Relative Refractory Period of Action Potentials

[Shakattack12]

Hello,

Quick question on the relative refractory period in neurons. I understand it is caused by the slow closing of voltage gated K+ channels, which leads to hyperpolarisation. This means a larger than normal stimulus is required to bring the membrane to threshold.

However, after reading my textbook it also says some Voltage gated Na+ channels are slow to open. I read elsewhere that the activation gate is voltage dependent while the inactivation gate is time dependent. How does this contribute to the membrane requiring a greater than normal stimulus?

 

[atyy]

In the rising phase of an action potential, voltage-gated sodium channels open and sodium enters the cell.

In the falling phase of an action potential, voltage-gated sodium channels remain open but become inactivated, preventing sodium from entering or leaving the cell through those channels. In the falling phase of the falling phase of an action potential, voltage-gated potassium channels also open and potassium leaves the cell.

The sodium channel inactivation contributes to an absolute refractory period, during which even if the voltage is above spike threshold, an action potential will not occur since its rising phase requires sodium entry through the voltage-gated sodium channel, which is inactivated.

The potassium channel opening contributes to a relative refractory period. In this time, the sodium channels are no longer inactivated, so it is possible for an action potential to start. However, to start an action potential the voltage must become more positive (ie. positively charged ions must enter the cell). Since the potassium channels are open, and potassium ions are leaving the cell (ie. positively charged ions are leaving the cell), this means that we have to have an even bigger amount of positively charged ions entering the cell (to counteract the potassium ions leaving the cell) in order to start an action potential.

 

[Shakattack12]

In the rising phase of an action potential, voltage-gated sodium channels open and sodium enters the cell.

In the falling phase of an action potential, voltage-gated sodium channels remain open but become inactivated, preventing sodium from entering or leaving the cell through those channels. In the falling phase of the falling phase of an action potential, voltage-gated potassium channels also open and potassium leaves the cell.

The sodium channel inactivation contributes to an absolute refractory period, during which even if the voltage is above spike threshold, an action potential will not occur since its rising phase requires sodium entry through the voltage-gated sodium channel, which is inactivated.

The potassium channel opening contributes to a relative refractory period. In this time, the sodium channels are no longer inactivated, so it is possible for an action potential to start. However, to start an action potential the voltage must become more positive (ie. positively charged ions must enter the cell). Since the potassium channels are open, and potassium ions are leaving the cell (ie. positively charged ions are leaving the cell), this means that we have to have an even bigger amount of positively charged ions entering the cell (to counteract the potassium ions leaving the cell) in order to start an action potential.

Sorry I may not have been clear enough. I understand everything you have said. I have attached a photo of the exact sentence in the textbook. How does this have anything to do with requiring a larger-than-normal stimulus or the relative refractory period?

 

[atyy]

Sorry I may not have been clear enough. I understand everything you have said. I have attached a photo of the exact sentence in the textbook. How does this have anything to do with requiring a larger-than-normal stimulus or the relative refractory period?

Perhaps a longer excerpt from the book will make it clearer to me. From the short bit, my guess is that they are saying that at the end of an action potential the sodium channels do not all recover from their inactivation simultaneously. Thus after an action potential, only some channels have recovered from the inactivation, and it is these channels that are available for starting the next action potential. Since there are fewer sodium channels available than if all the sodium channels had recovered, it takes a bigger stimulus to cause the next action potential.

 

[Shakattack12]

That's all the information the book includes. The rest continues with what you mentioned above regarding voltage gated K+ channels. However, I understand what you are saying. Thanks for the help.