How to Label an Action-Potential Graph Showing Depolarization

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  • 0:04 What Is an Action Potential?
  • 0:54 Action Potential Steps
  • 3:19 Graphing an Action Potential
  • 4:28 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, you'll be reviewing the parts of an action potential: depolarization, resting potential, threshold, and the refractory period. We'll look in detail about how to label these parts on an action potential graph as well.

What Is an Action Potential?

Right now, you're using your eyes to read this information. As you look at this page, your brain immediately interprets what you're seeing as words, and you make connections with other information stored as memories. Your brain also tells the muscles surrounding your eyes to contract or relax, allowing you to focus on the screen and track the words on the page. When you start to think about it, it's pretty amazing that you can read. How does all this happen so quickly?

The answer is action potentials. Your brain cells, called neurons, send lightning fast, electrical signals to each other to communicate through their axons, long connections between the cell body and the terminal. These signals are called action potentials. Scientists can measure how the electrical signals move through a neuron and create graphs with the information. But before we start on these graphs, let's review the steps in sending an action potential.

Action Potential Steps

Let's look back at the example of reading. When you read, light enters your eyes through the pupil. The light strikes sensory cells in your retina. These cells get the message that you're seeing something. So, here's how the steps of an action potential unfold as you're reading:

1. Depolarization

Sensory neurons, like the ones that sense light in your eyes, are activated by a stimulus, like light. Other neurons in your brain are activated by chemicals called neurotransmitters that are sent in between neurons.

So, as you're reading this, light is hitting sensory neurons in your eyes. The light causes channel proteins to open on the cell membrane. These proteins allow positively charged ions, like sodium and calcium, to enter the cell.

Your brain and other neurons are electrical organs. Electricity surges through their long axons, just like electricity moves through a wire in your house. All cells have a normal membrane potential, called the resting potential. When ions enter or leave the cell, the membrane potential changes.

So, when positive ions enter the cell, it causes an increase in the membrane potential of the cell. The membrane potential increases until the threshold potential is reached. Then voltage-gated sodium channels open and allow sodium to come rushing into the neuron at the junction of the axon and cell body.

At this point, the cell undergoes depolarization, which is a rapid increase in membrane potential. This response is all or none, meaning that once the depolarization starts, it goes on for a set amount of time, then the sodium channels slam shut, preventing the axon from depolarizing.

2. Repolarization

After the action potential is sent down the axon, the initial segment needs to be reset to start a new impulse. This phase is called repolarization. When the membrane potential increases to a certain level, voltage-gated potassium channels open. Potassium also has a positive charge, but when the channels open, potassium rushes out of the cell. Since a positive ion is leaving, it makes the cell more negative.

3. Hyperpolarization

Eventually the cell gets so negative, it actually overshoots the original resting potential. This is called hyperpolarization. During this phase, the membrane potential is more negative than it would normally be. This makes it harder for a neuron to reach the threshold to send a signal than normal, limiting the number of signals that can be sent back to back. This is called the refractory period, where it's more difficult for a cell to start an action potential.

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