Overlap of segmental populations and axon collaterals in the thoracic sympathetic system of the cat.

Sympathetic preganglionic axons are known to branch in the sympathetic chain, i.e. extraspinally. The possibility that the axons may branch within the spinal cord (intraspinally) and exit through different roots has not been well examined. The purpose of this study was to determine if intraspinal collaterals exist in the distribution of preganglionic neurons innervating the stellate ganglion of the cat using the double-labeled retrograde fluorescent dye technique (Diamidino Yellow and Fast Blue). The right stellate ganglion of pentobarbital-anesthetized cats was isolated; the sympathetic chain was ligated in two places just caudal to the T2 white ramus and then sectioned between the ligatures. One fluorescent tracer was injected into the stellate ganglion; the complementary-colored tracer was injected into the sympathetic chain below the cut. After a 6-day survival time, frozen serial 40-micron sections of spinal cord segments T1-T4 were cut and mounted. The longitudinal distribution of dyes: indicated a segmental preganglionic organization and overlapped approximately 700 micron. However, no sympathetic preganglionic neurons were double-labeled with both dyes. Additional experiments demonstrated a small number of extraspinal preganglionic axon collaterals.