Explain why the threshold voltage is not always the same value (between axons...

This has to do with the absolute refractory period. All the voltage-gated channels (both for sodium and for potassium) open if a threshold change in the electrical potential of the axon occurs.  If there's enough of an electrical stimulus from that electrical event, whatever it is, the voltage-gated channels will open and the entire action potential will occur.  If there's not enough of an electrical stimulus, nothing will happen.  If there's a really, overly-large electrical stimulus, an action potential will occur just like when an electrical stimulus occurs that is just at threshold.  There's no such thing as a big and a little action potential.  An action potential is an action potential, and it either occurs or it doesn't.  It is the all or none principle

Post Merge: 11 years ago

That was #3,
# 4 is that first the Voltage gated channels begin to inactivate on their own.   Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state.