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.
