Explain the steps of action potential across an axon membrane?

Resting Potential-3 Na+ out, 2 K+ in.Action potential- A stimulus is detected causing Sodium voltage gated channels to open, leading to Na+ to flow into the axon and therefor making the inside less negative.This change in charge causes more Sodium cottage gated channels to open, meaning more Na+ flows into the axon down their electrochemical gradient. The potential difference reaches +40Mv and Sodium channels close. The membrane is now more permeable to Potassium ions. K+ flow out of axon down their electrochemical gradient, leading to a less negative charge on the outside.Initially lots of K+ flow out leading to hyper-polarisation, this causes K+ channels to close and axon returns back to resting potential.