It is generally accepted that each dorsal root ganglion cell gives rise to a single central axon that enters the spinal cord via the dorsal root. This concept arose, in large part, from the reports that the numbers of dorsal root ganglion cells and dorsal root axons in mammals are equal. By contrast, in the frog a considerable excess of ganglion cells over axons was reported. None of these reports can be accepted, however, because the unmyelinated dorsal root axons could not be counted accurately with the light microscope. In the present study the counts in the frog are repeated, except that the axons are counted in the electron microscope, which has the necessary resolution for unmyelinated axons, and surgical controls are done to make certain that there are no extraneous axons in the root. When this is done, we show a considerable excess of axons to ganglion cells for all spinal segments in the frog, and there are no extraneous axons in the root. These data are interpreted as indicating that dorsal root axons branch either in the ganglion or in the root itself. Thus the concept that each frog dorsal root ganglion cell sends a single central process to the spinal cord needs to be modified. One of our control procedures was to make certain that no fibers survive in the proximal stump of a dorsal rhizotomy. This shows that there are no dorsal root efferents in frog dorsal roots, an important consideration in the interpretation of certain physiological phenomena that result from dorsal root stimulation. Finally, the study shows that there is a total of 43,000 dorsal root ganglion cells and 72,000 dorsal root axons in the bullfrog. These figures are considerably lower than similar figures from mammals and make an interesting comparison with some useful invertebrate preparations.
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