[[Image:Nerves.jpg|right|framed|Synapses allow neuron to communicate with one another through axons and dendrites, converting action potential into neurotransmitter ones.]] Synapses are specialized junctions through which cells of the nervous system signal to one another and to non-neuronal cells such as muscles or glands. Synapses form the circuits in which the neurons of the central nervous system interconnect. They are thus crucial to the biological computations that underlie perception and thought. They also provide the means through which the nervous system connects to and controls the other systems of the body. The word "synapse" comes from "synaptein" which Sir Charles Scott Sherrington and his colleagues coined from the Greek "syn-" meaning "together" and "haptein" meaning "to clasp". ==Anatomy== At a prototypical synapse, such as a dendritic spine, a mushroom-shaped bud projects from each of two cells and the caps of these buds press flat against one another. At this interface, the biological membranes of the two cells flank each other across a slender gap, the narrowness of which enables signalling molecules known as neurotransmitters to pass rapidly from one cell to the other by diffusion. This gap is sometimes called the synaptic cleft. Such synapses are asymmetric both in structure and in how they operate. Only the so-called ''pre-synaptic'' neuron secretes the neurotransmitter, which binds to transmembrane receptors facing into the synapse from the ''post-synaptic'' cell. The pre-synaptic ''nerve terminal'' (also called the ''synaptic button'' or ''bouton'') generally buds from the tip of an axon, while the post-synaptic target surface typically appears on a dendrite, a cell body, or another part of a cell. The parts of synapses where neurotransmitter is released are called the ''active zones''. At active zones the membranes of the two adjacent cells are held in close contact by cell adhesion proteins. Note: There also exists a less elaborate form of junction called an electrical synapse. == Signalling across chemical synapses == The release of neurotransmitter is triggered by the arrival of a nerve impulse (or action potential) and occurs through an unusually rapid process of cellular secretion: Within the pre-synaptic nerve terminal, vesicle (biology)s containing neurotransmitter sit "docked" and ready at the synaptic membrane. The arriving action potential produces an influx of second messenger through voltage-dependent, calcium-selective ion channels, at which point the vesicles fuse with the membrane and release their contents to the outside. Receptors on the opposite side of the synaptic gap bind neurotransmitter molecules and respond by opening nearby ion channels in the post-synaptic cell membrane, causing ions to rush in or out and changing the local transmembrane potential of the cell. The result is ''excitatory'', in the case of cell potential currents, or ''inhibitory'' in the case of cell potential currents. Whether a synapse is excitatory or inhibitory depends on what type(s) of ion channel conduct the post-synaptic current, which in turn is a function of the type of receptors and neurotransmitter employed at the synapse. == Synaptic strength == Synaptic strength is the amount of Current_(electricity), or, more strictly, the change in transmembrane potential of the synapse. It is subject to biological regulation. The variability of synaptic strength is often referred to as synaptic plasticity. One regulatory trigger of synaptic strength involves the simple coincidence sensory stimuli and action potentials in the synaptically linked cells. == Integration of synaptic inputs == Generally, if an excitatory synapse is strong, an action potential in the pre-synaptic neuron will trigger another in the post-synaptic cell; whereas at a weak synapse the excitatory postsynaptic potential will not reach the action potential for action potential initiation. In the brain, however, each neuron typically connects or "synapses" to many others, and likewise each receives synaptic "inputs" from many others. When action potentials "fire" simultaneously in several neurons that weakly synapse on a single cell, they may initiate an impulse in that cell even though the synapses are weak. On the other hand, a pre-synaptic neuron releasing an inhibitory neurotransmitter such as GABA can cause inhibitory postsynaptic potential in the post-synaptic neuron, decreasing its excitability and therefore decreasing the neuron's likelihood to fire an action potential. In this way the output of a neuron may depend on the input of many others, each of which may have a different degree of influence, depending on the strength of its synapse with that neuron. John Carew Eccles performed some of the important early experiments on synaptic integration, for which he received the Nobel Prize for Physiology or Medicine in 1963. Complex input/output relationships form the basis of transistor-based computations in computers, and so are thought to figure similarly in neural circuits. == Detailed properties and regulation == Following fusion of the synaptic vesicles and release of transmitter molecules into the synaptic cleft, the neurotransmitter is rapidly cleared from the space for recycling by specialized membrane proteins in the pre-synaptic or post-synaptic membrane. This "reuptake" prevents "desensitization" of the post-synaptic receptors and ensures that succeeding action potentials will elicit the same size EPSP. The necessity of re-uptake and the phenomenon of desensitization in receptors and ion channels means that the strength of a synapse may in effect diminish as a train of action potentials arrive in rapid succession--a phenomenon that gives rise to the so-called frequency dependence of synapses. The nervous system exploits this property for computational purposes, and apparently tunes its synapses through such means as phosphorylation of the proteins involved. The size, number and replenishment rate of vesicles also are subject to regulation, as are many other elements of synaptic transmission. The drugs known as selective serotonin re-uptake inhibitors or Selective serotonin reuptake inhibitors affect certain synapses by inhibiting the re-uptake of the neurotransmitter serotonin.
