Video Transcript
The diagram shows a membrane undergoing depolarization. Which of the following best explains how a membrane becomes depolarized? (A) Voltage-gated sodium ion channels open, and sodium ions move down their
concentration gradient and into the axon. (B) Voltage-gated potassium ion channels close, and potassium ions diffuse through
the membrane and out of the axon. (C) The sodium–potassium pump now transports three potassium ions into the axon and
two sodium ions out.
This question asks us about membrane depolarization, which is very important for the
transmission of an action potential in nerve cells. To answer this question, let’s review the movement of ions during membrane
depolarization.
Depolarization of the membrane is the first stage of an action potential, which is
caused by the receipt of chemical messengers by the dendrites of a neuron at the
synaptic cleft. Depolarization causes the membrane potential to reverse from negative to
positive. The membrane potential, which is the distribution of charged ions between the
extracellular space and the neuron’s cytoplasm, typically rests around negative 70
millivolts.
During transmission of an action potential, voltage-gated sodium ion channels that
were previously shut are triggered to open, making the membrane more permeable to
sodium ions. Sodium ions diffuse into the neuron’s cytoplasm where they are less concentrated due
to the action of the sodium–potassium pump. The increased concentration of sodium ions in the intracellular space makes the
neuron’s cytoplasm less negatively charged.
The increased positivity of the membrane potential causes more voltage-gated sodium
ion channels to open. This means that sodium ions diffuse into the neuron at a faster rate, which continues
until the membrane potential reaches around positive 40 millivolts. When the membrane potential reaches around positive 40 millivolts, it causes the
voltage-gated ion channels to close again. And the next stage of the action potential, repolarization, begins. So, depolarization occurs as the voltage-gated sodium channels open, causing the
membrane potential to reach its peak value of positive 40 millivolts.
After reviewing the movement of ions involved in depolarization, we are now able to
answer our question correctly. A membrane becomes depolarized when voltage-gated sodium ion channels open, and
sodium ions move down their concentration gradient and into the axon.
