Figure 1.

Unitary outward K⁺ currents via K_D channels versus E_M, from a representative “inside-out” patch of a S. saman extensor cell protoplast, during a slow voltage ramp. A, A linearly increasing voltage ramp applied to the patch membrane. B, Three traces of current (superimposed) during the voltage ramp. Note the “up and down steps” signifying opening and closing of K_D channels. Straight lines—fitted manually to open-channel currents—indicate idealized current levels through “n” (nos. to the right) simultaneously open K_D channels. Note the increase of “n” with the increased depolarization. I_Leak, Current recorded when all K_D channels are closed. E_rev, −79 mV. C, Initial analysis. I_S, Mean of three current records; i_S, idealized unitary current through a single open channel; I_Leak, as in B; 0, the level of zero current. D, Voltage dependence of K_D channel activation. The n̄ was calculated as a point-by-point ratio of currents in C (corrected for leak; Eq. 2). Note that n̄ increases with membrane depolarization. Dashed line, Boltzmann relationship (Eq. 4), with the following parameters (see “Materials and Methods”): n̄_max = 5.9; E_1/2= −37 mV; z = 1.4.