10 A. J. D. DE LORENZO 



Olfactory Glomeruli 



In the rabbit there are about 10*^ olfactory receptors (Allison, 1953). 

 In all bulbs carefully examined (Holmgren, 1920 for fishes, and Cajal, 

 191 1 for mammals) the olfactory fibers which enter the bulb do not divide 

 before entering the glomeruli, although they do branch freely once inside. 

 Therefore, any individual axon of a receptor cell does not terminate in 

 more than one glomerulus and thus each glomerulus receives impulses 

 from a distinct receptive field. Allison and Warwick (1949) estimated 

 that in the rabbit every glomerulus receives impulses from " on an average, 

 26,000 olfactory receptors and passes impulses on to twenty-four mitral 

 cells and sixty-eight tufted cells '\ This arrangement offers a unique 

 region for the study of synaptic junctions with the electron microscope, 

 especially since silver staining of this region is most limited due to the small 

 fibers and the structural complexity of the glomeruli. Figure 7 represents 

 a section through an olfactory glomerulus demonstrating a large number 

 of axon terminals. A few of these endings have been labeled (A). They 

 are in synaptic contiguity with two mitral cells (P) and demonstrate the 

 characteristic synaptic membrane thickening (arrows) and contain " synap- 

 tic vesicles '\ Quite often axons are seen in very close contiguity ( ^100 A) 

 with each other. Although in these cases, there are no membrane 

 specializations, " synaptic vesicles '' are present in the nerve fibers suggest- 

 ing a possible synaptic or ephaptic junction as reported in other systems 

 (de Lorenzo, 1959 and 1960). The oscillations in potential reported by 

 Adrian and others in the olfactory pathway suggest possible neural connec- 

 tions. The morphological evidence shows no obvious connections in 

 receptor cells other than the desmosomes described above. In the olfactory 

 fascicles, the close contiguity of nerve fibers (~100 A) and the vesicles 

 described might provide such a locus. An alternative explanation might 

 be derived from the arrangement of the olfactory glomerulus where 

 oscillations might be generated as dendritic potentials in the extensive 

 glomerular network. 



THE GUSTATORY PATHWAY 

 The serious hmitations enumerated for the study of olfactory neurons 

 are again manifest in the gustatory system. Although a detailed study of 

 the fine structure of taste buds has been published by de Lorenzo (1958) 

 the innervation of gustatory receptors remains obscure. The reasons for 

 this dilemma become apparent by referring to Figs. 8 and 9. Taste buds 

 (Tb) have been stained by the conventional silver methods. Although 

 nerve plexuses (N) can be seen, individual axons are extremely difficult to 

 follow even in the most successfully stained preparations (arrows Fig. 8). 

 Identifying synaptic junctions with any degree of reliability becomes 

 extraordinarily difficult and most times impossible. Likewise structural 



