neurotransmitter neurotransmitter a neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. the cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. for an introduction to concepts and terminology used in this article, see chemical synapse. structure of a typical chemical synapse postsynaptic density voltage-gated ca++ channel synaptic vesicle neurotransmitter transporter receptor neurotransmitter axon terminal synaptic cleft dendrite neurotransmitters are released from synaptic vesicles into the synaptic cleft where they are able to interact with neurotransmitter receptors on the target cell. the neurotransmitter's effect on the target cell is determined by the receptor it binds to. many neurotransmitters are synthesized from simple and plentiful precursors such as amino acids, which are readily available and often require a small number of biosynthetic steps for conversion. neurotransmitters are essential to the function of complex neural systems. the exact number of unique neurotransmitters in humans is unknown, but more than 100 have been identified. common neurotransmitters include glutamate, gaba, acetylcholine, glycine and norepinephrine. mechanism and cycle synthesis neurotransmitters are generally synthesized in neurons and are made up of, or derived from, precursor molecules that are found abundantly in the cell. classes of neurotransmitters include amino acids, monoamines, and peptides. monoamines are synthesized by altering a single amino acid. for example, the precursor of serotonin is the amino acid tryptophan. peptide transmitters, or neuropeptides, are protein transmitters that often are released together with other transmitters to have a modulatory effect. purine neurotransmitters, like atp, are derived from nucleic acids. other neurotransmitters are made up of metabolic products like nitric oxide and carbon monoxide. examples amino acid glycine, glutamate monoamines serotonin, epinephrine, dopamine peptides substance p, opioids purines atp, gtp other nitric oxide, carbon monoxide synaptic vesicles containing neurotransmitters storage neurotransmitters are generally stored in synaptic vesicles, clustered close to the cell membrane at the axon terminal of the presynaptic neuron. however, some neurotransmitters, like the metabolic gases carbon monoxide and nitric oxide, are synthesized and released immediately following an action potential without ever being stored in vesicles. release generally, a neurotransmitter is released at the presynaptic terminal in response to an electrical signal called an action potential in the presynaptic neuron. however, low level 'baseline' release also occurs without electrical stimulation. neurotransmitters are released into and diffuse across the synaptic cleft, where they bind to specific receptors on the membrane of the postsynaptic neuron. receptor interaction after being released into the synaptic cleft, neurotransmitters diffuse across the synapse where they are able to interact with receptors on the target cell. the effect of the neurotransmitter is dependent on the identity of the target cell's receptors present at the synapse. depending on the receptor, binding of neurotransmitters may cause excitation, inhibition, or modulation of the postsynaptic neuron. see below for more information. elimination acetylcholine is cleaved in the synaptic cleft into acetic acid and choline in order to avoid continuous activation of receptors on the post-synaptic or target cell, neurotransmitters must be removed from the synaptic cleft. neurotransmitters are removed through one of three mechanisms: diffusion – neurotransmitters drift out of the synaptic cleft, where they are absorbed by glial cells. these glial cells, usually astrocytes, absorb the excess neurotransmitters. in the glial cell, neurotransmitters are broken down by enzymes or pumped back into enzyme degradation – proteins called enzymes break the neurotransmitters down. reuptake – neurotransmitters are reabsorbed into the pre-synaptic neuron. transporters, or membrane transport proteins, pump neurotransmitters from the synaptic cleft back into axon terminals (the presynaptic neuron) where they are stored for reuse. for example, acetylcholine is eliminated by having its acetyl group cleaved by the enzyme acetylcholinesterase; the remaining choline is then taken in and recycled by the pre-synaptic neuron to synthesize more acetylcholine. other neurotransmitters are able to diffuse away from their targeted synaptic junctions and are eliminated from the body via the kidneys, or destroyed in the liver. each neurotransmitter has very specific degradation pathways at regulatory points, which may be targeted by the body's regulatory system or medication. cocaine blocks a dopamine transporter responsible for the reuptake of dopamine. without the transporter, dopamine diffuses much more slowly from the synaptic cleft and continues to activate the dopamine receptors on the target cell. discovery