Why does an action potential travel in one direction down an axon?
A. because the diffusion of charge can only occur in one direction
B. because voltage-gated potassium ion channels remain open longer
C. because voltage-gated sodium ion channels cannot immediately reopen
D. because it needs to reach the synapse to trigger release of neurotransmitters

Action potential occurs when there is a rapid change in the membrane potential of a cellular membrane as a result of impulses. For action potential to occur, there is the influx of sodium ions and efflux of potassium ions through the voltage-gated sodium and potassium channels respectively.

The action potential will travel in one direction because the voltage-gated sodium ion channels is already opened and active for awhile, it then becomes inactive as impulses pass into the axon terminals and cannot immediately reopen to cause excitation in the other direction. This is called absolute REFRACTORY PERIOD. Once that is done, the sodium gated channels begin to reset for reopening as the membrane potential starts going. At the re-polarization stage of an action potential, the potassium ions moves out o the membrane and leads to a decease membrane potential. The sodium channels can then be stimulated or another action potential to occur.