EXCITATION BY HYPERPOL ARIZING POTENTIALS 329 



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Fig. 3. Manifestation of electrical inexcitability of eccentric cell of Linni/iis. The 

 frequency of discharges in the axon was increased by depolarizing the cell with 

 an intracellularly applied current (dots, lower inset figure; abscissa in nA = 10"^ 

 A) or by lights which produced depolarizing generator potentials of different 

 amplitudes (circles; abscissa in mV of generator potential). {Main graph) — Another 

 experiment. The effects of the applied currents on the frequency are plotted as 

 the dots fitted by the "dark" curve. The circles of the other curves show the 

 generator potential produced by different intensities of light (I = 0-1-100 

 units) at various values of polarizing current. The dots show the frequencies of 

 the discharges in the axon under the same condition. The change in the slopes 

 of the straight lines shows that the membrane resistance of the "dark" cell is 

 decreased progressively by brighter illuminations. (Upper inset) — Equivalent 

 circuit of the electrically inexcitable Limuliis eccentric cell. Er and Eg are the 

 sources of the resting potential. One is in unreactive membrane whose resistance 

 {Rr) is unaffected by either illumination or polarization. The other is the generator 

 membrane whose resistance (Rg) is diminished by light, but not by applied 

 current (/). The voltage {V) resulting from the combined effects of current and 

 illumination, is the effective potential which determines the frequencies of the 

 discharges and may be calculated from the equivalent circuit. The calculated 

 lines for different illuminations and currents agree quite well with the frequencies 

 (dots) and generator potentials (circles) in main graph. (Figures combined from 

 Fuortes, 1959 and Rushton, 1959.) 



