Reversal potential is the name give for a specific channel at which point no NET current (ion flux) will occur. Each protein channel will have different reversal potentials based the ions in the environment, the ions "allowed" to pass through the channel, the electrochemical gradients for each of the ions.
The reason an action potential occurs is similar to that of other neurons. There will be a threshold membrane voltage that will lead to opening of sodium channels and this will propagate down the membrane.