One important excitatory neurotransmitter, acetylcholine, does not undergo reuptake, but instead is removed from the synapse by the action of the enzyme acetylcholinesterase. ---- By analogy to true synapses described above, the interface between an antigen presenting cell and lymphocyte is sometimes called an immunological synapse. ==References== * M.F. Bear, B.W. Conners, and M.A. Paradiso. 2001. ''Neuroscience: Exploring the Brain''. Baltimore: Lippincott. ISBN 0781739446 * Eric Kandel, James Schwartz, and Thomas Jessel. 2000. ''Principles of Neural Science''. 4th ed. McGraw-Hill, New York. ISBN 0838577016 Nervous system
---- That's as maybe, but I want also to add that there are different kinds of synapse. Most are chemical in nature, such as are already described. However, there is evidence for some very fast acting synapses which are electrical. These are reported to have been discovered in fish. Assuming that this has not been discredited, it would be worth having a section on electrical synapses. User:David Martland 00:06, 16 Jan 2004 (UTC) [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11389476&dopt=Abstract Electrical synapses between GABA-releasing interneurons. Nat Rev Neurosci. 2001 Jun; 2(6): 425-33. Galarreta M, Hestrin S.] *''Although gap junctions were first demonstrated in the mammalian brain about 30 years ago, the distribution and role of electrical synapses have remained elusive. A series of recent reports has demonstrated that inhibitory interneurons in the cerebral cortex, thalamus, striatum and cerebellum are extensively interconnected by electrical synapses. Investigators have used paired recordings to reveal directly the presence of electrical synapses among identified cell types. These studies indicate that electrical coupling is a fundamental feature of local inhibitory circuits and suggest that electrical synapses define functionally diverse networks of GABA-releasing interneurons. Here, we discuss these results, their possible functional significance and the insights into neuronal circuit organization that have emerged from them.'' Yes, I think electrical synapses deserve a section, if not some systemic changes to admit to the recently broadened concept of synapse. Gap junctions didn't used to be considered synapses, but evidently now they are. User:168... 16:31, 16 Jan 2004 (UTC) ==Synapses and circuits== "Synapses define the circuits in which the neurons of the central nervous system interconnect." was changed to "Synapses are circuits...." Synapses are functional contacts for communication between nearby cells. Most synapses are between adjacent neurons, but some synapses are from neurons onto other cell types such as muscle cells. Synapses are of two types: electrical synapses and chemical synapses. Electrical synapses provide cytoplasmic connections between adjacent cells where ions can carry charges from one neuron to another. Chemical synapses provide cell-to-cell communication that is mediated by neurotransmitters. A neurotransmitter is a chemical that is released from one neuron, moves to a nearby neuron, and acts to modulate the physiology of the target cell. Neurotransmitters often work by binding to receptor proteins on the surface of cells. However, some neurotransmitters pass through cell membranes and directly influence target proteins inside their target neurons. Structurally, synapses are usually regions of close contact between adjacent neurons. Most neurotransmitters are packaged into vesicles at the presynaptic side of synapses. Most neurotransmitter receptors are concentrated in post-synaptic densities on the post-synaptic side of synapses. When activated by neurotransmitters, neurotransmitter receptors typically change ion flow across the surface membrane of postsynaptic neurons. Neurotransmitter receptors can have other effects such as regulation of gene transcription and the regulation of synaptic plasticity (changes in the functional and structural properties of the synapses themselves). The interconnected networks of neuronal cell bodies, axons, synapses and dendrites can be thought of as circuits that carry signals using a mixture of chemical synapses, electrical synapses, action potential propagation, and signals in the form of graded potential changes at cell surface membranes. User:JWSchmidt 23:41, 29 Mar 2004 (UTC) This may be syntax curmudgeonery, but I think the original was more acurate. As you said, "The interconnected networks of neuronal cell bodies, axons, synapses and dendrites can be thought of as circuits" but that doesn't make synapses circuits themselves, as the sentence implies, only components of them. This stuck out to me immediately upon reading the article. I'm changing it to "synapses form the circuits" because that seems to say everything that's needed. User:24.0.213.140 20:28, 29 Aug 2004 (UTC) ==Synaptic Strength== *"Synaptic strength is the amount of current, or more strictly the change in transmembrane potential of the synapse." *"One regulatory trigger of synaptic strength involves the simple coincidence sensory stimuli and action potentials in the synaptically linked cells." 