Depolarization

The animation below illustrates how the flow of positively charged ions into the axon leads the axon to become positively charged relative to the outside. With each positively charged sodium ion that enters the axon, another positive charge is inside and one fewer negative charge is outside the axon. Thus, together the inside grows increasingly more positive and the relative concentration of sodium inside the axon relative to outside the axon grows greater.
This initial phase of the action potential is called the depolarization phase. Now as the depolarization phase progresses, the status of the two physical forces that have been discussed changes. At the end of the depolarization phase, the voltage of the inside of the axon relative to the outside is positive and the relative concentration of sodium ions inside the axon is greater than at the beginning of the action potential.

At the end of the depolarization phase, what is the tendency for movement of a sodium ion at the mouth of a sodium channel?

  1. The sodium ion will still be pulled in most of the time.
  2. The sodium ion will stay outside most of the time.

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