The action potential (or nerve impulse) is a chemical and electrical reaction that allows neurons to communicate with each other. Going through different stages, the electrical potential of the neuron changes sharply according to the ions entering and leaving the neuron. As it propagates along the neuron, the action potential reaches the terminal buttons and releases neurotransmitters. Then, when there are enough neurotransmitters that attach to another neuron and cause positive ions to enter, that neuron will also drive the nerve impulse and so on.

Neuron ions

If this is the first time you’ve heard of action potential, chances are these explanations are a little difficult to grasp. However, I will illustrate and define each term so that you have a good understanding of the action potential at the end of this article. I also invite you to ask questions in comments since it is a very important concept to understand in order to be able to read more easily about neurosciences.

Legend, Action potential
* Both ions are green because they are positively charged

1. Resting membrane potential

Basically, when the neuron is not transmitting a message, it’s around -70mV (millivolts). The channels which play a role in the action potential are therefore closed.

Channels: These are inputs designed for positive and negative ions to enter and exit the neuron. The displacement of these ions can make the neuron more positive or more negative.

Resting membrane potential, Action potential
We see here a neuron on which I zoomed at the level of the axon cone. We also see that the channels, shown in yellow, red and purple, are all closed.

2. Depolarization: Ligand-gated channels open

We remember that the action potential is used to release neurotransmitters. So when a neuron releases it, they attach themselves to another neuron. Then, when this neuron receives neurotransmitters on its receptors, it opens its chemo-dependent channels.

These channels allow the entry of positive and negative ions into the neuron. However, since the aim of this article is to explain the action potential, we will assume that the released neurotransmitters have opened channels that allow the entry of positive ions.

We can see this step as an ALL OR NOTHING phenomenon. Let me explain. In the axonic cone there is a summation of all positive and negative ions. It’s a bit like calculating an average. So if there is a large enough input of positive ions, there may be action potential. If there are not enough positive ions compared to negative ions, there is no action potential. This summation is done only below the excitability threshold which is -55mV, because if this electric potential is exceeded, the action potential is triggered.

Depolarization, Action potential
Chemo-dependent channels are shown in purple. We see that positive ions quietly enter the neuron thanks to the neurotransmitters that open the channels.

3. Action potential: Voltage-gated Na+ ion channels open

Reaching the excitability threshold (-55mV) triggers the opening of the voltage-dependent channels. These channels are a little different, because they open thanks to an electrical potential greater than -55mV unlike chemo-dependent channels which open thanks to neurotransmitters. So the depolarization continues, but this time at lightning speed. These channels allow the entry of a large quantity of specific positive ions: the Na + ions. Electrical potential of the neuron therefore goes very quickly from -55mV to + 30mV! The action potential is then officially engaged.

Action potential
The opening of voltage-gated Na + channels causes a massive entry of positive Na + ions.

4. Repolarization: Voltage-dependent K + channels open

When the electrical potential of the neuron reaches +30 mV, the Na + channels close and the K + channels open. Then, the K + ions leave the neuron through these channels making the neuron more and more negative.

Repolarization, action potential
The K + channels, shown in yellow, release the K + ions from the neuron.

5. Hyperpolarization: The neuron is more negative than at rest

By the time the K + channels close, so many positive ions have come out of the neuron that it becomes even more negative than at rest for a short time. During this time, the neuron will need a lot more positive ions so that it can depolarize again. So the neuron can rest for a short while while it restores its electrical potential to its resting state, i.e. -70mV.

In short.

To summarize, here is a small chart that illustrates the different steps.

Graph, Action Potential
We can see the electrical potential of the neuron as a function of time.

The nerve impulse travels along the axon like a chain of falling dominoes.

Nerve impulse

Then, once the nerve impulse reaches the opposite end of the neuron, Ca2+ ions enters the synaptic buttons. The arrival of Ca2+ ions ensure that neurotransmitters are released.

Neurotransmitters attach themselves to another neuron to open its chemo-dependent channels. If enough positive ions enter, the voltage-gated channels open, the action potential propagates along the neuron to the terminal buttons and releases neurotransmitters which attach themselves to ANOTHER neuron and so on. Until the nerve impulse reaches its destination.

Sources

Martini, F. (2015). L’essentiel de la biologie humaine, une approche visuelle. Pearson.

Le cerveau à tous les niveaux!. (s.d.). La communication neuronale. Université McGill. https://lecerveau.mcgill.ca/flash/a/a_01/a_01_cl/a_01_cl_fon/a_01_cl_fon.html

Kolb, B., Whishaw, I.Q. et Teskey, G.C. (2019). Cerveau et comportement. https://books.google.ca/books?id=4J6WDwAAQBAJ&pg=PA133&lpg=PA133&dq=canal+chimio+d%C3%A9pendant+influx+nerveux&source=bl&ots=_Q7Ej2IzYa&sig=ACfU3U0etV5GBytfEjrKG8-DkAD9kR8Akw&hl=fr&sa=X&ved=2ahUKEwiYnbmzipLrAhVnRN8KHT__Ch4Q6AEwCnoECAkQAQ#v=onepage&q=canal%20voltage%20d%C3%A9pendant&f=false

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