66 The Physiology of Sense Organs 



and (2) the major external ion species involved in depolarizing 

 receptor potentials is sodium. 



An increase in membrane permeability for diffusible ion 

 species (= increased membrane conductance) can most easily be 

 detected as a decrease in the overall electrical resistance of the 

 sensory cell membrane. A thorough investigation of such resist- 

 ance changes has been made by Fuortes'^ for the case of the 

 eccentric cell in the eye of Limulus following exposure to light. 

 While this preparation is not a primary sense cell, there is a good 

 deal of justification for assuming that the principles involved in 

 the production of the generator potentials in this eye are similar 

 to those responsible for other stimulus-induced potential changes 

 found in alternative sensory arrangements. 



It is not usually practical to make a direct measurement of 

 ionic current flow across the sensory cell membrane during 

 stimulation, although this would be the most satisfactory means 

 of determining the magnitude of increased membrane conductance. 

 Usually it is simpler to measure changes in overall membrane 

 resistance by recording the alterations which occur when a 

 potential drop is generated by passing a known constant current 

 across the membrane. The potential of the resting membrane is, 

 of course, changed by the passage of such a current, and this 

 voltage shpijljl summate with a receptor potential induced by the 

 stimulus, jln the Limulus eye, for example, light energy causes a 

 depolarization, and an artificially generated potential can summate 

 with it to produce impulses if the total depolarization surpasses 

 threshold values. In fact, any supra-threshold combination of 

 light and artificially-evoked decrease in resting potential will 

 generate equivalent impulse frequencies, so long as the absolute 

 level of membrane potential reached in all cases is the same. 

 However, as shown in figure 29, Fuortes found that the relation- 

 ship between experimentally applied depolarizing current and the 

 voltage drop generated by such current itself changes with 

 different values of stimulus light intensity, and the change is 

 steepest — ^with maximum slope — in the dark. Since the slope of 

 the relationship is equal to Efl = R, that is, overall membrane 

 resistance, it can be seen that this parameter decreases as light 

 intensity is increased. The conclusion drawn is that the end 

 result of stimulating the eye with light is an increased conductance 



