Segmental origins of cardiac sympathetic nerve activity in rats

The segmental origins of cardiac sympathetic nerve activity (CSNA) were investigated in 8 urethane-anesthetized, artificially ventilated rats. The left upper thoracic sympathetic chain was exposed retropleurally after removing the heads of the second to fourth ribs. The preganglionic inputs to the chain from segments T1-T3 and the trunk distal to T3 were marked for later sectioning. CSNA was recorded conventionally, amplified, rectified and smoothed. Its mean level was quantified before and after each preganglionic input was cut, usually in rostro-caudal sequence. The level after all inputs were cut (i.e. noise and residual ECG pickup) was subtracted from previous measurements. The signal decrement from cutting each preganglionic input was then calculated as a percentage. CSNA in all rats depended on preganglionic drive from two or more segments, which were not always contiguous. Over the population, most preganglionic drive came from T3 and below, while the least came from T1. But there was striking inter-individual variation, such that the strongest drive to CSNA in any one rat could come from T1, T2, T3, or below T3. These findings provide new functional data on the segmental origins of CSNA in rats.