How does a chemical synapse work?

Let's consider a synapse between an axon and a dendrite. To illustrate that an action potential is transmitted from the axon to the dendrite, the last portion of the axon, coming in contact with the dendrite at the synapse, is called the "presynaptic terminal", and the dendrite represents the "postsynaptic terminal". The small space between these terminals, only a few micrometres thick, is called the "synaptic cleft". Inside the presynaptic terminal there are numerous vesicles containing a certain neurotransmitter (say, acetylcholine). The action potential travels along the axon and reaches the terminal; when this happens, the membrane potential becomes more positive, because of Na+ ions coming inside the terminal. This in turn activates some Ca++ channels, called voltage gated Ca++ channels, and Ca++ ions enter. This causes the vesicles with acetylcholine to move towards the membrane and fuse with it, so that acetylcholine is released into the synaptic cleft. Acetylcholine then reaches the postsynaptic terminal and binds to receptors on its membrane; this binding will cause Na+ channels to open, Na+ goes in and depolarizes the postsynaptic terminal, so that a new action potential will be formed and will travel further down the dendrite.