The ratio of preganglionic axons to postganglionic cells in the sympathetic nervous system of the rat

Previously reported preganglionic-postganglionic ratios for the sympathetic system are a major part of the evidence for the widely accepted idea that the sympathetic innervation of the viscera is diffuse. Unfortunately, the previous reports did not assess the non-preganglionic fibers in the nerves examined, and the limitations of light microscopy precluded accurate unmyelinated fiber counts. Thus, a recalculation of these ratios is necessary. The present study recalculates these ratios for the cervical sympathetic system of the rat. All fiber counts are done with the electron microscope which has the resolution necessary for accurately determining axon numbers. Selective surgical procedures establish that 84% of the axons in the cervical sympathetic trunk are preganglionic, 11% are postganglionic, and 5% are sensory. Thus, the numbers of preganglionic fibers can now be accurately assessed and compared to the number of postganglionic neurons in the superior cervical ganglion. When this is done, a preganglionic/postganglionic ratio of approximately 1:4 is obtained. This ratio differs considerably from those previously published.