During iN cell reprogramming, both passive and active membrane properties gradually approach the levels seen in primary cultured neurons (Vierbuchen et al., 2010). The resting membrane potential becomes more hyperpolarized and the capacitance increases as the cell volume increases. The input resistance also decreases, presumably as a result of the appearance of more neuronal membrane channel proteins. As for passive membrane properties, under current clamp recording mode, action potential-like responses induced by depolarization grow to resemble the mature stereotypic shape with increasing amplitudes and narrowing widths. A mature, all-or-none action potential is characterized by constant and high amplitude irrespective of the induction-trigger shape and current, with a fast depolarization and fast repolarization. The rapid repolarization ensures the regeneration of voltage-gated Na+ channels, which enables mature cells to fire a series of action potentials (Koester and Siegelbaum; Lockery et al., 2009). Therefore, the presence of repetitive action potentials is a clear sign that the ion-channels responsible for generating action potentials are properly orchestrated. Repetitive action potentials occur spontaneously or can be evoked by injecting currents. However, the presence of spontaneous action
