Neurotransmitters

Different types of molecules can act as neurotransmitters. Some of these molecules are inorganic, while others are organic chemicals. Please use your book to understand the different types, although I have also summarized them here.

Please note that a neuron may have more than one neurotransmitter within it. If so, it releases all the types of neurotransmitters that it contains at all of its presynaptic terminals.

Types of neurotransmitters:

Amino Acids:

Some plain amino acids can work as neurotransmitters. For example, glutamate and glycine are both amino acids that are ordinarily used to build proteins. But in neurons, they can also get packaged into synaptic vesicles and get released as neurotransmitters.

Glutamate receptors typically mediate an excitatory response. In fact, there are certain types of glutamate receptors that are thought to be involved in the learning process that occurs in our brains. Meanwhile, glycine receptors typically mediate an inhibitory response... they are abundant in the spinal cord, as you will see when we do reflexes.

Monoamines:

The term "monoamine" simply indicates a modified amino acid. For example, the amino acid tyrosine gets modified through chemical reactions and is turned into dopamine. Dopamine is a monoamine that acts as a neurotransmitter. When someone has Parkinson's disease, the neurons that produce dopamine in the brain slowly degenerate, and that is a major cause of loss of motor skills.

Dopamine can also get modified further (in neurons that do not use it as a neurotransmitter) to get turned into epinephrine (also called adrenaline) and even norepinephrine and serotonin. All of these are then also "monoamines."

Peptides:

A peptide is a short chain of amino acids... not quite long enough to be considered a full-fledged protein. Your book describes these in even more detail on page 365, under the heading of "neuropeptides." It notes there that these peptide neurotransmitters can have complex effects on the postsynaptic neuron.

You have learned that neurotransmitter can cause the opening of a ligand-gated channel. Well, some neurotransmitters, especially many of the peptides, don't necessarily work in that same fashion. They can work through their own receptors to alter the way the ligand-gated channels of other neurotransmitters will work. This is why your book calls them neuromodulators.

Other:

Acetylcholine and nitric oxide are just two other molecules that can be used by neurons as neurotransmitters, but don't fit into the categories listed above. There are many others, but we do not have to discuss them for our purposes.