Difference Between Depolarization and Repolarization

The main difference between depolarization and repolarization is that the depolarization is the loss of resting membrane potential due to the alteration of the polarization of cell membrane whereas repolarization is the restoration of the resting membrane potential after each depolarization event. Furthermore, the inner membrane is less negatively charged during depolarization while the negative charge of the inner membrane is restored during repolarization. 

Depolarization and repolarization are two sequential events that occur in the cell membrane during the transmission of nerve impulses. 

Key Terms 

1. What is Depolarization
     – Definition, Resting Membrane Potential, Action Potential
2. What is Repolarization
     – Definition, Potassium channels, Importance
3. What are the Similarities Between Depolarization and Repolarization
     – Outline of Common Features
4. What is the Difference Between Depolarization and Repolarization
     – Comparison of Key Differences

Key Terms 

Action Potential, Depolarization, Potassium Channels, Repolarization, Resting Membrane Potential, Sodium Channels

What is Depolarization 

Depolarization is the change in the resting membrane potential to a more positive value. The resting membrane potential is the potential across the cell membrane at rest, which is -70 mV. This means the cell interior is more negatively-charged when compared to the cell exterior. The resting membrane potential is maintained by: 

1. the continuous diffusion out of potassium ions from the cell; 
2. the action of the sodium-potassium pump, which pumps 3 sodium ions out of the cell while taking two potassium ions into the cell; and  
3. the presence of more negatively charged ions such as proteins and phosphate ions in the cell interior.
   

   Figure 1: Generation of an Action Potential

When an action potential wants to fire, a depolarization current is generated by the opening of sodium channels, which allows more sodium ions to enter into the cell. This results in the decrease of the negative charge in the cell interior. When the membrane potential reaches -55 mV, the action potential is fired. During the transmission of a nerve impulse in the form of an action potential, the membrane potential across the cell membrane is +30 mV.  

What is Repolarization 

Repolarization is the event through which the membrane potential is reconverted into the resting membrane potential, following the depolarization of the cell membrane. Following depolarization, the sodium channels, which cause the less negative charge inside, are closed while the potassium channels are opened due to the presence of more positive ions inside. This results in the movement of potassium ions out of the cell, making the cell interior more negative. Finally, the repolarization process restores the resting membrane potential. 

Figure 2: Movement of Ions During an Action Potential

Repolarization does not trigger any mechanical activity by signaling to the effector organs such as muscles, unlike during the depolarization event. However, repolarization is essential in order to make the cell membrane ready for the transmission of a second nerve impulse by depolarizing for the second time. 

Similarities Between Depolarization and Repolarization 

• Depolarization and repolarization are two events that occur on the cell membrane of nerve cells during the transmission of a nerve impulse. 
• Both are regulated by the opening and closing of ion channels. 
• Sodium-potassium pump is active during both events. 

Difference Between Depolarization and Repolarization 

Definition 

Depolarization refers to the movement of a cell’s membrane potential to a more positive value while repolarization refers to the change in membrane potential, returning to a negative value. 

Change in the Membrane Potential 

The inner membrane becomes less negative during depolarization while repolarization turns the inner membrane negative. 

Membrane Potential 

While depolarization increases the membrane potential, repolarization decreases the membrane potential, restoring the resting membrane potential. 

Action Potential 

Depolarization facilitates the firing of an action potential while repolarization prevents the firing of an action potential. 

Ion Channels 

The opening of sodium channels causes depolarization while the closing of sodium channels and the opening of the potassium ion channels cause repolarization.  

Importance 

Whereas depolarization results in stimulating effector organs such as muscular contractions, repolarization does not result in the stimulation of the effector organ. 

Conclusion 

Depolarization is the process by which the resting membrane potential is decreased, facilitating the firing of an action potential. However, repolarization is the subsequent process through which the resting membrane potential is restored. The opening of sodium channels is responsible for the depolarization of the cell membrane while the opening of the potassium channels is responsible for the repolarization. The main difference between depolarization and repolarization is the influence on the resting membrane potential. 

Reference:

1. Emily. “Q & A: Neuron Depolarization, Hyperpolarization, and Action Potentials.” Khan Academy, Khan Academy, Available Here
2. Samuel, Leslie. “010 Repolarization: Phase 2 of the Action Potential.” Interactive Biology, with Leslie Samuel, Interactive Biology, with Leslie Samuel, 10 Jan. 2016, Available Here

Image Courtesy:

1. “1221 Action Potential” By OpenStax (CC BY 4.0) via Commons Wikimedia 
2. “The process of how action potential passes through a neuron” By Giovanni Guerra – Own work (CC BY-SA 4.0) via Commons Wikimedia