520 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



leaves salt and sour unaffected (191). The effect may 

 be demonstrated most simply after chewing a few of 

 the dried leaves. A similar action is described for 

 verba santa, an extract of the leaves of Eriodicyton 

 calif ornicum. Topical application of w^eak concentra- 

 tions of cocaine depress taste in the order, bitter > 

 sweet > salt > sour. Stovaine in proper concentra- 

 tions will eliininate sweet and Ijitter and reduce 

 sensitivity to salt and acid followed by hypergeusia 

 for salt. Cocaine ageusia may be followed ijy hyper- 

 geusia for sweet and bitter. The Sudanese plant 

 Bumelia duliifica is said to change sweet and bitter to 

 sour. 



The extract from the Gvnimmn leaves, after purifi- 

 cation and recrystallization, yields a white powder 

 with a melting point of i99°C and a molecular weight 

 of 805. This appears to be a glycoside which yields 

 glucose, arabinose and a small quantity of glucuro- 

 nalactone upon hydrolysis. The hydrolysate has no 

 effect on taste. Other sul^stances in the crude extract 

 do not influence the inhiljitory action. Not only is 

 the sweet taste of such diverse substances as sucrose 

 and saccharin suppressed, both are equalh' reduced 

 in sweetness, i.e. at suprathreshold levels equal sweet- 

 ness matches remain unchanged (Warren, R. M. & 

 C. Pfaffmann, unpublished observations). 



The differential action of this drug is clearly shown 

 in the electrical activity of the chorda tympani nerve 

 (fig. 12), although the effect does not last as long as 

 the perceptual effect in man. A similar reversible ef- 

 fect can be produced by the salts of heavy metals, 

 silver and mercury, but not by arsenic, arsenous acids 

 or potassium ferricyanide. This suggests competitive 

 blocking of a mechanism which is nonenzymatic (88). 

 These results are in striking agreement with those 

 obtained in invertebrate contact chemoception (67, 

 68). The evidence from functional studies contra- 

 indicates an enzymatic process in stimulation gener- 



!i!SO 



>eo. 



3 



r-^ 



10 MIN. 



i-i .1 No CI 



•—» .1 sue. 



T r 



TIME (mir ) 



1 r 



l-IO min H 



FIG. 12. Differential suppression of taste response to sucrose 

 («/<r.) and sodium chloride (chorda tympani discharge) by 

 gymnemic acid after 10 min. application to surface of tongue. 

 [From Hagstrom (88).] 



ally in spite of the demonstration of enzymes in the 

 neighborhood of taste cells (14). 



Action potential studies have shown that an indi- 

 \idual afferent receptor neural unit might respond 

 not only to sugar but also to a salt like .sodium chlo- 

 ride. Thus, specificity to a chemical agent must be 

 specificity of sites within or on the individual sense 

 cells. Presumably, different sites are specifically 

 sensiti\e to sugar on the one hand and salt on the 

 other (in addition to a wide variety of other sub- 

 stances). Gymnemic acid does not block the receptor 

 cell as a whole but onl\' the sucrose site. 



Threshold and suprathreshold ecjual-sweetne.ss com- 

 parison methods have been employed to study rela- 

 tive sweetness of different stimuli. Although the 

 exact values may vary from experimenter to experi- 

 menter, the relative order of sweetness among the 

 sugars, for example, is the same. In equimolar con- 

 centrations the order of sweetness is: sucro.se > fruc- 

 tose > maltose > glucose > lactose (44). The rela- 

 tion between concentration and sweetness changes 

 with concentration (see fig. 13). There are very wide 

 differences in the stimulating efficiency of sweet 

 stimuli. Close to threshold, saccharin is 500 to 700 

 times less concentrated than sucrose of equal sweet- 

 ness. As yet there is no indication why .some syn- 

 thetic agents are so effective. In general, the available 

 evidence suggests that the initial step in sweet stimu- 

 lation ma\' be a process like that already elaborated 

 for salts. The response to sugar is resistant to enzyme 

 poisons and pH change but not to surface active 

 competiti\e inhibitors. 



BITTER. Bitter, like sweet, is elicited by members of 

 many chemical classes and is often found in associa- 

 tion with sweet and other taste qualities. Increasing 

 molecular weight of inorganic salts is associated with 

 increasing bitterness (see p. 516). An increase in 

 length of the carbon chain of the organic molecules 

 may he associated with a change from sweet to Ijitter. 

 Many sweet substances have a concomitant bitter 

 taste or aftertaste (e.g. saccharin). This douiile or 

 multiple taste quality is especially apparent as the 

 stimulus moves from the front to the back of the 

 tongue where bitter sensitivity is especially developed. 

 The best known class of bitter substances is the 

 alkaloids which are complex nitrogenous compounds, 

 often highly toxic, such as quinine, caffeine, strych- 

 nine and nicotine (144). Nitro compounds are often 

 bitter (such as picric acid) especially if three or some- 

 times two nitro groups are present. The following 

 groups are often associated with bitter taste : (NO2) > 



