What is the Difference Between Absolute and Relative Refractory Period

The main difference between absolute and relative refractory period is that absolute refractory period is the period of time during which a second action potential absolutely cannot be initiated whereas relative refractory period is the interval immediately after the absolute refractory period. Furthermore, the absolute refractory period occurs due to the position of the time-gated ion channels while the initiation of another action potential is possible during the relative refractory period only under a greater stimulation for the depolarization.  

Absolute and relative refractory period are two types of refractory periods that simultaneously occur after the generation of an action potential. 

Key Areas Covered 

1. What is the Absolute Refractory Period 
     – Definition, Features, Importance 
2. What is the Relative Refractory Period 
     – Definition, Features, Importance 
3. What are the Similarities Between Absolute and Relative Refractory Period 
     – Outline of Common Features 
4. What is the Difference Between Absolute and Relative Refractory Period 
     – Comparison of Key Differences

Key Terms 

Absolute Refractory Period, Action Potential, Depolarization, Relative Refractory Period, repolarization

What is the Absolute Refractory Period 

Absolute refractory period (ARP) is the time just after the firing of an action potential. Generally, just after the firing of an action potential, sodium channels undergo inactivation spontaneously and rapidly at the peak of the action potential. However, when the sodium channels are inactivated, they are unable to reactivate immediately. This recovery from inactivation is a time and voltage-dependent process. Moreover, the full recovery of the reactivation usually takes about 4-5 msec. However, the initial time period after the peak of the action potential is the absolute refractory period.

Figure 1: Refractory Period

Furthermore, during the absolute refractory period, there is no way to fire a second action potential, no matter how strong the stimulus is. The time period through which the absolute refractory period exists is about 1-2 msec. 

What is the Relative Refractory Period 

Relative refractory period (RRP) is the time when the firing of a second action potential is possible. Generally, during the relative refractory period, sodium channels begin to recover from their inactivation. Therefore, if the stimulus is strong enough, the excitable membrane can fire a second action potential. Here, the stimulus has to be stronger than the stimulus, which can fire an action potential when the excitable membrane is at rest.  

Moreover, the full recovery of sodium channels occurs at the end of the relative refractory period. However, a continuous flow of potassium ions from inside to the outside of the cell is there. Therefore, there is a tendency to oppose any depolarization. That is why it requires a stronger stimulus to fire an action potential during the relative refractory period.  

Similarities Between Absolute and Relative Refractory Period  

• Absolute and relative refractory periods are two types of refractory periods which occur after an action potential. 
• Moreover,  they are two of the several phases of an action potential. 
• They are the time taken for an excitable membrane to be ready for a  second stimulus. 
• Thus, the main function of refractory periods is to repolarize and hyperpolarize an excitable membrane. 

Difference Between Absolute and Relative Refractory Period 

Definition 

Absolute refractory period refers to the period immediately following the firing of a nerve fiber when it cannot be stimulated no matter how great a stimulus is applied while relative refractory period refers to the period shortly after the firing of a nerve fiber when partial repolarization has occurred, and a greater than normal stimulus can stimulate a second response. 

Occurrence 

The absolute refractory period occurs first while the relative refractory period occurs after the absolute refractory period. 

Significance 

The absolute refractory period occurs due to the inactivation of sodium channels while both the inactivation of sodium channels and the P_k value greater than the P_k value of the resting phase are responsible for the relative refractory period.  

The Behavior of Ion Channels 

While the absolute refractory period contains inactivated sodium channels, the relative refractory period contains recovering sodium channels and opened potassium channels. 

Time Period 

Moreover, the absolute refractory period exists for 1-2 msec, while the relative refractory period exists for about 3-4 msec. 

Firing a Second Action Potential 

The initiation of a second action potential is not possible during the absolute refractory period while the initiation of another action potential is possible during the relative refractory period only under a greater stimulation for the depolarization.  

Conclusion 

The absolute refractory period is the initial time period just after the firing of an action potential. Generally, at the peak of the action potential, sodium channels undergo inactivation. Therefore, during the absolute refractory period, it is unable to fire a second action potential. On the other hand, the relative refractory period is the second refractory period, which allows the recovery of sodium channels. During this second refractory period, potassium channels remain open; therefore, it is possible to fire a second action potential only if the stimulus is stronger than a stimulus which can fire an action potential when the excitable membrane is at rest. Therefore, the main difference between absolute and relative refractory period is their features and their ability to generate an action potential. 

References:

1. “Refractory Periods – Neuronal Action Potential.” PhysiologyWeb, Available Here.

Image Courtesy:

1. “Action potential” By Original by en:User:Chris 73, updated by en:User:Diberri, converted to SVG by tiZom – Own work (CC BY-SA 3.0) via Commons Wikimedia