Olfactory Nerve Fibers by Herbert

Unmedullated nerve fibers physiologically carrying signals in the afferent direction became of interest as a group apart, when it was learned that those in the skin nerves, having their origin in cells of the dorsal root ganglia (d.r.C), possess properties different from those with cells of origin in the sympathetic ganglia (s.C). I t has long been known that the fibers of the olfactory nerve are unmedullated. Therefore, the question naturally suggested itself as to whether the olfactory fibers are relatable to the d.r.C fibers. In as much as the fibers in mammals are too short for investigation with the methods used for other fibers, it was decided to start with observation of their dimensions with the aid of the electron microscope. A picture was revealed unlike that of any nerve previously examined: extremely small fibers in great numbers. How such fibers might arise from the bipolar cells of the olfactory mucosa was a problem that could only be answered by an examination of the mucosa itself. In the course of the examination characteristics of the mucosa, as yet undescribed, came into view as a by-product. Descriptions are here given of the fibers in relation to the mucosa; and finally of some observations on the olfactory nerve of the pike, which will show that the fibers do not correlate with the d.r.C fibers. Although classification of the olfactory fibers with the d.r.C fibers is not permi.~sible each provides the best morphological analogy for the other. Embryologically both arise from thickenings of the ectoderm, placodes by the forebrain for the olfactory fibers, the neural crest for the d.r.C fibers. As the neurons form, an axonal projection is directed toward the central nervous system, and a dendritic projection toward the periphery, there to terminate in free endings of great fineness. Small as are the diameters of the cilia on the olfactory dendrites they are matched by the terminals of the d.r.C fibers in the skin (Gasser, 1954). In a previous paper it was shown that the axonal branches of the d.r.C fibers are much smaller than the dendritic branches, and that they are often connected in bunches to the Schwann membrane by their mesaxons. The pattern first revealed in connection with the d.r.C neurons is