07. Neurotransmitters

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Nerve cells communicate with each other and with other cells through the release of biochemical substances called neurotransmitters [ 5 , 38 , 41 , 54 ].

Many neurons secrete more than one type of neurotransmitters [ 38 , 39 , 41 ]. Some neurotransmitters may have different actions on the same target cell. The response time is also variable.

Each terminal contains several thousand button vesicles [ 1 ], each is filled with approximately 10 000 molecules of neurotransmitters [ 4 , 100 , 136 ].

1. Neurotransmitters:

1.1. Criteria of neurotransmitters:

Today there are more than 50 substances [ 52 , 82 ] that meet the criteria of neurotransmitters. These main criteria are as follows [ 38 , 39 , 41 ]:

• A neurotransmitter must be present in the presynaptic neuron.
• Its release should be in response to a presynaptic depolarization.
• There must exist in the postsynaptic neuron specific receptors for the neurotransmitter.

Other widespread in the body such as ATP and nitric oxide have recently considered neurotransmitters [molecules 57 , 110 ] although they have very specific properties.

1.2. Classification of neurotransmitters:

There are several classifications of neurotransmitters according to their biochemical structure and action [ 10 ].

1.2.1. Depending on their structure:

Is generally divided into two main categories according to their structure [ 38 , 41 , 52 ]: The neuropeptides and small molecules.

• Neuropeptides [ 2 , 57 ] are made ​​essentially of chains of amino acids, they contain a wide variety of molecules to various effects such as the endorphins [ 76 ] and somatostatin [ 5 ], they are synthesized in the cell body.
• Small molecules [ 38 ] The bulk of the conventional neurotransmitters such as acetylcholine [ 41 ], glutamate [ 38 ], GABA [ 48 ] and catecholamines [ 52 ]. They are mainly synthesized in nerve endings and much faster than neuropeptides action.

Some of the place of synthesis of neurotransmitters, enzymes are always sought synthesized in the cell body [ 80 ].

1.2.2. According to their action:

Neurotransmitters can be excitatory or inhibitory, they can also have a direct or indirect action on their receptors.

2. Neuromodulators:

While the major role of neurotransmitters is to transmit nerve impulses from one nerve cell to another, neuromodulators [ 1 ] (which are also secreted by neurons) have the function to change the function and metabolism of the cell target for a very considerable time. These neuromodulators usually affect a population of neurons and harmonize their operations.

3. Neuro-hormones:

Neurohormones [ 1 ] as some catecholamines (adrenaline and noradrenaline) are substances released by nerve cells in the bloodstream, so they act remotely on their target cells scattered in the whole organism.

A neurotransmitter or neuromodulator can be as neuro-hormone according to its place of production and distribution.

4. Neurotransmitter receptors:

All biochemicals above can have an effect on sites can be identified and receive: receptors [ 41 ].

The receptors are specific macromolecular structures that are located in specific regions of the plasma membrane of target cells.

We call the molecules which bind to and activate a receptor (ligand or agonist). Substances capable of binding to a receptor and are referred to the block (antagonists).

There are two types of receptors [ 41 ]:

• Receptors associated with a channel (also called ionotropic) [ 4 , 39 ] which allow for direct and fast-acting neurotransmitters. These receptors open a channel causing an ion diffusion and without a direct and rapid change in the potential of the postsynaptic membrane.
• The metabotropic receptor (G-protein) [ 4 , 39 ] that determine slow synaptic responses produced by the G protein and intracellular second messengers.

All chemicals and receptors that activate or inactivate a major target for therapeutic drugs used in [ 38 , 41 ] and of most drugs.

Tables Appendices:

• [ Chart different synaptic neurotransmitter ]
• [ Table of neurotransmitters, their production sites and their action ]
• [ Different neurotransmitter systems ]
• [ Main active ingredients found in the nervous system ]