Absorption of Stimulus Energy 91 



stimulating salts and the sensory membrane. Some objections 

 can be made to it. For example, most of the early experimental 

 work with Phormia gave little indication that the anion of a salt 

 was of any importance in determining response magnitude; 

 however, this conclusion may not be strictly correct. ^^ In 

 addition, values for the equilibrium constant, calculated from 



500 



Fig. 40. A plot of the equation, a/R=a/Rm-\-i/RmKy using data 

 obtained from the response of a blowfly chemosensory neuron to 

 different concentrations of sodium chloride. (From Evans and 

 Mellon," Fig. 3.) 



data obtained from the Phormia sense organs, were found to vary 

 over a considerable range. This finding casts doubt upon the 

 assumption that the reaction of a stimulating substance with the 

 membrane is in a true state of thermodynamic equilibrium. The 

 objection might be overcome if it could be shown that the reaction 

 is in a steady-state condition, but further evidence will be necessary 

 before this can be resolved. 



One curious finding with the blowfly salt receptor has been 

 that potassium and sodium salts are about equally effective in 

 eliciting a response from the sensory neuron. Even neglecting 

 the slight contribution of the anion to the response, one finds the 

 maximum response to highly-concentrated solutions of both salts 

 S.O.— G* 



