468 Comparative Animal Physiology 



lation are not related to structural formulae. It appears, therefore, tfrat 

 chemical structure as a correlate of stimulating effectiveness may be dis- 

 missed for the gustatory chemoreceptors as well as for the olfactory receptors. 



The correlation between stimulating effectiveness and lipoid solubility or 

 oil-water distribution coefficients is in accord with all known facts of gusta- 

 tory sense organ stimulation. This is illustrated by several groups of data. 

 In man taste of the inorganic acids seems to be a simple matter of pH, but 

 in the case of organic acids or of buffered solution the stimulating efficiency 

 seems to be related also to the presence of undissociated acid. The simplest 

 explanation for these findings is that the pH of the membrane or the cyto- 

 plasm of the sense cell is the determining factor in stimulation. Undissoci- 

 ated molecules penetrate the membrane more rapidly and then may become 

 ionized in the membrane or in the cytoplasm, thus producing an eflfective 

 change in pH.^'' In this connection it has been demonstrated that the in- 

 troduction of polar groups (-OH, -CI, -Br, -COOH, or -NH2) into the acid 

 molecule reduces its stimulating effectiveness, whereas the introduction of ad- 

 ditional methyl groups or the lengthening of the carbon chain increases 

 the stimulating effectiveness.'"'^ 



Among the insects Frings'^'^ has shown that the stimulating effectiveness 

 of several series of inorganic salts with a common anion increased in the 

 following order, Li + < Na+ < Mg++ <Ca++ = Sr++ < K+ < Cs+ = 

 Rb+ < NH4+ <<< H + , and this scheme is in agreement with most of 

 the reported data on other animals, including man. The order of effectiveness 

 of the salts is in accord with their ionic mobiHties and also with their partition 

 coefficients. Dethier and Chadwick^" measured the rejection thresholds for 

 a series of aliphatic alcohols, and they obtained a high correlation between 

 stimulating effectiveness and boiling point, molecular area, molecular mo- 

 ments, vapor pressures, activity coefficients, and oil-water distribution co- 

 efficients. Recently the same authors-^ determined that the glycols are rejected 

 by blowflies at logarithmically decreasing concentrations as the chain length 

 is increased. 



SUMMARY 



The history of chemoreception is studded with man's attempts to explain 

 and rationalize the mechanism involved in the activation of these sense or- 

 gans. Yet today this is still one huge question mark. True, the work relating 

 physical and chemical properties of compounds to their stimulating effect- 

 iveness may soon demonstrate those properties of the stimulus essential for 

 activation of chemoreceptors, but the question of mechanism of activation 

 will still remain unanswered. This answer must be sought through investi- 

 gation of cellular changes in the sensory end-organ itself, a difficult but 

 challenging problem. 



REFERENCES 



1. Adrian, E. D., and LuDwiG, C, /. P/iysio/. 94:441-460(1938). Nerve discharges 

 from olfactory organs: fish. 



2. Allun, F., and Weinberg, M., Quart. ). Exper. Physiol. 15:385-420 (1925). Elec- 

 trical stimulation of taste buds: man. 



3. Bauer, L., Ztschr. vergl. Physiol. 26:107-120 (1938). Physiology of taste in 

 beetles: Hydrous. 



