EDUCATION: How does a nerve fire

When a nerve impulse is transmitted it moves because of the action of sodium channels. The outside of the cell is normally more electrically positive than the inside of the cell. 

So when the stimulus comes along sodium ions which are electricialy positive ,start to move into the cell to make the inside of the cell more electrically positive and the outside more negative and it moves along at sodium channels (purple dots below). Then there is a reset.

This is how the nerve impulse travels and it travels quicker with myelin because the impulse jumps along the nerve faster


The inside of the cell is about 60-70 thousandth of a volt less electrically positive or 60-70 thousandth more negative

When this difference between the outside and the inside of the cell is -55 thousands of a volt, voltage-gated sodium channels (sodium channels that respond to voltage) open and sodium ions rush into the nerve making the inside of the nerve positive. This is called DEPOLARISATION. This sets off the same reaction at the next sodium channel along the nerve (see above)

When the electrical difference between the inside and the outside is 30 to 40 thousandth of a volt more positive on inside the cell than the outside the sodium channels close. The system then resets itself with voltage gated potassium channels by potassium ions moving out of the cell. The potassium ions also carry a positive charge, so as they move out, the inside becomes more negative and the outside becomes more positive. This is called REPOLARIZATION. As the voltage difference between the outside and inside resets, the potassium channels close.

When the inside of the cell gets to the stage where the difference between the outside and inside of the cell is more than 60-70 thousandths of a volt and this is called HYPERPOLARISATION making it more difficult for the threshold, required to send the next nerve impulse by sodium channels, to be reached. This makes it more difficult for the nerve to be excited and so less likely that the nerve impulse is fired.

Then potassium is pumped in and sodium is pumped out (by the potassium sodium pump) and because three sodiums (+++) are pumped out for every two potassium (++) the outside becomes positive relative to the inside of the cell again.

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The reset button causing the potassium to move are due to voltage-gated potassium channels. 

But there are others that add to this effect.BKCa is on such set of Channels

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  • The membrane would depolarize if charged ions moved freely across the membrane. The resting potential of the membrane occurs when there has been no stimulation or inhibiting. At rest the membrane is polarized which means sodium and potassium channels are closed. The only sodium allowed to move are is that which is being forced out by the sodium-potassium pump. Outside of the cell, sodium ions are more concentrated while inside of the cell potassium ions are more concentrated.
    F. Henderson
    (Not critiquing, class assignment)

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