Action Potential: Definition & Steps

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  • 0:01 Neurons
  • 1:10 Action Potential
  • 3:17 Polarization
  • 4:47 Lesson Summary
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Lesson Transcript
Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

How do brain cells communicate with each other? View this lesson to find out about action potentials of neurons, how that helps them to communicate with each other, and what happens when neurons become more or less polarized.


Angela is playing a game with her friend Jodie. They are each going up to the top of their apartment buildings, which are across the street from each other. On the roofs, they will use mirrors to reflect the sun and send each other messages.

What Angela and Jodie are doing is not that different from what happens in your brain all the time. Neurons are nervous system cells, including those in the brain. Brain activity is caused by many neurons passing messages from cell to cell. Just like Angela and Jodie are passing messages from the top of their buildings, neurons send information from one cell to the next.

Each neuron has an axon, which is a long part of the cell that carries messages towards another neuron.

For the message to be sent, it has to go all the way down the neuron's axon until it gets to the end. It's like an arm reaching out to hand a note to another person. But how, exactly, does a message travel down the axon?

To understand that, let's look closer at an action potential, and what happens when a neuron is depolarized or hyperpolarized.

Action Potential

Ok, so Angela and Jodie want to pass messages, but first, Angela needs to get to the roof of her apartment building.

An action potential is a chain reaction down the length of an axon, which causes the neurotransmitter to fire at the neighboring neuron. It's kind of like Angela walking up the stairs in her building, from floor to floor, until she reaches the roof and is able to send Jodie her message. How does an action potential work? To understand, let's think about Angela. Before she can get to the roof, before she can even begin to climb the stairs, she has to start somewhere. If she's in the lobby of her building, she's in the normal starting place.

A resting potential is the normal state of an axon. It's like the neuron is at rest, and therefore it's called the resting potential. During resting potential, there are lots of ions that are traveling in and out of the axon, kind of like the lobby of Angela's building. Some of the ions travel easily, and some have it a little harder. During a resting potential, there is a higher concentration of potassium ions inside the axon and a higher concentration of sodium ions outside the axon. This makes the inside of the axon have a more negative charge at rest than the outside of the axon.

But what happens when an action potential begins? If a neuron has a message that needs to be sent, sodium channels in the axon open, and sodium rushes into the axon. This makes the inside of the axon more positive relative to the outside. This sets off the chain reaction, as more sodium channels open up a little further down the axon, causing that part of the axon to become more positive, and so on.

It's kind of like Angela deciding to go up to the roof and pass her message to Jodie. She can't just magically teleport to the roof. Instead, she has to climb up one flight of stairs at a time. She's getting closer and closer to the point where she can pass the message on to Jodie.


Now, as we've said, if Angela wants to go to the roof of her building, it makes sense for her to start in the lobby of the building. That's kind of like the resting potential of a neuron. But what if Angela starts somewhere else, like her apartment on the third floor or the basement down below the lobby?

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