synapse
The definitions used in this glossary of terminology either have been provided by the authors of the articles, or have been extracted wholly or in part, or paraphrased from the following sources: The American Medical Association Encyclopedia of Medicine, Charles B. Clayman, MD, Medical Editor, Random House, New York, 1989; Biotechnology from A to Z, 2d Edition, William Bains, Oxford University Press, New York, New York, 2002; A Dictionary of Genetics, 6th Edition, Robert C. King and William D. Stansfield, Oxford University Press, New York, New York, 2002; Dorland's Illustrated Medical Dictionary, 29th and 30th Editions, W. B. Saunders Company, Philadelphia, 2000, 2003; Genes VII, Benjamin Lewin, Oxford University Press, New York, New York, 2000; The Gale Encyclopedia of Genetic Disorders, Volumes I and II, Stacey L. Blachford, Ed., Thomson Learning, New York, New York, 2002; The Merriam-Webster Dictionary, Merriam-Webster, Inc., Springfield, Massachusetts, 1997; Molecular Biology of the Cell, 3rd Edition, Bruce Alberts, et al., Garland Publishing, 1994; The Random House Dictionary of the English Language, Unabridged Edition, 1966; Webster's Ninth New Collegiate Dictionary, 1991.
DEFINITION:
- synapse
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A junctional connection between two neurons (nerve cells) across which a signal can pass. A single neuron may form thousands of these connections with adjacent nerve cells.
A typical neuron has one long fiber (axon) that projects from its cell body; it splits into several smaller branches and twigs, each ending in a terminal that forms a synapse, usually close to the cell body of an adjacent neuron. At a synapse, the two neurons do not come directly into contact; their surface membranes are separated by a gap called the synaptic cleft. When an electrical signal passes along a neuronal axon and reaches a synapse, it cannot bridge the cleft directly; instead, it causes the release of a chemical, called a neurotransmitter. The chemical travels across the cleft and is received at the surface membrane of the next neuron, where it changes the electrical potential of the membrane.
The axonal membrane from which the neurotransmitter is released is called the presynaptic membrane; the neuronal membrane at which it is received is called the postsynaptic membrane.
A synapse may be excitatory or inhibitory:
excitatory synapse - When a neurotransmitter passes across an excitatory synapse, the effect is to excite the postsynaptic membrane, making it more likely that the receptor neuron will "fire" and propagate an electrical impulse in turn.
inhibitory synapses - Inhibitory synapses decrease the excitation of the next neuron.
Most drugs affecting the nervous system work through their effects on synapses. They may modify neurotransmitter release or may modify the effects of neurotransmitters on postsynaptic membranes.
Used in 18 Term definitions
Used in 18 Term definitions