Chemoreception 463 



The relative sweetness of sugars has been determined by a number of authors, as illus- 

 trated in the following list," based on a value of 100 for sucrose: 



fructose 173 rhamnose 32 



glucose 74 galactose 32 



xylose 40 raffinose 23 



maltose 32 lactose 16 



These values vary somewhat with the concentration. 



The bitter taste is elicited by a wide variety of chemical compounds. Some 

 salts— e.g., caesium chloride, many iodides, many calcium, ammonium, or 

 magnesium salts (MgS04, or epsom salts)— are bitter, as are picric acid and 

 the alkaloids, the latter being the most bitter ot all compounds. A list of 

 some of the substances which elicit a bitter or bitter-sweet taste, compiled 

 by Cohn,'** reveals the wide variety of their chemical structure: 



Bitter: picric acid, phenyl urea, glucose triacetate, strychnine 

 Bitter then sweet: o-benzoyl benzoic acid, p-amino-azobenzene 



sulphonic acid, Meucyl-d-tryptophane, phenolphthalein, sodium- 



naphthoyl o-benzoate 

 Bitter and sweetish: tetrachlorethyl ether, 2, 3-dichlorhexane 

 Bitterish and sweet: hexenylglycerin 

 Bitterish then sweetish : sodium ethyl sulfonate. 

 Bittersweet, pungent: nitrostyrene 



The method of choice in the study of the physiology of chemoreception 

 is the recording of nerve spike potentials in the afferent nerves connected 

 with the peripheral sensory end-organs. Such studies among the insects 

 have met with consistent failure,-^ which is attributed to the low magnitude 

 of these electrical transients in the small sensory' nerve fibers; they are prob- 

 ably below the noise level of electron tube amplifiers (i.e., several microvolts). 

 Among the vertebrates several successful studies have been published. Pum- 

 phrey''- recorded from nerve fibers innervating the anterior portion of the 

 tongue of the frog. He obtained records of action potentials (Fig. 151) from 

 fibers which could be excited by applying acid and salt solutions to the 

 tongue. PfaflFman''^ recorded action potentials from single nerve fibers in- 

 nervating at least three different chemoreceptors, some of which could be 

 stimulated by acid, others by acid and salts, and still others by acid and 

 quinine. Notably absent were the chemoreceptors stimulated by sugar. Only 

 one study has been reported in which sugar produced a response,**^ and this 

 was observed only once. These results cast some doubt on the discrete na- 

 ture of the chemoreceptors which mediate the different taste sensations. 



The evidence for four specific receptors may be listed as follows: (1) taste 

 qualities are zonally distributed; (2) taste sensations are abolished by co- 

 caine in the following order: bitter, sweet, salt, and finally sour;""' (3) gym- 

 nemic acid selectively abolishes sweet and bitter but not salt and sour; *^ 



(4) papillae can be found that are sensitive to only one of the four tastes; 



(5) the threshold for electrical stimulation of the taste buds is different for 

 the four different tastes;- (6) in the cat recorded action potentials from single 

 fiber preparations indicate at least three different sensitivities: sour, sour and 

 salt, and sour and bitter.*'^ 



If there are four, and only four, tastes, then it should be possible to dupli- 

 cate any complex taste by mixture of four types of agent— sodium chloride, 



