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Question Video: Identifying the Stage of an Action Potential Where the Membrane Has Depolarized Biology

The graph provided shows how the potential difference across an axon membrane changes during the course of an action potential. What is happening during stage 2?

04:15

Video Transcript

The graph provided shows how the potential difference across an axon membrane changes during the course of an action potential. What is happening during stage 2? (A) Voltage-gated potassium ion channels open, and potassium ions diffuse out of the axon. (B) The membrane is at resting potential, maintained by the sodium–potassium pump. (C) A stimulus has triggered the opening of voltage-gated sodium ion channels, and sodium ions depolarize the membrane. Or (D) the inside of the axon has become more negative than usual, causing hyperpolarization.

First, let’s work out what the graph is showing us. On the 𝑥-axis, we have the time in milliseconds. We can see that the entire action potential takes only six milliseconds. That’s incredibly fast. On the 𝑦-axis, we have the potential difference across the membrane of the axon, measured in millivolts. Although potential difference sounds like a complicated term, it just means the difference in charge between the space inside the neuron and the extracellular space outside the neuron. The potential difference across the membrane of a neuron is known as the membrane potential. If the inside is more negative than the outside, then this value will be negative. And if the inside is more positive than the outside, then it will be positive.

The question is asking us about stage 2, where we can see that the potential difference increases from about minus 65 millivolts to around plus 40 millivolts. Why does this happen? Let’s have a look at the events that are occurring across the axon membrane to find out. This diagram represents the membrane of the axon just before stage 2 of the action potential. We can see that there are three ion channel proteins embedded in the membrane. The first, labeled x, is a potassium ion channel, which is open and therefore allows potassium ions, represented here as pink dots, to diffuse out of the axon down their concentration gradient.

The second, labeled y, is a sodium–potassium pump. The sodium–potassium pump uses energy in the form of ATP to transport three sodium ions, represented here as green dots, out of the axon and two potassium ions in, both against their concentration gradients. The third ion channel protein, labeled z, is a voltage-gated sodium ion channel, and it only opens in response to a stimulus that causes a change in the membrane potential. You may recall that a stimulus is any change in the environment that the body detects and responds to. When a stimulus is detected, the potential difference across the axon membrane changes, causing voltage-gated sodium ion channels to open.

Due to the activity of the sodium–potassium pump, there is a much higher concentration of sodium ions outside the axon than inside. So, when the voltage-gated sodium ion channels open, sodium ions rapidly diffuse down their concentration gradient into the axon. Because sodium ions, like potassium ions, are positively charged, this influx of sodium ions causes the potential difference across the axon membrane to increase and become positive. This process is called depolarization. And it’s reflected in the upward curve of the graph that we can see during stage 2 of the action potential. The more depolarized the membrane becomes, the more voltage-gated sodium ion channels open.

This chain-reaction effect is why the graph increases so steeply during stage 2 of the action potential. It also explains how the action potential travels from one end of the axon to the other, which can sometimes be as far as a couple of meters. We have therefore determined that the correct answer to the question is (C). During stage 2 of an action potential, a stimulus has triggered the opening of voltage-gated sodium ion channels, and sodium ions depolarize the membrane.

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