Action potentials first begin when a graded potential is strong enough to reach its trigger zone. This causes voltage-gated sodium channels to open, resulting in increased sodium permeability. This overall process causes the membrane potential to become more positive (normally it is at -70, resting membrane potential). This happens until the charge of the cell reaches the threshold (-50 mv). At around +30 mv, the voltage-gated channels for potassium open. This causes increased potassium permeability. The cell begins to become more negative again due to this. Eventually, resting membrane potential is restored.
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