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So typically, the outer portion of a cell membrane is more positive and the inner portion is more negative. During an action potential, voltage-gated Na+ ion channels in the cell membrane open in response to this difference in charge (membrane potential) switching into a depolarized positive cell interior and negative cell exterior. More Na+ inside the cell makes the interior positive relative to the exterior and this causes the voltage-gated Na* ion channels to open and dump more Na+ further along in the axon. This propagation of an action potential continues until it reaches the terminal branches and the neuron releases it’s neurotransmitter.
So basically, we are stuck with a bunch of sodium in our axons and it’s useless till we get that out. The Na+/K+ ATPase enzyme (sodium-potassium pump) facilitates this by breaking down adenosine triphosphate into adenosine diphosphate, using the energy in the Pi (inorganic phosphate) ion to activate a change of shape in this carrier protein which kicks 3 Na+ inside of the cell out and brings 2 K+ outside of the cell in. This along with ion leak channels restores the neuron membrane to a resting state allowing more jerking.