When an electrical impulse reaches the pre-synaptic terminal, this is known as an action potential and is the start of the process, this impulse then triggers the release of neurotransmitters from the vesicles within the terminal. These neurotransmitters then diffuse across the synapse (the gap in between the pre- and post-synaptic membranes), where they then binds to receptors on the post-synaptic membrane. This causes a stimulation which can either be excitatory or inhibitory.As a basic answer this should be enough for a 4 mark question, however if there are more marks then more detail would be needed, like describing the process of neurotransmitter re-uptake, and more explanation about the excitatory and inhibitory post-synaptic potential, and what this means in context of neurons firing.