CHEMORECEPTOR MECHANISMS 27 



Mullins (1955a), on the basis of threshold measurements in man of 

 ^-paraffins, ;z-alcohols, and ;?-alkyl chlorides, in which at any given number of 

 carbon atoms the thresholds are about the same for any series, has shown 

 that thresholds (pt/ps) are lowest for the 4-5 carbon members and rise for 

 longer and shorter membered chains. On the basis of these and other more ex- 

 tensive data he has proposed that two molecular parameters are of importance 

 in olfactory stimulation: (1) molecular shape and (2) cohesive energy density 

 (the energy of vaporization of a liquid per unit volume). This second factor is 

 roughly related to the oil/water partition coefficient (for full discussion consult 

 Mullins, 1954). 



Obviously only a beginning has been made in analyzing the process of olfac- 

 tion by critical experiment, but even at this early stage indications are emerging 

 which suggest that the process is less likely to be a chemical than a physical one. 



Conclusion 



The attempts to analyze the mechanism of chemoreception at the molecular 

 level at which the receptor is conceived of as a transducer are of relatively re- 

 cent origin. Obviously they have not progressed far enough to supply us with 

 satisfactory answers to the questions which opened this discussion. Yet they 

 have supplied us with some broad and very suggestive hints. In the light of our 

 present knowledge it is highly probable that the process of olfaction and the 

 process of taste are more nearly biophysical than biochemical reactions. The 

 case of sugars may be a special one which involves a more specific type of reac- 

 tion. That the sugars might be exceptional is not unexpected in view of their 

 special and important role in carbohydrate metabolism in many tissues of the 

 body. 



Acknowledgment 



The records in Figure 15 were obtained by M. Wolbarsht on equipment 

 made available through the courtesy of the United States Naval Medical Re- 

 search Institute. 



References 



Adrian, E. D. 1950. Sensory discrimination with some recent evidence from the olfac- 

 tory organ. Brit. Med. Bull. 6: 330-333. 



Allison, A. C. and R. T. T. Warwick. 1949. Quantitative observations on the olfactory 

 system of the rabbit. Brain 72: 186-197. 



Andersson, B., S. Landgren, L. Olsson and Y. Zotterman. 1950. The sweet taste 

 fibres of the dog. Acta physiol. scand. 21: 105-119. 



process. Adaptation after olfactory stimulation may be considered to be an equilibrium 

 effect. Evidence is presented to support the suggestion that the excitatory action of 

 molecules is by virtue of their ability to produce a local disorder in the oriented molec- 

 ular structure of the cell membrane. 



