Neurotransmitter

Basic sequence of synaptic transmission

An action potential arriving at the axon terminal triggers the fusion of synaptic vesicles with the presynaptic cell membrane, releasing neurotransmitter into the synaptic cleft. Molecules diffuse across the gap and interact with specialised receptor proteins on the postsynaptic membrane. The immediate effects depend on receptor type: some open ion channels and produce rapid electrical changes, while others activate intracellular signalling cascades that have slower, longer-lasting consequences. After acting, neurotransmitters are cleared by enzymatic breakdown, uptake into neurons or glial cells, or simple diffusion away from the synapse; these termination processes help shape signal duration and intensity.

Receptor classes and actions

Receptors are commonly divided into fast, ionotropic receptors that form ion channels, and slower, metabotropic receptors that signal via G-proteins and second messengers. Ionotropic receptors mediate rapid excitatory or inhibitory postsynaptic potentials, while metabotropic receptors modulate neuronal excitability, gene expression and synaptic plasticity. The same neurotransmitter can act at multiple receptor subtypes with distinct physiological effects.

Major categories and examples

Neurotransmitters are chemically diverse and grouped by origin and function. Important categories include:

• Amino-acid transmitters: Small molecules derived from common amino acids. Glutamate is the principal excitatory transmitter throughout much of the brain; GABA (gamma-aminobutyric acid) is the principal fast inhibitory transmitter in many regions, and glycine acts similarly in the spinal cord.
• Monoamines: Synthesised from amino-acid precursors, these modulatory transmitters include dopamine, noradrenaline (norepinephrine) and serotonin. Noradrenaline is central to arousal and the fight-or-flight response, while dopamine is linked to reward, motivation and motor control.
• Acetylcholine: A widespread transmitter important for voluntary muscle activation, autonomic signalling and attention-related circuits.
• Neuropeptides and gaseous messengers: Short peptide chains (for example endorphins, substance P) and small diffusible gases such as nitric oxide act as neuromodulators, often released alongside classical transmitters to shape circuit function.

Synthesis, storage and release

Many small-molecule neurotransmitters are synthesised from dietary amino acids via a few enzymatic steps; for example, tyrosine is a precursor for catecholamines. Once formed, transmitters are concentrated into membrane-bound vesicles. The probability and amount of release depend on presynaptic calcium entry, firing patterns and the availability of precursors.

Functional diversity and co-transmission

Neurons may release more than one type of signalling molecule (co-transmission), combining fast synaptic actions with slower modulatory influences. In addition to point-to-point synaptic transmission, some transmitters act at a distance in a form of "volume transmission," influencing populations of cells beyond a single synapse. These mechanisms allow flexible control of information flow and contribute to learning-related changes such as long-term potentiation and depression.

Roles in health, disease and therapy

Proper neurotransmitter signalling is essential for normal nervous system function. Alterations in transmitter systems are implicated in movement disorders, mood disorders, epilepsy, schizophrenia, cognitive decline and pain. Many treatments act by modifying neurotransmitter levels, receptor activity or reuptake: for example, drugs that increase synaptic serotonin are widely used in depression, dopamine-replacement strategies are central to treating some movement disorders, and agents that enhance GABAergic inhibition are used for anxiety and seizures.

History and research methods

The concept of chemical transmission emerged from classic physiological experiments showing that substances released by one tissue can change the activity of another; careful studies identified acetylcholine and later many other transmitters. Modern research uses electrophysiology, imaging, biochemical assays, microdialysis and genetic or optical tools to study release, receptor function and the role of transmitters in behaviour and disease.

For further reading on related topics, see entries on neurons, chemical synapses and the role of amino acids in transmitter synthesis.