The Nervous System
The characteristic of nerve cells which sets them apart from the other cells of the body, and which allows them to transmit signals, is their ability to depolarize.
When the electrical impulse reaches the end of the axon it is transferred to the next nerve cell so the message can continue. At the end of the axon the cell widens out and forms a bulb which comes close to but does not touch the next nerve cell. It is this tiny space between the two cells that is referred to as a synapse. Transmission of the signal across a synapse involves the release of a chemical, called a neurotransmitter, which passes from one cell to the other across the synapse. The neurotransmitter acts on receptor sites on the second cell's membrane, changing its permeability to sodium ions and thus stimulating depolarization. In this manner the nerve impulse is transferred to the second cell.
After the neurotransmitter has had its intended effect on the neighboring cell, the cell which released the neurotransmitter will re-uptake any remaining. In this way the neurotransmitter is recycled and the cell is ready for the next nerve impulse.
Scientists have identified more than 50 neurotransmitters but there are certainly more. The most common are acetylcholine and adrenaline. Neurotransmitters are divided into four main groups:
Acetylcholine was the first neurotransmitter to be discovered. It has an excitatory effect at the junction between nerves and muscles. Nerve pathways which use acetylcholine as a neurotransmitter are generally involved in memory, motivation and arousal.
- amino acids such as gamma-aminobutyric acid (GABA), glycine and glutamate
- monoamines such as noradrenaline, dopamine and serotonin, and
- neuropeptides (chains of amino acids) such as endorphins.
Dopamine is another neurotransmitter. A shortage of dopamine in the brain causes Parkinson's disease. Over-activity of the nerve cells that release dopamine seems to be related to schizophrenia.
Serotonin is an important neurotransmitter that has received a lot of attention by the scientific community because it has been discovered that deficiencies in serotonin in the brain are associated with depression. Additionally, deficiencies of serotonin, or disturbances in serotonin transmission, seem to underlie many psychiatric problems such as bulimia and kleptomania.
A chemical, such as serotonin, that is produced in the nerve cell and released across the synapse to transmit a signal to a neighboring nerve cell is referred to as a(n) __________.
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