Biology a level - nervous system + depolarisation

For the nervous system
in terms of the resting potential
there is the active transport of K+ and Na+ ions
and the membrane is more permeable to potassium ions leaving (leaky potassium ion channels in which potassium ions diffuse down an electrochemical gradient)
eventually hits threshold in which the voltage gated Na+ channels open? - Na+ diffuses in allowing for depolarisation
But my question is - when do voltage gated K+ ion channels open and how does that contribute to the action potential