Neurotransmitters in General

Cross references:   Ligands   ,    Neuromodulators in General   ,    
NeuropeptidesGABA  ,     Catecholamine             

Neurotransmitter (Wiki) 

"Neurotransmitters are endogenous chemicals which transmit signals from a neuron to a target cell across the synapse.[1]    
    Neurotransmitters are packaged into synaptic vesicles that cluster beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to receptors in the membrane on the postsynaptic side of the synapse.  
    Release of neurotransmitters usually follows arrival of an action potential at the synapse, but may follow graded electrical potentials.  
    Low level "baseline" release also occurs without electrical stimulation.

There are many different ways to classify neurotransmitters. Dividing them into amino acids, peptides, and monoamines is sufficient for some classification purposes.

Major neurotransmitters:

In addition, over 50 neuroactive peptides have been found, and new ones are discovered regularly. Many of these are "co-released" along with a small-molecule transmitter, but in some cases a peptide is the primary transmitter at a synapse.

Catecholamine (Wiki) 

"Catecholamines are sympathomimetic[1] "fight-or-flight" hormones released by the adrenal glands in response to stress.[2] They are part of the sympathetic nervous system.
  They are called catecholamines because they contain a catechol or 3,4-dihydroxylphenyl group and an ethylamine moiety.

"In the human body, the most abundant catecholamines are epinephrine (adrenaline), 

 norepinephrine (noradrenaline)

and dopamine,

all of which are produced from phenylalanine and tyrosine. Various stimulant drugs are catecholamine analogs.

Serotonin (Wiki) 
"Serotonin or 5-Hydroxytryptamine (5-HT) is a monoamine neurotransmitter. "


Approximately 80 percent of the human body's total serotonin is located in the enterochromaffin cells in the gut, where it is used to regulate intestinal movements.[1][2] The remainder is synthesized in serotonergic neurons in the CNS

Serotonin is mainly metabolized to 5-HIAA, chiefly by the liver.
... In addition to animals, serotonin is also found in fungi and plants.[3]

Neurotransmitter - Wikipedia   
Neurotransmitters, also known as chemical messengers, are endogenous chemicals that enable neurotransmission. They transmit signals across a chemical synapse, such as a neuromuscular junction, from one neuron (nerve cell) to another "target" neuron, muscle cell, or gland cell.[1] Neurotransmitters are released from synaptic vesicles in synapses into the synaptic cleft, where they are received by receptors on the target cells. Many neurotransmitters are synthesized from simple and plentiful precursors such as amino acids, which are readily available from the diet and only require a small number of biosynthetic steps for conversion. Neurotransmitters play a major role in shaping everyday life and functions. Their exact numbers are unknown, but more than 100 chemical messengers have been uniquely identified.[2]


Chemical synapse - Wikipedia   
Chemical synapses are biological junctions through which neurons' signals can be exchanged to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie perception and thought. They allow the nervous system to connect to and control other systems of the body.

At a chemical synapse, one neuron releases neurotransmitter molecules into a small space (the synaptic cleft) that is adjacent to another neuron. The neurotransmitters are kept within small sacs called vesicles, and are released into the synaptic cleft by exocytosis. These molecules then bind to receptors on the postsynaptic cell's side of the synaptic cleft. Finally, the neurotransmitters must be cleared from the synapse through one of several potential mechanisms including enzymatic degradation or re-uptake by specific transporters either on the presynaptic cell or possibly by neuroglia to terminate the action of the transmitter.

The adult human brain is estimated to contain from 1014 to 5 × 1014 (100–500 trillion) synapses.[1] Every cubic millimeter of cerebral cortex contains roughly a billion (short scale, i.e. 109) of them.[2]

The word "synapse" comes from "synaptein", which Sir Charles Scott Sherrington and colleagues coined from the Greek "syn-" ("together") and "haptein" ("to clasp"). Chemical synapses are not the only type of biological synapse: electrical and immunological synapses also exist. Without a qualifier, however, "synapse" commonly means chemical synapse.