>>12278128hopefully im not doing your homework for you.
I have to start by explaining how action potentials work.
A neuron has a resting potential along its dendrites and axon
Once a neurotransmitter binds on a receptor of the dendrite, it opens a sodium ion channel, and sodium ions can enter the cell, causing the change in voltage in your picture.
Neurotransmitters are also broken down or re-uptaken, and so if the threshold voltage is not met before the NTs are removed, you get a failed initiation.
But if the threshold is reached, then voltage-gated sodium channels are opened further along the cell. So there is a chain reaction of sodium entering the cell at distance x from the synapse, and the voltage it produces causes ion channels to open at distance x+1, leading to a chain reaction.
But for the part of the cell at x, eventually it reaches a threshold for potassium ion channels to open, and potassium is higher in the cell than out it, so it leaves the cell. So that part of the cell at X has roughly equal sodium in it and out it, and roughly potassium out it and in it.
During the refractory period potassium-sodium pumps swap the ions, to restore more potassium in and more sodium out.
Now, say an action potential has made it so potassium and sodium levels are roughly equal in the cell at X and outside the cell. A second action potential could not pass that point, since it would have to open the sodium ion channels, but the levels of sodium in the cell and out it are roughly the same.
This also explains why point x+1 doesnt induce a chain reaction in x, then x-1, then x-2, and so on. Because they are in the refractory period.
By the time an action potential could spread again, the entire neuron is at resting potential again.
So a neuron can't fire rhythmically partly because of the refractory period. There is not time point where part of the cell x is at resting potential, and part x+1 of the cell is depolarized.
