Activation and inactivation of the Na+ current in immature IHCs.

<p><b>a</b>, Normalized activation and inactivation curves from P0–P4 apical IHCs. Continuous lines are calculated from eqn. 3 (activation) and eqn. 4 (inactivation). Fitting parameters for the activation curves are: apical IHCs (<i>n</i> = 14) <i>V</i>_½ = −29.4 mV, <i>S</i> = 6.5 mV; <i>I</i>_max = −4289 pA; basal IHCs (<i>n</i> = 13) <i>V</i>_½ = −31.6 mV, <i>S</i> = 6.6 mV; <i>I</i>_max = −3198 pA. Fitting parameters for the inactivation curves are: apical IHCs (<i>n</i> = 12) <i>V</i>_½ = −67.0 mV, <i>S</i> = 5.4 mV; <i>I</i>_max = −4341 pA; basal IHCs (<i>n</i> = 10) <i>V</i>_½ = −68.8 mV, <i>S</i> = 4.9 mV; <i>I</i>_max = −2687 pA. <b>b</b>, <i>I</i>_Na was recorded from apical and basal P0 IHCs during 10 ms depolarizing test steps to −21 mV, following 50 ms conditioning steps (not shown) from −131 mV to −1 mV in 5 mV increments and used to derive the inactivation curves. Some of the conditioning steps are shown next to the traces. Apical and basal IHCs are as in <b>Fig. 4a</b>,<b>b</b>, respectively.