The characteristic feature of a chemical synapse is that communication between the two neurones is enabled by the release of a chemical (known as the neurotransmitter) from the presynaptic neurone.
When an action potential arrives in the axon terminal of the the presynaptic neurone the neurotransmitter is released by exocytosis and subsequently diffuses across the synaptic cleft which in this instance is about 30 nm wide.
The neurotransmitter then binds to complementary receptors on the postsynaptic membrane. 
The binding of the neurotransmitter to the receptor then initiates changes in the excitability of the postsynaptic neurone.
Neurotransmitters are usually synthesised by the presynaptic neurone and stored in synaptic vesicles that are present in large numbers in axon terminals adjacent to the presynaptic membrane.
Electron microscopic analysis of synaptic terminals has revealed two major classes of synaptic vesicle within axon terminals. The most prolific of these are of small diameter and have a clear (translucent) core whilst the others are large diameter dense-cored vesicles.
Evidence suggests that these two types of vesicles contain different classes of neurotransmitter:
Small Molecule (Classical) Neurotransmitters
| Class | Neurotransmitters |
|---|---|
| Acetylcholine | This is the only neurotransmitter in this class. |
| Biogenic Amines | Dopamine Noradrenaline (norepinephrine) Adrenaline (epinephrine) Serotonin (5-hydroxytryptamine) |
| Amino Acids | gamma-Aminobutyric acid (GABA) Glycine Glutamate |
| Purines | Adenosine triphosphate |
As their alternative name suggests this family of neurotransmitters were the first to be described and indeed it was the identification of some of these molecules that led to Otto Loewi
and Henry Dale
Small molecule neurotransmitters are packaged in small diameter clear vesicles and include four major classes of chemical.
The four classes and some examples of neurotansmitter in each are detailed in the table opposite.
Note that acetylcholine is the only neurotransmitter in its class and that some neurotransmitters in other classes perform important cellular functions in addition to their roles as neurotransmitters.
Peptide Neurotransmitters
| Class | Neurotransmitters |
|---|---|
| Opioids | Enkephalin Dynorphin |
| Tachykinins | Substance P Bombesin |
| Secretins | Vasoactive intestinal polypeptide Secretin |
| Insulins | Insulin Insulin-like growth factors I & II |
This family of neurotransmitter consists of over 50 different substances that are (as their name suggests) made up of relatively short chains of amino acids. These neurotransmitters are usually stored in large-diameter dense-cored vesicles within axon terminals. Peptide neurotransmitters are split up into a number of classes and examples of members of some classes are provided in the table opposite.
Although at one time it was believed that a single neurone only released one neurotransmitter, electron microscopic images such as the one above clearly indicate the presence of more than one type of synaptic vesicle type and consequently neurotransmitter coexistence. Although some neurones do contain only one small molecule neurotransmitter, many also contain one (or more) peptide neurotransmitters in addition.