3. Action potential

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An action potential is a travelling wave of electrical excitation or a nerve impulse that can carry a message without the signal weakening from one end of a neuron to another.

 An action potential is triggered by a sudden local depolarization of the plasma membrane, i.e., by a shift in the membrane potential. A stimulus that causes sufficient large depolarization to surpass a certain threshold value causes voltage gated Na+ channels to open, thus allowing Na+ ions to enter the cell down their electrochemical gradient.

©  Molecular Devices

 The influx of positive charge depolarizes the membrane further, causing the membrane potential to become less negative. More voltage gated sodium channels open, causing more Na+ ions to enter and therefore leading to further depolarization. The membrane potential becomes less and less negative, until it becomes positive. 

 The membrane potential reaches to a peak of about +40 mV, which is close to that at which the electrochemical driving force for the movement of Na+ across the membrane is zero. In other words, Na+ ions have no further tendency to enter or leave the cell.

Sodium channels have an automatic inactivating mechanism, by which they rapidly adopt a special inactive conformation, causing the channel to be unable to open again.

 The membrane potential then returns to its resting value during repolarization. Voltage gated K+ channels open, allowing K+ ions to flow out of the cell down their electrochemical gradient. This brings the membrane back to its resting state more quickly than could be achieved by K+ outflow through potassium leak channels.

 The third phase, known as hyperpolarization, is when the cell becomes more negative than its typical resting membrane potential. As the action potential passes through, potassium channels stay open a little bit longer, and continue to let positive ions exit the neuron. This means that the cell temporarily gets even more negative than its resting state. As the potassium channels close, the sodium-potassium pump works to re-establish the resting state.

 At last, the membrane potential returns to the resting voltage that occurred before the stimulus occurred, known as the resting state. 




References
Anon (n.d.) What is an action potential? [Online]. Molecular Devices. Available from: https://www.moleculardevices.com/applications/patch-clamp-electrophysiology/what-action-potential [Accessed: 11 April 2020].

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