The absolute refractory period:
is there any reason for this period (the cause)?
The absolute refractory period is considered to be a time during which no stimulus, no matter how great, can cause an action potential to be generated. This occurs after each nerve depolarization/action potential generation process. The way it accomplishes the depolarization is via the influx of sodium ions across the cell membrane facilitated by ion channels specific to sodium. As the action potential travels down the axon, channels are opened in a wave-like fashion to facilitate the conduction or movement of the signal from the cell body to the nerve terminal. Since these action potential are driven by ion gradients across the membrane, the cell must re-establish those gradients to repeat the process. This repositioning of ions, again accomplished by channels takes time and therefore accounts for the period during which further signals cannot be sent. A crude, but familiar, example of this might be flushing the toilet. You can only do it once and then must wait for the tank to refill (and regain the potential of the stored water) before it can be done again. This period is followed by the relative refractory period during which only a stronger than normal stimulation can elicit an action potential.
Hey, good example! I'll have to remember that when teaching, just for getting the general concept across.
In a large enough class, you can also demonstrate action potentials by lining up chairs and assigning groups of students to be the various ions. Then they get to run in and out of the "pores" between chairs (I also assign some as "ion pumps" for active transport...they get to shove their classmates through the pores...gently of course). Each ion type (group of students) have their rules to follow. Then they can actually see where they are during different stages of an action potential. Besides, once they get over thinking it's a stupid game, it gets them out of their seats and keeps them from falling asleep. For getting across the most basic concepts, it does work.
both analogies are great! thanks.
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