Action potential
The action potential - nature, formation, propagation. The Hodgkin-Huxley equations. Ion channels - model structure, effects of toxins and drugs.
An action potential is a short, rapid rise and fall of the membrane potential that follows a fixed pattern. It occurs in excitable cells: neurons, muscle cells and some endocrine cells. In neurons it carries signals along the axon to the synapses, and in muscle cells it is the first step towards contraction.
Action potentials depend on voltage-gated ion channels and run in stages:
- Threshold: a stimulus depolarizes the membrane to its threshold, and voltage-gated sodium channels open.
- Depolarization: sodium flows in, which depolarizes the membrane further and opens more channels. This positive feedback reverses the polarity of the membrane.
- Repolarization: sodium channels inactivate and potassium channels open, so potassium flows out and the potential falls.
- Hyperpolarization: potassium channels close slowly, so the potential briefly drops below the resting level.
During the refractory period the membrane cannot readily fire again, which stops the signal from traveling backward. The sodium-potassium pump restores the ion balance.
Propagation: local currents depolarize the neighboring membrane, so the action potential spreads along the axon. In myelinated axons it jumps between the nodes of Ranvier (saltatory conduction), which is much faster.
The Hodgkin-Huxley equation:
The Hodgkin-Huxley model describes how action potentials start and spread. It treats the membrane as an electrical circuit: the lipid bilayer acts as a capacitor, and the ion channels act as conductors in parallel. In the equations below, Y stands for the leak current.
Equation 1:
Equation 2 (for Hodgkin-Huxley ionic currents):