Avian sympathetic cardiac fibers and their cells of origin: anatomical and electrophysiological characteristics.

The sympathetic cardiac innervation of the pigeon was investigated to describe certain anatomical and physiological properties of the cardiac nerve fibers and their postganglionic cells of origin. The compound action potential of the right cardiac nerve has two major components, one conducting at 2.0-5.6 m/sec with no chronotropic effect on the heart and the other conducting at 0.4-1.0 m/sec with a cardioacceleratory effect. Postganglionic neurons responding antidromically to cardiac nerve stimulation were then studied in ganglion 14 which contains most cells of origin of the cardiac fibers. These neurons have refractory periods of approximately 4 msec, following frequencies of less than 4 HZ, and axons conducting at 0.4-2.0 m/sec; this conduction velocity range corresponds to the slower compound action potential component. Electron microscopy of the cardiac nerve revealed unmyelinated fibers ranging in diameter from 0.2 to 1.2 micrometer and a population of myelinated fibers 1.0-3.6 micrometer in diameter. The unmyelinated fibers account for the slower compound action potential component and are largely postganglionic cardioaccelerator axons. The myelinated fibers account for the faster compound action potential component which has no chronotropic effect and is not reflected in postganglionic antidromic latencies; it is suggested that these myelinated fibers are cardiac sympathetic afferents. This study thus establishes electrophysiological criteria for identifying cardiac postganglionic neurons and describes the anatomical basis of these criteria.