4. Resting and Action Potentials in Excitable Cells Membrane Potential —I L

B Single-Channel Conductance Changes

C Average

Na+-Conductance _

10 msec

FIGURE 13 Single-channel changes in Na+ conductance. In response to a pulse depolarization, the probability of single Na+ channels opening is increased. (A) Voltage step used to depolarize the membrane. (B) Successive traces obtained in response to multiple presentations of the voltage step in A; with any single trace the relationship is not particularly clear. (C) When the traces are averaged, a conductance change resembling that of the macroscopic conductance (see Fig. 11) change is observed. (Modified from Sigworth FJ, Neher E. Nature 1980; 287:447-449.)

membrane-spanning regions designated S1 to S6. The functional significance of specific regions has been elucidated. For example, region S4 contains a high density of positively charged residues and is believed to represent the voltage sensor of the channel. The channel pore is believed to be formed by the four homologous regions between S5 and S6 (also designated the SS1 and SS2 regions, the H5 region, or the P region). Channel inactivation (see below) seems to be associated with the region that links domains III and IV. In particular, a hinged-lid structure formed by the amino terminus of domain IV has properties consistent with an ability to

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