1) I'm not sure why these two "sentences" seem to have a charmed life on the synapse page. They both should be fixed. 2) If you look in a neuroscience textbook ([http://www.ncbi.nlm.nih.gov:80/books/bv.fcgi?call=bv.View..ShowTOC&rid=neurosci.TOC&depth=2 example]) you will find discussion of synaptic strength mostly in the context of synaptic plasticity. I tried adding some basic information about synaptic plasticity. It was removed, leaving these two (above) problem sentences. In summary, the "Synaptic Strength" section needs help but I'm not sure I should make any more effort to help it. User:JWSchmidt 00:47, 30 Mar 2004 (UTC) ==Lack of references== Hi this article no longer meets the Wikipedia:What is a featured article for a featured article because it does not Wikipedia:Cite sources. Please help fix this so that all featured articles can meet the same standards. Best would be the most trusted resources in the field being added, some print resources especially, but also online references are better than none. Those sources would likely help with good material to further improve the article anyway. - User:Taxman 23:00, Oct 26, 2004 (UTC) :I added two of the most common neuroscience textbooks (Kandel and Bear) as references. If anyone who contributed to this article had more specific references in mind, please add them. User:Sayeth 17:43, Nov 1, 2004 (UTC) ::Would you say you are very confident that the material in those books agrees with what is here? Otherwise that is potentially dishonest to list them as references. - User:Taxman 19:11, Nov 1, 2004 (UTC) :::Yes, both books state what is now the established dogma of synaptic transmission and action potential propagation, which is repeated in the wikipedia articles. In all likelihood, at least some of the authors of these articles used the Kandel or Brown textbooks as their source, since these are the primary textbooks for graduate and undergraduate courses in neuroscience. If the articles delved into more the esoteric mechanics of these neural functions, then there might be some conflict with the books, but as it stands now, they agree. User:Sayeth 17:03, Nov 2, 2004 (UTC) ::::Ok, sounds good to me. Thanks for helping and being willing to explain. - User:Taxman 18:35, Nov 2, 2004 (UTC) I used Kandel & Schwartz in graduate school and think it's a perfect reference. == Dimension of a synapse ? == Which is the average size of a synapse ? It is probably smaller than a micron since the neurons size is between 5 and 150 microns, but I would like to know their real size. : It's hard to tell exactly what dimensions you're speaking of, but I'd bet you're asking about the distance between pre- and postsynaptic neurons. The gap is called the synaptic cleft and it is typically 20-30 nanometer across. Here's a [http://synapses.mcg.edu/anatomy/chemical/cleft.htm nice visual]. --User:Diberri | User talk:Diberri 22:09, Apr 13, 2005 (UTC)
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[[Image:Nerves.jpg|right|framed|Synapses allow neuron to communicate with one another through axons and dendrites, converting action potential into neurotransmitter ones.]] Synapses are specialized junctions through which cells of the nervous system signal to one another and to non-neuronal cells such as muscles or glands. Synapses form the circuits in which the neurons of the central nervous system interconnect. They are thus crucial to the biological computations that underlie perception and thought. They also provide the means through which the nervous system connects to and controls the other systems of the body. The word "synapse" comes from "synaptein" which Sir Charles Scott Sherrington and his colleagues coined from the Greek "syn-" meaning "together" and "haptein" meaning "to clasp". ==Anatomy== At a prototypical synapse, such as a dendritic spine, a mushroom-shaped bud projects from each of two cells and the caps of these buds press flat against one another. At this interface, the biological membranes of the two cells flank each other across a slender gap, the narrowness of which enables signalling molecules known as neurotransmitters to pass rapidly from one cell to the other by diffusion. This gap is sometimes called the synaptic cleft. Such synapses are asymmetric both in structure and in how they operate. Only the so-called ''pre-synaptic'' neuron secretes the neurotransmitter, which binds to transmembrane receptors facing into the synapse from