CHEMORECEPTION 123 



over the middle range of chain length (Fig. 74). Olfactory thresholds 

 for fatty acids measured in the dog (Neuhaus, 1953 a, 1953 b) and the 

 honeybee (Schwarz, 1955) do not show such a precise relationship. 

 For these two animals the thresholds, expressed as molecules per c.c. 

 of air, are lowest in the region Co to C4. Dethier (1954 b) pointed out 

 that the relationship observed with alcohols suggests that olfaction in 

 these cases may involve the establishment of an equilibrium, and as 

 such represents a physical rather than a chemical process which in- 

 volves specific receptor substances (Dethier, 1956). Mullins (1954) 

 further analysed the date of Dethier and Yost (1952) by plotting the 

 product of the thermodynamic activity at threshold and the molal 

 volume of the compounds against chain length and assigning a cal- 

 culated membrane solubility parameter of 11 -5. The analysis suggested 

 that the vapour of the first three alcohols might not have been in 

 equilibrium with the organism. Additional data and analyses indicated 

 that olfaction does not involve an equilibrium process and that excita- 

 tion is probably a result of the ability of the odour molecules to pro- 

 duce local disorder in the oriented molecular structure of the cell 

 membrane (Mullins, 1955 a, 1955 b). 



The theories of Ehrensvard (1942) and Davies (1953 a, 1953 b) 

 postulate that odour molecules must first adsorb into the plasma mem- 

 brane of the olfactory neuron. Ehrensvard suggested that the potential 

 changes due to the adsorption actually initiated impulses in the re- 

 ceptor. The Davies theory envisions the adsorbed molecules as dislo- 

 cating the membrane, thus permitting an exchange of sodium and 

 potassium ions across it, with the consequent initiation of impulses. 

 The effectiveness of compounds depends not only upon the concen- 

 tration of adsorbed molecules but also upon their size and shape 

 (Davies and Taylor, 1959). Olfactory threshold data for Phormia, as 

 well as for dogs and man, confirm that both shape and adsorption are 

 important factors (Fig. 75). 



Electrophysiological studies thus far have not clarified our under- 

 standing of the receptor mechanism. Summed spontaneous activity in 

 antennal nerves has been recorded by Boistel and Coraboeuf (1953), 

 Boistel, Lecompte, and Coraboeuf (1956), Roys (1954), Smyth and 

 Roys (1954), Schneider (1955, 1957 a, 1957 b), Schneider and Hecker 

 (1956), and Morita and Yamashita (1961). Firing from individual 

 neurons has occasionally been detected in the antennae of Bombyx 

 mori and related species (Schneider, 1957 b), but it has not been pos- 

 sible to ascertain from which type of sensillum the activity originated 

 (Schneider, 1961). When the olfactory stimulus is extract of female sex 



