Action potential

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Action potential

In: Biology

2 Answers

Anonymous 0 Comments

This is a really huge question.

An action potential essentially describes the change in potential (or voltage) across the cell membrane at a single point. It has a characteristic curve shape that is different for neurones (nerve cells) and cardiac/skeletal muscle, but essentially the theory is the same. Propagation of action potentials allows a wave of “excitation” to spread through a set of cells, resulting in, essentially, a signal moving from one point to another.

The membrane potential (the difference in voltage between the inside of the cell and the outside) is normally negative – this is called the resting membrane potential. Because it is negative you could refer to it as “polarised”. For neurones it is around -70mV, for skeletal muscle around -90mV. Cardiac is variable depending on which type of cell.

The resting membrane potential is a huge topic. It is contributed to by the Gibbs-Donnan effect and channel open/close states; way too much to delve into here.

An action potential occurs when a sequence of preceding events result in “depolarisation”. This could be another neurone stimulating certain channels, a cellular signalling machinery, or simply an incoming action potential. Either way, the membrane becomes less negative.

Once it reaches a certain level (-40mV in neurones, I think), it causes sodium channels to open. This results in sodium moving into the cell, because sodium is present in vast amounts outside the cell in comparison to inside. The net flow of positive charge into the cell causes a spike in the potential to above positive. This is very short lived because the sodium channels close very rapidly.

At some point in this sequence, potassium channels open. Because the potassium situation is opposite to sodium, potassium moves out of the cell, causing a net outflow of positive charge. So the membrane becomes more negative, resulting in “repolarisation”. It actually overshoots in the negative phase, resulting in “hyperpolarisation”.

The period where the action potential is occurring and the hyperpolarisation just after is known as the refractory period – this is because new action potentials either can’t be generated (absolute) or take a higher stimulus to be generated (relative).

The membrane then returns to resting membrane potential, so this can all happen again! It all happens in 3 milliseconds in a neurone. Cardiac is slower at 300ms and more complex because there are more channels; same goes for skeletal muscle.

One last thing is that the action potential created is just at one point in time and place. For the signal to travel along a nerve, multiple APs need to be generated in sequence. The way this happens in nerves is via stimulation by the previous action potential – the depolarisation opens the next set of sodium channels, so the cycle continues.

Anonymous 0 Comments

Think of cells as batteries and action potentials as the electricity generated by it. When cell membranes open, voltage is generated, generating an action potential. They are called as such because the voltage generated has the potential to generate an action.