HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



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FIG. II. Ratio of molar concentration and magnitude of integrated cliorda tympani response 

 plotted against molar concentration of stimulus. Explanation in text. [From Beidler (19).] 



equation expressing the i^inding of ions by proteins. 

 The equation describes the responses to different 

 organic sodium salts (see fig. 11). From the low values 

 of AF, the change in free energy, the relative inde- 

 pendence of response magnitude of temperature or 

 pH, the conclusion is drawn that the ions are loo.seiy 

 bound to the taste receptor surface by a nonenzymatic 

 process much like that which occurs in the binding 

 of ions by proteins or naturally occurring polyelec- 

 trolytes, such as nucleic acid or polysaccharides (19). 

 Such binding may be the initial step in a series of 

 reactions leading ultimately to stimulation of the 

 receptor and depolarization of the associated afferent 

 nerve fiber. Species differences are attributed to dif- 

 ferences in the detailed configurations of the reacting 

 molecular sites on the receptor surfaces. 



SWEET. The sweet taste appears to be associated 

 primarily with organic compounds, except for certain 

 inorganic salts of lead and beryllium. The aliphatic 

 hydroxy compounds which include alcohols, glycols, 

 sugars and sugar derivatives constitute one of the 

 better known classes. Other stimuli are aldehydes, 

 ketones, amides, esters, amino acids, sulfonic acids, 

 halogenated hydrocarbons, etc. A sampling of thresh- 

 old values for the commoner sweet stimuli is £;iven in 

 table 6. 



The complex relations between structure and the 

 .sweet taste cannot be adequately explained by any 

 present systematization (sO- Oertly & Myers C148) 

 listed a number of sweet-producing molecular ar- 

 rangements and postulated that, to be sweet, a .sub- 

 stance must contain a 'glucophore' and an 'auxogluc' 

 Examples of their analysis are given in table 7. How- 

 ever, saccharin and dulcin are but two of the many 

 exceptions. 



In an homologous series, the taste of the members 

 often changes from sweet to bitter with increase in 

 molecular weight. In the higher members of an 

 homologous series, it is said (144) that taste eventually 

 disappears when the products become insolufjie. On 

 the other hand, as previously noted, an increa.se in 

 molecular weight associated with increasing chain 

 length in an homologous series is paralleled by a 

 decrease in water solubility and an increase in taste 

 stimulating efhciency. This was shown for alcohols 

 and glycols at threshold (66). Some (62) have hy- 

 pothesized that sweetening power is often a.ssociated 

 with low water solubility, but numerous exceptions 

 make this a difficult rule to maintain. 



The importance of the spatial arrangement of the 

 molecule is strikingly clear in the case of homologues 

 in which small changes may produce striking dif- 



