How is an action potential generated?

Firstly the resting membrane is set by Na+/K+ ATPase so that 3 Na+ moves out for every 2K+ that moves in; this creates a negative membrane potential of around -65mV. To initiate an action potential the membrane must depolarise to the threshold value. When the threshold value is reached Na+ channels open, allowing a rapid inflow of Na+ ions making the membrane potential more positive (around +40mV). At this point Na+ channels begin to close as K+ channels open, meaning that K+ ions move out of the cell making the membrane potential more negative. The membrane hyperpolarises (goes below the resting membrane potential) because the K+ channels are slow to close meaning more K+ ions move out. The neuron goes through a refractory period to equilibrate ions across the membrane and allow another action potential can be fired.