A neurotransmitter is a molecule used for signalling between nerve cells or neurons. Neurotransmitter molecules pass between neurons at synapses. Within the cell they are packaged in vesicles and released by rapid exocytosis upon the arrival of a nerve impulse, after which they diffuse across the synaptic gap to bind neurotransmitter receptors or other ligand gated ion channels.
Many neurotransmitters are removed from the synaptic gap, after they have activated their specific receptors, by transport proteins residing in neuronal and glial plasma membranes. At cholinergic synapses where acetylcholine (ACh) is the neurotransmitter, the enzyme acetylcholinesterase rather than a transport protein is responsible for removing the ACh. A similar event occurs at dopamine neurons through the enzyme known as a monoamine oxidase. Drugs known as MAOIs, usually anti-depressants and anti-Parkinson's Disease drugs, act here. It is important to remove neurotransmitters from the synaptic gap so that they do not continue to stimulate or inhibit the firing of the postsynaptic neuron.
Neurotransmitters may be either excitatory or inhibitory; that is, they may be of a type that fosters the initiation of a nerve impulse in the receiving neuron, or they may inhibit such an impulse (more at synapse). Most are small molecules derived from or related to amino acids. GABA and glycine are well-known inhibitory neurotransmitters.
There are many neurotransmitters; some of the important ones are:
- Amino acids:
- Monoamines (in the order of their synthesis):