E2 Generation of action potential (Hodgkin-Huxley model)

Topic

The action potential is a transient, regenerative depolarization of the neuronal membrane, generated by the sequential, voltage-dependent opening and closing of the membrane's sodium and potassium channels.

The Hodgkin-Huxley model describes the total current flowing across the neuronal membrane during an action potential as the sum of three distinct currents: a fast sodium current, governed by the activation and inactivation gates of the sodium channels; a slower potassium current, governed by the activation gate of the potassium channels; and a passive leak current, which is independent of any gates. The kinetics of each activation and inactivation gate depend on the instantaneous voltage of the neuronal membrane.

Each activation or inactivation gate of the neuronal membrane relaxes exponentially toward a steady-state value determined by the membrane voltage, with a time constant that also depends on that voltage: the faster the time constant, the more rapidly the gate approaches its steady-state value in response to a voltage change.

This relaxation of the various gates toward their steady-state values ​​sequentially produces the distinct phases of the neuronal membrane's action potential: first, the opening of the sodium channel activation gate produces the rapid depolarization phase; next, the inactivation of that same sodium channel gate, combined with the delayed activation of the potassium channel gate, causes repolarization of the neuronal membrane; and finally, while the sodium channel inactivation gate has not yet recovered, the neuronal membrane undergoes a refractory period during which it cannot generate a new action potential. This action potential model employs the same variables previously defined for the resting neuronal membrane: the observed membrane potential, neuronal membrane capacitance, the conductance of each ion channel, the Nernst potential for each ion, and an external current applied from outside the neuronal membrane. The difference is that, in this action potential model, the conductance of each ion channel is no longer constant; instead, it depends on both time and the neuronal membrane voltage, mediated by the activation and inactivation gates described earlier. Ultimately, it is this very neuronal membrane—with its sodium and potassium channels governed by voltage-dependent gates—that generates the action potential, including its phases of rapid depolarization, repolarization, and the refractory period.

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