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
three? (A) The inside of the axon has
become more negative than usual, causing hyperpolarization. (B) A stimulus has triggered the
opening of voltage-gated sodium ion channels, and sodium ions depolarize the
membrane. (C) The membrane is at resting
potential, maintained by the sodium–potassium pump. Or (D) voltage-gated potassium ion
channels open, and potassium ions diffuse out of the axon.
First, let’s interpret the graph
we’ve been provided with. On the 𝑥-axis, we have the time in
milliseconds. You may recall that a millisecond
is one thousandth of a second, so action potentials happen incredibly quickly. On the 𝑦-axis, we have the
potential difference across the membrane of the axon, measured in millivolts. Also known as the “membrane
potential,” the potential difference across the membrane of a neuron is the
difference in charge between the space inside the neuron and the extracellular space
outside the neuron. If the space inside is more
negative than the extracellular space, the membrane potential will be negative. And if the space inside is more
positive than the extracellular space, it will be positive.
This question is asking us about
stage three, where we can see that the potential difference across the axon membrane
decreases from about plus 40 millivolts to around minus 75 millivolts. Why does this happen? To find out, let’s consider the
events that are occurring across the axon membrane. This diagram represents the
membrane of the axon at the end of stage two of an action potential, just before
stage three. We can see that there is a higher
concentration of potassium ions, represented here as pink dots, and sodium ions,
represented here as green dots, inside the axon than outside. This is because the voltage-gated
sodium ion channel, labeled here as y, is open. So, sodium ions are able to diffuse
into the axon down their concentration gradient.
The sodium–potassium pump, labeled
here as x, is also pumping more potassium ions into the axon than are diffusing out
through the open potassium ion channel, labeled here as w. Because both sodium and potassium
ions are positively charged, this means the membrane potential has increased to
around plus 40 millivolts, as we can see here on the graph. When the axon membrane reaches plus
40 millivolts, the voltage-gated sodium ion channel closes and the voltage-gated
potassium channel, labeled here as z, opens. Due to the action of the
sodium-potassium pump, there is a higher concentration of potassium ions inside the
axon than outside. So, potassium ions diffuse out of
the axon down their concentration gradient.
Because potassium ions are
positively charged, this efflux causes the extracellular space to become more
positive than the axon cytoplasm. So, the membrane potential
decreases and becomes negative. This process is called
repolarization, and it’s reflected in the downward curve of the graph that we can
see during stage three of the action potential. We have therefore determined that
the correct answer to the question is (D). During stage three of an action
potential, voltage-gated potassium ion channels open, and potassium ions diffuse out
of the axon.
