CHEMORECEPTOR MECHANISMS 



21 



and Harcliman (1955) are thus explainable in terms of difference in the detailed 

 configuration of reacting receptor molecules in different species. 



Olfaction 



The olfactory receptors in insects are only superficially different from the 

 taste receptors (Fig. 18). In vertebrates the difference is of a more fundamental 

 nature (Fig. 19). In mammals olfactory receptors occur more or less evenly 

 distributed in a small patch (2.5 cm. 2 ) of epithelium covering the medial wall 

 of the superior concha and the lateral wall of the nasal septum. In the rabbit 

 there is a moderate density gradient diminishing centrifugally (Allison and 

 Warwick, 1949). Proximal processes of the olfactory receptors synapse in the 

 glomeruli of the bulb (Fig. 20). In the rabbit approximately 26,000 receptors 

 synapse with each glomerulus. There are approximately 1,900 glomeruli which 

 synapse with about 24 mitral and 68 tufted cells. Thus, each glomerulus, al- 

 though connected with other glomerulus systems by short-axon granular cells 

 and collaterals from mitral and tufted cells, receives impulses from a segregated 

 and independent group of olfactory fibers and projects them through a limited 

 number of mitral and tufted cells to the secondary olfactory centers at the base 

 of the telencephalon. 



The question of whether there are a few basic types of receptors analogous to 

 the specific taste receptors, or a multitude of receptors or receptor units each 

 sensitive to different chemicals or whether all receptors are sensitive to all chem- 



Fig. 18. Olfactory pit on antenna of dipterous insect (Liebermann, 1926) 



