Once inactivation has occurred, additional prolonged depolarization produces only minor changes in distribution between fast and slow recovery. (A) A standard paired pulse protocol (5 ms to +0 mV) was used to examine recovery from inactivation at −80 mV with recovery in response to the second pulse shown on the right. (B) Following inactivation at +0 mV, a 50-ms step to +70 mV preceded the recovery step to −80 mV, with individual recovery tests on the right. (C) Fractional recoveries for the cell shown in A and B are plotted, along with an additional control protocol without the +70 mV step. Lines are fits of a double exponential function to the recovery time courses. With no step to +70 mV, Af =0.46 ± 0.01 (mean ± confidence limit), τf = 14.8 ± 0.7 ms, As = 0.56 ± 0.01, and τs = 363.0 ± 14.6 ms; with a step to +70 mV, Af = 0.39 ± 0.01, τf = 20.5 ± 1.2 ms, As = 0.63 ± 0.01, and τs = 388.7 ± 16.6 ms; after return to a protocol with no step to +70 mV, Af = 0.40 ± 0.01, τf = 19.6 ± 1.4 ms, As = 0.6 ± 0.01, and τs = 354.9 ± 19.2 ms. (D) Average recovery time courses are shown for six cells for the indicated conditions. Initial control recovery after a 5-ms step to 0 mV: Af = 0.51 ± 0.02, τf = 14.1 ± 1.0 ms, As = 0.51 ± 0.01, and τs = 328.1 ± 25.5 ms. Recovery following both the 5-ms inactivation step and the 50-ms step to +70 mV: Af = 0.40 ± 0.01, τf = 22.3 ± 2.1 ms, As = 0.60 ± 0.01, and τs = 377.3 ± 25.8 ms. A repeat of recovery following just the 5-ms inactivation step to 0 mV: Af = 0.44 ± 0.02, τf = 15.6 ± 1.3 ms, As = 0.56 ± 0.01, and τs = 337.9 ± 24.0 ms. (E) Mean and individual values for Af, τf, and τs are shown for the set of six cells from D. There are small differences in Af and τf observed with the +70 mV step, but similar to that which would expected from a 55-ms inactivation step to 0 mV (Fig. 4). All error bars in D and E are SD. Except as indicated in the middle panel, all t-test comparisons yielded P > 0.1. w/o, without.