Fig. 1. A schematic diagram of the voltage-gated Na⁺ channel subunit. The four homologous domains are labeled I–IV, and the six transmembrane segments in each domain are labeled S1–S6. The critical motif of the fast inactivation particle (IFM) is shown on the cytoplasmic linker connecting domains III and IV. The channel is aligned with the extracellular side of the membrane on the top. The lengths that are shown for the N and C termini and the interdomain cytoplasmic loops are consistent with the sequence of the Na_v1.2 mammalian Na⁺ channel, but these linkers (except for that between domains III and IV) vary greatly in length and sequence among the different Na⁺ channel isoforms. P, P loop.