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Question Video: Stating the Sequence of Stages in an Action Potential Biology

The diagram provided shows the stages of an action potential, with each stage assigned a number. State the correct sequence of numbers.


Video Transcript

The diagram provided shows the stages of an action potential, with each stage assigned a number. State the correct sequence of numbers. (A) 5, 2, 6, 3, 1, 4. (B) 1, 2, 6, 2, 5, 3. (C) 4, 2, 6, 1, 5, 3. (D) 4, 5, 1, 2, 3, 5.

This question asks us to list out the steps of an action potential in a neuron. So to answer this question, let’s review how these ions move during an action potential.

Prior to transmitting an action potential, the membrane potential is at a resting potential between negative 65 and negative 70 millivolts. At rest, the cytoplasm of the neuron is much more negative than the extracellular space and the sodium ion channels are closed, which prevents the sodium from moving down its concentration gradient. When a neuron is not at rest, the neuron is transmitting an action potential.

An action potential can be further broken down into its separate stages: depolarization, repolarization, hyperpolarization, and the refractory period.

Depolarization starts when a stimulus, such as a neurotransmitter, reaches the dendrites of a resting neuron and causes the membrane potential to flip from negative to positive. The membrane potential flips from negative to positive because the voltage-gated sodium ion channels open, allowing sodium ions to diffuse into the cytoplasm. When the membrane potential reaches its peak positive value at positive 40 millivolts, it causes the voltage-gated sodium ion channels to close again. As sodium ion channels close, preventing any more sodium ions from entering the neuron, voltage-gated potassium channels open, initiating repolarization.

In repolarization, the open voltage-gated potassium ion channels allow potassium ions to diffuse out of the neuron where it is more concentrated and into the extracellular space. When voltage-gated potassium ion channels open, so much potassium diffuses out of the neuron that the membrane potential becomes even more negative than the resting potential. This is called hyperpolarization.

After hyperpolarization, the membrane potential becomes so negative that the neuron cannot be stimulated to transmit another action potential for a brief period of time. This is called the refractory period. The refractory period resolves when the sodium–potassium pumps return the membrane potential to the resting potential of negative 70 millivolts. Only then will the neuron be ready to transmit another action potential.

Now that we’ve reviewed the stages of the action potential in neurons, we are able to answer our question. The correct sequence of numbers for an action potential is 4, 2, 6, 1, 5, 3.

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