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Neurotransmitters are chemicals that are used to relay, amplify and modulate signals between a neuron and another cell. It does this through the process of receptor binding to a neuroreceptor, which triggers aresponse in the neuron to alter its functioning.
According to the prevailing beliefs of the 1960s, a chemical can be classified as a neurotransmitter if it meets the following conditions:
- There are precursors and/or synthesis enzymes located in the presynaptic side of the synapse;
- The chemical must be present in the presynaptic element
- It is available in sufficient quantity in the presynaptic neuron to affect the postsynaptic neuron;
- There must be postsynaptic receptors and the ability for the chemical to bind to said receptors
- A biochemical mechanism for inactivation must be present.
Types of neurotransmitters
Some more precise divisions are as follows:
- Around 10 small-molecule neurotransmitters or small signalling molecules are known:
- 3 or 4 excitory amino acids, depending on exact definition used: (primarily glutamic acid, gamma aminobutyric acid (GABA), aspartic acid & glycine)
- Over 50 neuroactive peptides (vasopressin, somatostatin, neurotensin, etc.) have been found, among them hormones such as Luteinizing hormone (LH) or insulin that have specific local actions in addition to their long-range signalling properties. This group also includes hypothalmic releasing hormones, the neurohypophyseal hormones, pituitary peptides and the gastrointestinal peptides.
- Single ions, such as synaptically-released zinc, are also considered neurotransmitters by some.[How to reference and link to summary or text]
- Gaseous including :Nitrogen monoxide (NO) also called Nitric Oxide and Carbon monoxide (CO))]
Some examples of neurotransmitter action:
- Acetylcholine - voluntary movement of the muscles
- Norepinephrine - wakefulness or arousal
- Dopamine - voluntary movement and motivation, "wanting", pleasure, associated with addiction and love
- Serotonin - memory, emotions, wakefulness, sleep and temperature regulation
- GABA - inhibition of motor neurons
- Glycine - spinal reflexes and motor behaviour
- Neuromodulators - sensory transmission - especially pain
- Main article: Neurotransmitter systems
Neurons expressing certain types of neurotransmitters sometimes form distinct systems, where activation of the system causes effects in large volumes of the brain, called volume transmission.
Drugs targeting the neurotransmitter of such systems affects the whole system, and explains the mode of action of many drugs;
- Cocaine, for example, blocks the reuptake of dopamine, leaving these neurotransmitters in the synaptic gap longer.
- Prozac is a selective serotonin reuptake inhibitor (SSRI), hence potentiating the effect of naturally released serotonin.
- AMPT prevents the conversion of tyrosine to L-DOPA, the precursor to dopamine; reserpine prevents dopamine storage within vesicles; and deprenyl inhibits monoamine oxidase (MAO)-B and thus increases dopamine levels.
Diseases may affect specific neurotransmitter systems. For example, Parkinson's disease is at least in part related to failure of dopaminergic cells in deep-brain nuclei, for example the substantia nigra. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.
A brief comparison of the major neurotransmitter systems follows:
|Noradrenaline system||locus coeruleus||
|Lateral tegmental field|
|Dopamine system||dopamine pathways:||motor system, reward, cognition, endocrine, nausea|
|Serotonin system||caudal dorsal raphe nucleus||Increase (introverson), mood, satiety, body temperature and sleep, while decreasing nociception.|
|rostral dorsal raphe nucleus|
|Cholinergic system||pontomesencephalotegmental complex|
|basal optic nucleus of Meynert|
|medial septal nucleus|
Production and destruction cycles
When neurotransmitters are released into the synapse only some binds with receptors, the rest may be broken down by enzymes in the synaptic fluid or is reabsorbed quickly into the presynaptic neuron via a reuptake transporter pump. This allows for the neurotransmitter to be repackaged in the vesicles and to be recycled. So, for example, monoamine transporter are structures in nerve-cell membranes which function as neurotransmitter transporters transferring monoamine neurotransmitters in or out of cells. Some psychiatric drugs work by effecting this process
Genetic control of neurotransmitters
Many of these neurochemicals have been studied with a view to identifying the genetic mechanisms underlying their production and control
- Factors affecting neurotransmitter action
- Nervous system
References & Bibliography
- Kopn,L.J. (ed.) Neurotransmitters, Baltmore: Williams & Wilkins.
- Molecular Expressions Photo Gallery: The Neurotransmitter Collection
- Brain Neurotransmitters
- Endogenous Neuroactive Extracellular Signal Transducers
- MeSH Neurotransmitter
Cell physiology: cell signaling
|Types of proteins|
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