Neurotransmitters are biochemical substances that are present in the nervous system, which allow signals to pass from one nerve cell (neuron) to another. There are approximately 50 identified neurotransmitters. Nerve cells communicate messages by releasing neurotransmitters. Neurotransmitters can stimulate or block nerve cells. Each neurotransmitter can affect neurons, either directly or indirectly, in a particular portion of the brain, thus affecting behavior.
A nerve impulse passes through a nerve in a cellular structure that's known as an axon – a single smooth body arising from the nerve cell. The impulse finally reaches the presynaptic membrane. This membrane holds neurotransmitters to be released in a free space called the synaptic cleft. Here, receptors pick up the free flowing neurotransmitter molecules in a structure called the postsynaptic membrane of another neuron that's close at hand. Once the neurotransmitter is picked up by receptors, the molecule is internalized in the neuron and the impulse continues. This complete process is very rapid.
Once the neurotransmitter is set free, the molecules should move to receptor sites situated on the postsynaptic membrane. However, if the flow of the neurotransmitter is faulty, the molecules flow back to their originating site, i.e. the presynaptic membrane instead of the receptors on the postsynaptic membrane that pass on the impulse to a nearby neuron. The mechanism of action and localization of neurotransmitters in the brain has furnished important information regarding many mental disorders and has thus helped in the research and development of medications that allow normal flow and movement of neurotransmitter molecules.
Acetylcholine, which has many functions, was the first neurotransmitter to be discovered. It is responsible for the stimulation of muscles, as well as stimulation of the gastro-intestinal system. It is found in the autonomic nervous system and in sensory neurons. A link has been found between Alzheimer's disease and acetylcholine. There is about 90% loss of acetylcholine in the brains of people suffering from Alzheimer's disease.
There is a lot of evidence suggesting a relation between neurotransmitter imbalances and mental problems such as schizotypal personality disorder, borderline personality disorders, social phobia, somatization disorder, and others. Some of the most important neurotransmitters are adenosine, dopamine, epinephrine (adrenaline), norepinephrine (noradrenaline), histamine, serotonin, gama aminobutyric acid (GABA), melatonin, serine, glutamate, aspartate, and glycine. While a faulty dopamine system has been found to be associated with the development of schizophrenia and Parkinsonism, alterations in the level of serotonin is speculated to play a vital role in depression.
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