Synaptic Transmission?

Question

What channels in the presynaptic neuron open up in response to an action potential?

Answer 1

There are several.  The answer depends on the context of your question.

Sodium channels and potassium channels open and are responsible for the action potential itself.  

However, in the presynaptic terminal you also have voltage-gated calcium channels that open in response to the action potential.  The influx of calcium causes fusion of presynaptic vesicles with the membrane, releasing the neurotransmitter.

Answer 2

Sodium channels.