Neurotransmitters. (Illustration and description)

Neurotransmitters are endogenous chemicals that transmit signals across a synapse from one neuron (brain cell) to another 'target' neuron. Neurotransmitters arepackaged into synaptic vesicles clustered beneath the membrane in the axon terminal, on the presynaptic side of a synapse. Neurotransmitters are released into and diffuse across the synaptic cleft, where they bind to specific receptors in the membrane on the postsynaptic side of the synapse. Many neurotransmitters are synthesized from plentiful and simple precursors, such as amino acids, which are readily available from the diet and which require only a small number of biosynthetic steps to convert.

Most neurotransmitters are about the size of a single amino acid, but some neurotransmitters may be the size of larger proteins or peptides. A neurotransmitter is available only briefly – before rapid deactivation – to bind to the postsynaptic receptors. Deactivation may occur due to: the removal of neurotransmitter by re-uptake into the presynaptic terminal; or degradative enzymes in the synaptic cleft. Nevertheless, short-term exposure of the receptor to neurotransmitter is typically sufficient for causing a postsynaptic response by way of synaptic transmission.

In response to a threshold action potential or graded electrical potential, a neurotransmitter is released at the presynaptic terminal. Low level "baseline" release also occurs without electrical stimulation. The released neurotransmitter may then move across the synapse to be detected by and bind with receptors in the postsynaptic neuron. Binding of neurotransmitters may influence the postsynaptic neuron in either an inhibitory or excitatory way. This neuron may be connected to many more neurons, and if the total of excitatory influences is greater than that of inhibitory influences, it will also "fire". That is to say, it will create a new action potential at its axon hillock to release neurotransmitters and pass on the information to yet another neighboring neuron
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CHEMICAL STRUCTURES OF NEUROTRANSMITTERS [PDF]:
- http://www.compoundchem.com/wp-content/uploads/2014/02/Neurotransmitters.pdf
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Neurotransmitters:
• Lots of different kinds, over 100 or so.
• There are two main types- small molecule neurotransmitters and neuropeptides.
• Abnormalities of neurotransmitter function contributes to wide range of neurological diseases and psychiatric disorders

Major categories of neurotransmitters:
• Small molecule neurotransmitters- amino acids, purines, biogenic amines.
• Peptide neurotransmitters: 3-36 amino acid polypeptides, often derived from longer
polypeptides.
--> http://bio.classes.ucsc.edu/bio125/lecture7.pdf