Neurotransmitter
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. 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.
Drugs may alter the way neurotransmitters function. Cocaine is one example that blocks the reuptake of dopamine, leaving it in the synaptic gap longer. AMPT prevents the conversion of tyrosine to l-DOPA; Reserpine prevents dopamine storage within vesicles; and monoamine oxidase (MAO) in the axon terminal degrades dopamine (deprenyl blocks MAO-B and increases dopamine).
Neurotransmitters may be either excitatory (EPSPs) or inhibitory (IPSPs); 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). GABA and glycine are well-known inhibitory neurotransmitters.
Common neurotransmitters
See also: Nervous system