganglionic fibers that are firing, we measured cardiac sympathetic nerve activity (CSNA) before and after the successive sectioning of T1-T5 thoracic rami in anesthetized cats. Total activity from the area was measured under the mean CSNA curve. Peak amplitude, width and periodicity of the synchronized discharge was analyzed from the CSNA curve by the method we developed. Total CSNA decreased to 91+/-6%, 63+/-6%, 27+/-10%, 8+/-6% and < 1% of the control due to successive section of the T5, T4, T3, T2 and TI rami, respectively. The peak amplitude of synchronized CSNA decreased to 95+/-6%, 73+/-8%, 40+/-5% and < 10% of the control value, due to section of the T5, T4, T3 and T2 rami, respectively. The control width was 107+/-8 ms and decreased to 106+/-1 ms, 92+/-6 ms and 68+/-5 ms by successive section of the respective T5, T4 and T3 rami. However, periodicities of 80-120 ms (Tc rhythm) and 140-500 ms (Tb rhythm) of synchronized CSNA remained unchanged after section of the T3-T5 rami. The total CSNA decreased gradually due to decreases in the peak amplitude and width of synchronized CSNA with the successive section of preganglionic fibers. These results indicate that the peak amplitude of synchronized CSNA reflects the number of pre- and post-ganglionic fibers that are firing and suggest that the number of preganglionic neurons which activate the cardiac fibers naturally was largest in the T3 segment. The periodicity of the Tc and Tb rhythms of the synchronized discharges is, however, constant between the various preganglionic inputs.