the ''post-synaptic'' cell. The pre-synaptic ''nerve terminal'' (also called the ''synaptic button'' or ''bouton'') generally buds from the tip of an axon, while the post-synaptic target surface typically appears on a dendrite, a cell body, or another part of a cell. The parts of synapses where neurotransmitter is released are called the ''active zones''. At active zones the membranes of the two adjacent cells are held in close contact by cell adhesion proteins. Note: There also exists a less elaborate form of junction called an electrical synapse. == Signalling across chemical synapses == The release of neurotransmitter is triggered by the arrival of a nerve impulse (or action potential) and occurs through an unusually rapid process of cellular secretion: Within the pre-synaptic nerve terminal, vesicle (biology)s containing neurotransmitter sit "docked" and ready at the synaptic membrane. The arriving action potential produces an influx of second messenger through voltage-dependent, calcium-selective ion channels, at which point the vesicles fuse with the membrane and release their contents to the outside. Receptors on the opposite side of the synaptic gap bind neurotransmitter molecules and respond by opening nearby ion channels in the post-synaptic cell membrane, causing ions to rush in or out and changing the local transmembrane potential of the cell. The result is ''excitatory'', in the case of cell potential currents, or ''inhibitory'' in the case of cell potential currents. Whether a synapse is excitatory or inhibitory depends on what type(s) of ion channel conduct the post-synaptic current, which in turn is a function of the type of receptors and neurotransmitter employed at the synapse. == Synaptic strength == Synaptic strength is the amount of Current_(electricity), or, more strictly, the change in transmembrane potential of the synapse. It is subject to biological regulation. The variability of synaptic strength is often referred to as synaptic plasticity. One regulatory trigger of synaptic strength involves the simple coincidence sensory stimuli and action potentials in the synaptically linked cells. == Integration of synaptic inputs == Generally, if an excitatory synapse is strong, an action potential in the pre-synaptic neuron will trigger another in the post-synaptic cell; whereas at a weak synapse the excitatory postsynaptic potential will not reach the action potential for action potential initiation. In the brain, however, each neuron typically connects or "synapses" to many others, and likewise each receives synaptic "inputs" from many others. When action potentials "fire" simultaneously in several neurons that weakly synapse on a single cell, they may initiate an impulse in that cell even though the synapses are weak. On the other hand, a pre-synaptic neuron releasing an inhibitory neurotransmitter such as GABA can cause inhibitory postsynaptic potential in the post-synaptic neuron, decreasing its excitability and therefore decreasing the neuron's likelihood to fire an action potential. In this way the output of a neuron may depend on the input of many others, each of which may have a different degree of influence, depending on the strength of its synapse with that neuron. John Carew Eccles performed some of the important early experiments on synaptic integration, for which he received the Nobel Prize for Physiology or Medicine in 1963. Complex input/output relationships form the basis of transistor-based computations in computers, and so are thought to figure similarly in neural circuits. == Detailed properties and regulation == Following fusion of the synaptic vesicles and release of transmitter molecules into the synaptic cleft, the neurotransmitter is rapidly cleared from the space for recycling by specialized membrane proteins in the pre-synaptic or post-synaptic membrane. This "reuptake" prevents "desensitization" of the post-synaptic receptors and ensures that succeeding action potentials will elicit the same size EPSP. The necessity of re-uptake and the phenomenon of desensitization in receptors and ion channels means that the strength of a synapse may in effect diminish as a train of action potentials arrive in rapid succession--a phenomenon that gives rise to the so-called frequency dependence of synapses. The nervous system exploits this property for computational purposes, and apparently tunes its synapses through such means as phosphorylation of the proteins involved. The size, number and replenishment rate of vesicles also are subject to regulation, as are many other elements of synaptic transmission. The drugs known as selective serotonin re-uptake inhibitors or Selective serotonin reuptake inhibitors affect certain synapses by inhibiting the re-uptake of the neurotransmitter serotonin.
