Module 1 Lesson 6 - 3

The Action Potential

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Read pages 374 - 377

When a neuron becomes stimulated sufficiently by a threshold stimulus, the point of stimulation becomes depolarized and the depolarization spreads along the length of the unmyelinated neuron. This depolarization is produced by a rapid change in membrane permeability and a corresponding change in the balance of ions maintained at the resting state. In the neuron, after the wave of depolarization, an immediate recovery called repolarization occurs.

Depolarization

Let's return to the resting membrane potential. The net charge on the outside of a resting neuron is positive and the net charge on the inside of a resting neuron is negative. The resting neuron is polarized.

The sodium channels (purple) and potassium channels (yellow) in a neuron are voltage-gated. This means the channels open and close depending on the changes of the voltage on the membrane.



An external stimulus applied raises the membrane potential and makes it more positive toward the direction of 0. Not all stimuli are intense enough to trigger a nerve impulse. The minimum voltage required to start an action potential is called the threshold potential. In the diagram, the stimulus must reach –55 mV to start an action potential. In a typical neuron, the threshold potential is –55 mV, but the threshold potential can vary depending on the neuron.

A stimulus of –60 mV will not fire a neuron if the threshold is –55 mV. A stimulus of –56 mV will also not fire a neuron. However, a stimulus of –54 mV will fire a neuron.



When the stimulus makes the membrane potential more positive and the threshold potential is reached, the voltage-gated sodium channels (purple) open, allowing the sodium ions from extracellular fluid to enter the cell. This is called depolarization. The voltage-gated potassium channels also begin to open, but they are slower than the voltage-gated sodium channels are.

The sodium ions continue to rush in and increase the membrane potential to +30 mV. This is the maximum depolarization. At this point, the voltage-gated sodium channels begin to close, but the voltage-gated potassium channels (yellow) remain open.



This graph depicts an action potential in a neuron. It shows the change in the membrane potential of the neuron as the action potential progresses. So far, the left side of the peak has been considered. 

A Recap of Depolarization

1. Resting membrane potential: -70 mV
2. Stimulus: **ZAP**
3. Threshold potential reached: -55 mV
4. Na+ ions rush in.
5. Membrane potential reaches +30 mV
6. Na+ channels close.

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As the membrane potential nears the threshold potential, some of the voltage-gated sodium channels with lower threshold begin to open. This allows some of the sodium ions into the neuron. The movement of sodium ions into the neuron helps to increase the membrane potential to reach the threshold potential. When the threshold potential is reached, the remaining voltage-gated sodium channels open to allow the action potential to occur.

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1. What is illustrated by Number 1? Explain what is happening in Number 1.
   
   
2. What is shown by Number 2? Explain what is happening to Number 2 at this time.
   
   
3. What is indicated by Number 3? Identify one characteristic of Number 3 and identify the function of Number 3.
   
   
4. What structure is shown by Number 4? Explain its function.
   
   
5. What are Numbers 5 and 6? What is their function in nerve impulse transmission?
   

Repolarization

At maximum depolarization, the voltage-gated sodium channels are closed but the voltage-gated potassium channels are open. The sodium ions stop flowing into the cell, and the potassium ions begin to leave the cell. As the positive potassium ions leave, the inside of the cell becomes less positive and returns to the resting potential. This is called repolarization.

As the potassium ions exit, the membrane potential returns to its resting state of -70 mV. However, the voltage-gated potassium channels are slower to react than the voltage-gated sodium channels are. Although the voltage-gated potassium channels start to close at -50 mV, their actions are delayed. This results in a slight hyperpolarization in which the membrane potential becomes more negative than the resting state is.

After repolarization, the location of the charged particles has reversed. The sodium ions are inside the cell, and the potassium ions are outside the cell. This problem can be resolved with the help of sodium-potassium exchange pump. Three sodium ions are taken out into the extracellular space while two potassium ions are brought into the cytoplasm.

A Recap of Repolarization

1. Maximum depolarization: +30 mV
2. Na+ channels close.
3. K+ channels open.
4. K+ ions rush out.
5. Membrane potential: - 70 mV
6. K+ channels close slowly.
7. Hyperpolarization

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1. What is shown by Number 1 in this diagram? How do you know this?
   
   
2. What is happening in Number 2 in this process?
   
   
3. What is the function of Structure 3?
   
   
4. What functions does Structure 4 serve in the process illustrated?
   
   
5. What do Numbers 5 and 6 tell you about Structure 4?
   

Watch and Listen

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1. You should now realize that communication through the neuron involves a series of action potentials. For further review of the events of the action potential, watch the following animation on The Action Potential.  This video also gives you a preview of how impulses are transmitted along the neuron which you will learn about later in this lesson and how the impulses are transferred across the synapse which you will learn about in the next lesson.