Patch model reproduces features of gating in macropatches. (A) Patch-gating model that accounts for macropatch currents. Model states are as follows: R, resting closed; CL, singly bound closed, one agonist site; CL₂, doubly bound closed, two agonist sites; O, open; D_s, slow desensitized; D_f, fast desensitized; BL₃, triply bound blocked, two agonist and one inhibitory; BL₄, quadruply occupied, two agonist and two inhibitory; BL₅, pentuply occupied, two agonist and three inhibitory. Model parameter values are given in a table with standard errors (SE). Model fits of empirical macropatch target data used in parameter estimation values are as indicated (B: left, 500 μM; right, 3,000 μM). Open bars above indicate delivery of PB at specified concentration. (Insets) Fits of activation and tail current time course on expanded timescale for clarity. Straight lines indicate baseline. (C, top) Comparison of families of the patch model (left) and empirical (right; reproduced from Fig. 4 A) responses. Individual traces selected for clarity (concentrations indicated) and families were scaled to normalize to the peak of a 3,000-μM response. Upper trace indicates PB application. (Bottom, left) Concentration–response relationships for patch model I_PB peak, plateau, and tail amplitudes in the same fashion as used for the empirical results in Fig. 4 A. (Right) Fraction of activated channels blocked versus PB concentration in patch model. Activated channels represent those that are open (O) or have sojourned through the open state (D_s, D_f, BL₃, BL₄, and BL₅). Blocked activated channels approximates the conditional probability of blocked states, given that the channel is activated and is represented by the expression [P{(BL₃, BL₄, or BL₅) | (O, D_s, D_f, BL₃, BL₄, or BL₅)}] (see Results).