Outline the process that occurs when an action potential arrives at a presynaptic neuron for the transmission to the postsynaptic neuron.

An action potential arrives at the presynaptic membrane of the pre-synaptic neuron causing the calcium ion channels to open. This opening causes vesicles filled with neurotransmitter to fuse with the pre-synaptic membrane. The neurotransmitter is released via exocytosis into the synaptic cleft (the space between the two neurons). The neurotransmitter travels to their specific receptors on the post-synaptic membrane of the second neuron via diffusion (a passive process - requiring no ATP). The binding of the neurotransmitter to the receptor causes the associated sodium ion channels to open, resulting in another action potential along the post-synaptic membrane. The electrical impulse carries on until another synapse is reached.