Refractory Periods

Absolute Refractory Period

There are specific periods following the stimulation when a neuron cannot have another action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission.. This happens after a neuron experiences an action potential.  Here is our action potential graph. I want you to think about which channelsProtein passages in the cell membrane that allow specific molecules to pass through. are opening and which are closing as the action potential proceeds.  Here in depolarizationThe loss of electrical charge across a membrane, triggering an action potential., we are opening sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. channels.  The peak of this graph represents when all sodium channels are open.  If you were to send the neuron another stimulus at this point, the neuron would respond by saying it has all channels open. 

Sorry, I can’t do more for you until some close.”  This is a period of time called the absolute refractory periodTime during which no new action potential can be initiated, no matter how strong the stimulus..  During this time, the neuron is unable to respond to another stimulus.  Go ahead and poke the bear, it won’t respond.  The neuron CAN’T respond because all sodium channels are already open.  As the cell starts to repolarize, sodium channels will start to close.  Once they do, the neuron can then depolarize in response to another stimulus.  So, the absolute refractory period is dependent on the sodium channels.

Relative Refractory Period

The relative refractory periodThe period after an action potential when a stronger-than-usual stimulus is needed to trigger anothe follows the absolute refractory period. It is a time when the neuron can respond to another stimulus. However, the stimulus has to be stronger than the first.  At this point, we are in the repolarizationThe return of membrane potential to a negative value after depolarization. downhill of the line on the graph.  Sodium channels are starting to close. This allows the neuron to respond to another stimulus. Meanwhile, the potassium(K⁺): Major ICF cation; essential for muscle and nerve function. channels are also starting to open here.  This is different than the first stimulus. 

The first stimulus arrived when all channels are closed. Now, some of those potassium channels are open. We have to overcome that to have another action potential.  The relative refectory period begins here. Sodium channels are starting to close. It extends to here. Potassium channels are starting to close.  During this time, the neuron can have another action potential. However, because potassium channels are open, we need a stronger stimulus to excite this neuron.