*A*, membrane potential response, shown as mean (black line) and standard deviation (grey band), to a ZAP current (*I*_{in}, fixed amplitude and logarithmically increasing frequency, shown at bottom) is depicted for each age (*n*= 22 (P7), 24 (P14), 22 (P21), and 32 (P28)). Neurons were first hyperpolarized to −70 mV with direct current, and ZAP current amplitude was adjusted for each neuron to elicit a 20 mV depolarizing deflection. Instantaneous frequency of the injected current is highlighted with grey bars above each trace, and the mean, peak resonance frequency for each age is depicted amid the grey bars with a filled triangle. *B*, relative impedance for input from 0.1–10 Hz, calculated by normalizing the power spectra of the voltage responses in *A* to the power spectra of injected current, were fitted with polynomials and plotted as the mean (*n* consistent with *A*). *C*, peak resonance frequency is plotted as means ± SEM, measured at the maximum of each neuron's individual power spectrum (*n*= 21 at P7, 24 at P14, 21 at P21, and 29 at P28). Statistical significance was assessed with a one-way ANOVA using Bonferroni's *post hoc* test to compare all data sets (***P* < 0.001 *versus* all other groups).

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