Most neurotransmitters are synthesized in the presynaptic neuron, and packaged in synaptic vesicles. They are released by the axon terminal upon arrival of an action potential arrives. Following release, they bind to receptors on the cell membrane of the postsynaptic cell, and in doing so alter the electrical activity of the postsynaptic cell. Neurotransmitters are removed from the synaptic cleft immediately after postsynaptic action so that the synapse becomes free to carry a new message if required. This removal is necessary as neurotransmitters would continue to stimulate the receptors indefinitely so that no new messages could get through. Deactivation of neurotransmitters is achieved in one of two ways: 1. by enzymes that breakdown the neurotransmitter in the synaptic cleft [e.g., the enzyme acetylcholinesterase (AchE) breaks down the neurotransmitter Ach shortly after release], and 2. by re-uptake in which the pre-synaptic cell re-absorbs the neurotransmitter it has released by means of transporter molecules in the pre-synaptic membrane. The effect is to attract the neurotransmitter molecules back into the pre-synaptic terminal for re-cycling. In terms of development, gamma amino butyric acid makes an early appearance and glutumate a later one. In addition, neurotransmitters serve a neurotrophic role early in development, examples being serotonin with regard to the establishment of cortical connections and acetylcholine and norepinephrine in terms of neural plasticity. The existence of neurotransmitters was first demonstrated by Oscar Loewi (1873-1961) in 1921. He stimulated the vagus nerve in the frog, which slowed down the heart rate of two hearts in fluid-filled chambers containing an electrolyte solution, one of which was connected to it. This outcome had only one interpretation: stimulation of the vagus nerve resulted in the release of a chemical messenger that slowed down the unconnected heart by diffusing through the fluid.
See Acetylcholinesterase (AcHE), Acethycholine (AcH), Action potential, Aspartate (or aspartic acid), Attention deficit hyperactivity disorder (ADHD), Catecholamines, Cerebral cortex (development), Cholinergic neurotransmitter system, Climbing fibers, Dendrite, Dopamine, Entrorhinal cortex, Enzyme, Epinephrine (or adrenaline), Gamma amino butyric acid (GABA), Glutumate (or glutamate), Glycine, Epinephrine (or adrenaline), Ligands, Locus coeruleus (or ceruleus), Mesencephalic reticular activating system, Motor end plate, Nerve growth factor (NGF), Neuromuscular junction, Nissl substance, Norepinephrine (or noradrenaline), Oxytocin, Plasticity, Serotonin (5-hydroxytriptamine, or 5-HT), Synapse, Synaptic cleft, Vagus (or pneumogastric) nerve, Vesicles