Question

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

Answer 1

Sodium and potassium channels

Explanation:

A stimulus would first cause the sodium channels allowing `Na^+` to rush into the neuron. The neuron becomes depolarized. The peak voltage of the action potential causes the potassium channels to open and `K^+` rushes out thus reversing the depolarization.
Around this time, sodium channels begin to close and the action potential will gradually go back to -70 mV (refractory period).