When the inside of the axon becomes sufficiently positive, about 30 mV is given as an average value, the sodium channels close. This closing of the sodium channels will greatly limit the ability of sodium ions to enter the axon. In addition to the sodium channels closing, the potassium (K+) channels open. Now potassium is free to cross the axon. 

The next page will now show the action potential moving down the axon.

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