Relationship between available Na⁺ conductance and the effects of noise in pyramidal neurons. A, Scatter plot showing gain change by noise versus maximal rate of rise of the first action potential (AP) during a just-suprathreshold, noiseless current step. The four layer 5 (L5) pyramidal cells with the largest gain increases by noise (circled) had relatively slow-rising action potentials, suggesting a possible relationship between available Na⁺ conductance and gain change. L2/3, Layer 2/3. B, Voltage traces showing the first action potential during a just-suprathreshold current step, in control solution and while applying a subsaturating concentration of TTX (10 nm). TTX increased spike threshold and reduced spike amplitude. C, f–I relationships (in the absence or presence of noise) in control solution and with TTX. Application of TTX raised the current threshold and lowered the current at which spike failure occurred, but it did not cause noise to increase gain. D, Bar graph showing the mean additive effect of noise, with and without TTX and the asAHP. TTX did not change the additive effect of noise in control conditions or while applying the asAHP. E, Mean gain change by noise, with and without TTX and asAHP. TTX did not alter the gain change by noise in control or with the asAHP. Ctrl, Control.