CHEMORECEPTOR MECHANISMS 



TABLE 1 



Scheme illustrating possible relationship between afferent impulse 



pattern and evoked taste sensation in the cat (Cohen, 



Hagiwara, and Zotterman, 1955) 



there remains a question of whether two specific receptors are involved or a 

 single less specific one. Pfaffmann (1941) first demonstrated by recording from 

 single fibers in the chorda tympani nerve of the cat that there were at least three 

 physiologically different types of receptors (the term "receptor-unit" as applied 

 by Beidler, Fishman, and Hardiman (1955) is preferable). One type responded 

 only to acids, a second to acids and salts, and a third to acids and quinine. No 

 fibers were then found which responded when sugars were applied to the tongue. 

 Subsequently, "sweet" fibers were demonstrated (Beidler, 1952); and recently 

 Cohen, Hagiwara, and Zotterman (1955) further characterized four physiolog- 

 ically different fibers. Their findings are summarized in Table 1. 



In the frog, Zotterman (1949, 1950) and Andersson and Zotterman (1950) 

 found some fibers which responded specifically to water and to dilute NaCl 

 (< 0.025%), some which were specific for acids, and others which responded to 

 hypertonic NaCl and CaCU . In the chorda tympani of the dog, Andersson et al. 

 (1950) detected fibers which were specific for sugars. In the rat, Beidler (1953) 

 found fibers which responded specifically to salt, to salt and acid, and to sugar. 



The foregoing results clearly indicate that there is a degree of true specificity 

 in taste receptors of mammals and that there exists at the peripheral level a 

 basis for the four modalities. There is evidence, furthermore, that the sensitivity 

 of the several receptors represents a specialization of these cells and is not 

 merely a consequence of exposure and opportunity. Beidler (1953) has shown, 

 for example, that salt receptors in the rat have a lower threshold for NaCl 

 (0.002 M) than other receptors innervated by the lingual nerve (+1.0 M) and 

 than bare axon. 



Granting, therefore, that there may be found in the mammal taste receptor- 

 units which are specifically sensitive to acid, salt, acid and salt, acid and quinine, 

 water, and sugar, what further information can be adduced concerning the char- 

 acteristics of these units? Most of the experimental work which has been done 

 has involved stimulation of large populations of receptors, i.e., areas of the 

 tongue or individual papillae. Some of the electrical recording has been con- 



