12 A. J. D. DE LORENZO 



Nerve Fibers 



Myelinated nerve fibers about 1 to 6// in diameter are seen in the con- 

 nective tissue underlying each taste bud. They lose their myelin, for the 

 most part, in the nerve plexus near the basement membrane. These un- 

 myelinated fibers then enter the bud and establish synaptic contiguity with 

 the receptors. The center of the bud demonstrates much more mixing of 

 nerve fibers, deriving from many loci in the plexus, whereas the peripheral 

 or lateral margins of the buds are more simply organized. Examination 

 of these nerve fibers with the electron microscope quickly reveals the reason 

 they are most difficult to resolve with conventional light microscopic 

 techniques. Figure 12 demonstrates a region in a nerve plexus. In the 

 upper left a gustatory receptor can be seen whose nucleus (Nu) has been 

 identified. This cell rests upon a basement membrane (B). The spaces 

 beneath the basement membrane are filled with connective tissue (Ct) con- 

 taining collagen. A capillary (Ca) has beeen sectioned longitudinally and 

 its lumen contains a red cell seen at the upper right. Directly beneath the 

 capillary is a typical nerve plexus consisting of numerous unmyelinated 

 nerve fibers (N) in close contiguity with Schwann cells and exhibiting the 

 typical mesaxons (arrows) characteristic of most unmyelinated fibers. 

 Note this typical arrangement in contrast to the organization of olfactory 

 fascicles (Figs. 5 and 6). The most striking observation is the overall 

 diameter of these fibers. There appear to be two kinds of nerve fibers — 

 one type about 0.5// to 1.0// in diameter, the other below 0.5//. Many of 

 the smallest fibers measure only 50 millimicrons in diameter and are 

 clearly enclosed in Schwann cell mesaxons (Fig. 12). These observations 

 were first published by de Lorenzo in 1958. It becomes highly proble- 

 matical whether these small fibers can conduct an impulse due to their 

 minute dimensions. They may be small branches of the larger fibers. 



These fibers enter the bud and assume a most unique structural relation- 

 ship with the receptor cells (Fig. 13). In this figure some 30 nerve fibers, 

 consisting of both the large and small types push into a single receptor cell, 

 whose nucleus (Nu) is located at the right of the figure. The fibers now 

 are embraced, as it were, by the membranes of the receptors in a manner 

 identical with mesaxons deriving from Schwann cells. These receptor 

 mesaxons are indicated with the arrows. Close scrutiny of this figure will 

 reveal that all the fibers bear this relationship. Compare Fig. 13 with 

 Fig. 12 to clarify this relationship. Some of these fibers may be terminating 

 as evidenced by their " synaptic vesicles ". Figures 11, 14 and 15 demon- 

 strate how the large fiber groups form synaptic junctions with receptor 

 cells. The large fibers always seem to terminate in relationship to two or 

 more receptor cells, i.e. they may innervate more than one receptor or may 

 be free nerve endings, since they are between cells, near pores and more 

 readily available to the extracellular environment. Figure 14 demonstrates 



