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
1? (A) A stimulus has triggered the
opening of voltage-gated sodium ion channels, and sodium ions depolarize the
membrane. (B) Voltage-gated potassium ion
channels open, and potassium ions diffuse out of the axon. (C) The membrane is at resting
potential, maintained by the sodium–potassium pump. (D) The inside of an axon has
become more negative than usual, causing hyperpolarization.
To help us answer the question,
let’s look at what is meant by the potential difference across the membrane and how
an action potential changes the potential difference at each stage.
The potential difference refers to
the membrane potential, which is the distribution of charged ions between the
extracellular space and the neuron’s cytoplasm. When a neuron is not firing an
action potential, the membrane potential resides at the resting potential. So, before an action potential
changes the membrane potential of a neuron, the potential difference across an axon
membrane is the resting potential. When an action potential is being
transmitted, there is a change in the electrical potential, which is caused by the
movement of ions in and out of the neuron as the action potential travels along the
axon of a neuron.
There are four stages of an action
potential: depolarization, repolarization, hyperpolarization, and the refractory
period. Let’s review how the membrane
potential changes during these four stages and how the movement of ions create these
changes in the membrane potential.
The resting potential is maintained
by the sodium–potassium pumps and the potassium leak channels. The sodium–potassium pump actively
transports three sodium ions out of the neuron for every two potassium ions
transported in, while the leak channels allow for the passive diffusion of potassium
ions out of the neuron into the extracellular space. This movement of ions means that at
rest, the cytoplasm of the neuron is more negatively charged compared to the
extracellular space. This results in the resting
potential being around negative 70 millivolts.
The start of an action potential is
depolarization. This is when the membrane potential
reverses from negative to positive because the cytoplasm of the neuron becomes more
positively charged compared to the extracellular space. When the neuron receives a
stimulus, voltage-gated sodium ion channels open and sodium ions flood into the
cytoplasm down their concentration gradient. As sodium ions diffuse into the
cytoplasm, it causes more voltage-gated sodium ion channels to open, which changes
the potential difference to a positive 40 millivolts.
Once the membrane potential reaches
a positive 40 millivolts, the voltage-gated sodium ion channels shut and the
voltage-gated potassium channels open, which allows for potassium ions to rapidly
diffuse out of the cytoplasm and into the extracellular space. This stage of the action potential,
called repolarization, is where the cytoplasm of the neuron becomes negative
again. However, since so many potassium
ions rush out so quickly, the membrane potential overshoots the resting potential;
this is known as hyperpolarization.
During hyperpolarization, the
potential difference reaches around negative 75 millivolts. At this potential difference,
another impulse cannot be sent, so messages do not merge together and it keeps
impulses moving in one direction. When the potential difference of
around negative 75 millivolts is reached, the voltage-gated potassium channels shut
and the sodium–potassium pumps begin to restore the resting potential. This stage of the action potential
is known as the refractory period. Soon after this stage, the neuron
is ready for the next action potential.
Now that we have gone through the
key points about the change in charge during an action potential, we can see that in
stage 1 of the graph, the potential difference is negative 70 millivolts, otherwise
known as the resting potential. With this information, we are now
ready to answer the question. Therefore, during stage 1, the
membrane is at resting potential, maintained by the sodium–potassium pump.
