What effect does an action potential have on a vesicle?

An action potential causes the vesicles in the axon terminal of a neuron to move towards the cell membrane, where they meet with it and merge with it. After merging, loads of neurotransmitters diffuse into the synaptic cleft. Here they diffuse and move to the post-synaptic dendrites receptors. This initiates a graded potential which moves down the dendrites to the axon hillock, where another action potential takes place.