Voltage-Gated Ion Channels

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The bit of information in nerves is the action potential, a fast electrical transient in the transmembrane voltage that propagates along the nerve fiber. In the resting state, the membrane potential of the nerve fiber is about ¡ 60 mV (negative inside with respect to the extracellular solution). When the action potential is initiated, the membrane potential becomes less negative and even reverses sign (overshoot) within a millisecond and then goes back to the resting value in about 2 ms, frequently after becoming even more negative than the resting potential. In a landmark series of papers, Hodgkin and Huxley studied the ionic events underlying the action potential and were able to describe the conductances and currents quantitatively with their classical equations (Hodgkin and Huxley, 1952). The generation of the rising phase of the action potential was explained by a conductance to NaC ions that increases as the membrane potential is made more positive. This is because, as the driving force for the permeating ions (NaC) was in the inward direction, more NaC ions come into the nerve and make the membrane more positive initiating a positive feedback that depolarizes the membrane even more. This positive feedback gets interrupted by the delayed opening of another voltage-dependent conductance that is K-selective. The driving force for KC ions is in the opposite direction of NaC ions, thus KC outward flow repolarizes the membrane to its initial value. The identification and characterization of the voltage-dependent NaC and KC conductances was one of the major contributions of Hodgkin and Huxley. In their final paper of the series, they even proposed that the conductance was the result of increased permeability in discrete areas under the control of charges or dipoles that respond to the membrane electric field. This was an insightful prediction of ion channels and gating currents.