One important excitatory neurotransmitter, acetylcholine, does not undergo reuptake, but instead is removed from the synapse by the action of the enzyme acetylcholinesterase. ---- By analogy to true synapses described above, the interface between an antigen presenting cell and lymphocyte is sometimes called an immunological synapse. ==References== * M.F. Bear, B.W. Conners, and M.A. Paradiso. 2001. ''Neuroscience: Exploring the Brain''. Baltimore: Lippincott. ISBN 0781739446 * Eric Kandel, James Schwartz, and Thomas Jessel. 2000. ''Principles of Neural Science''. 4th ed. McGraw-Hill, New York. ISBN 0838577016 Nervous system
Synapse
---- That's as maybe, but I want also to add that there are different kinds of synapse. Most are chemical in nature, such as are already described. However, there is evidence for some very fast acting synapses which are electrical. These are reported to have been discovered in fish. Assuming that this has not been discredited, it would be worth having a section on electrical synapses. User:David Martland 00:06, 16 Jan 2004 (UTC) [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11389476&dopt=Abstract Electrical synapses between GABA-releasing interneurons. Nat Rev Neurosci. 2001 Jun; 2(6): 425-33. Galarreta M, Hestrin S.] *''Although gap junctions were first demonstrated in the mammalian brain about 30 years ago, the distribution and role of electrical synapses have remained elusive. A series of recent reports has demonstrated that inhibitory interneurons in the cerebral cortex, thalamus, striatum and cerebellum are extensively interconnected by electrical synapses. Investigators have used paired recordings to reveal directly the presence of electrical synapses among identified cell types. These studies indicate that electrical coupling is a fundamental feature of local inhibitory circuits and suggest that electrical synapses define functionally diverse networks of GABA-releasing interneurons. Here, we discuss these results, their possible functional significance and the insights into neuronal circuit organization that have emerged from them.'' Yes, I think electrical synapses deserve a section, if not some systemic changes to admit to the recently broadened concept of synapse. Gap junctions didn't used to be considered synapses, but evidently now they are. User:168... 16:31, 16 Jan 2004 (UTC) ==Synapses and circuits== "Synapses define the circuits in which the neurons of the central nervous system interconnect." was changed to "Synapses are circuits...." Synapses are functional contacts for communication between nearby cells. Most synapses are between adjacent neurons, but some synapses are from neurons onto other cell types such as muscle cells. Synapses are of two types: electrical synapses and chemical synapses. Electrical synapses provide cytoplasmic connections between adjacent cells where ions can carry charges from one neuron to another. Chemical synapses provide cell-to-cell communication that is mediated by neurotransmitters. A neurotransmitter is a chemical that is released from one neuron, moves to a nearby neuron, and acts to modulate the physiology of the target cell. Neurotransmitters often work by binding to receptor proteins on the surface of cells. However, some neurotransmitters pass through cell membranes and directly influence target proteins inside their target neurons. Structurally, synapses are usually regions of close contact between adjacent neurons. Most neurotransmitters are packaged into vesicles at the presynaptic side of synapses. Most neurotransmitter receptors are concentrated in post-synaptic densities on the post-synaptic side of synapses. When activated by neurotransmitters, neurotransmitter receptors typically change ion flow across the surface membrane of postsynaptic neurons. Neurotransmitter receptors can have other effects such as regulation of gene transcription and the regulation of synaptic plasticity (changes in the functional and structural properties of the synapses themselves). The interconnected networks of neuronal cell bodies, axons, synapses and dendrites can be thought of as circuits that carry signals using a mixture of chemical synapses, electrical synapses, action potential propagation, and signals in the form of graded potential changes at cell surface membranes. User:JWSchmidt 23:41, 29 Mar 2004 (UTC) This may be syntax curmudgeonery, but I think the original was more acurate. As you said, "The interconnected networks of neuronal cell bodies, axons, synapses and dendrites can be thought of as circuits" but that doesn't make synapses circuits themselves, as the sentence implies, only components of them. This stuck out to me immediately upon reading the article. I'm changing it to "synapses form the circuits" because that seems to say everything that's needed. User:24.0.213.140 20:28, 29 Aug 2004 (UTC) ==Synaptic Strength== *"Synaptic strength is the amount of current, or more strictly the change in transmembrane potential of the synapse." *"One regulatory trigger of synaptic strength involves the simple coincidence sensory stimuli and action potentials in the synaptically linked cells." 1) I'm not sure why these two "sentences" seem to have a charmed life on the synapse page. They both should be fixed. 2) If you look in a neuroscience textbook ([http://www.ncbi.nlm.nih.gov:80/books/bv.fcgi?call=bv.View..ShowTOC&rid=neurosci.TOC&depth=2 example]) you will find discussion of synaptic strength mostly in the context of synaptic plasticity. I tried adding some basic information about synaptic plasticity. It was removed, leaving these two (above) problem sentences. In summary, the "Synaptic Strength" section needs help but I'm not sure I should make any more effort to help it. User:JWSchmidt 00:47, 30 Mar 2004 (UTC) ==Lack of references== Hi this article no longer meets the Wikipedia:What is a featured article for a featured article because it does not Wikipedia:Cite sources. Please help fix this so that all featured articles can meet the same standards. Best would be the most trusted resources in the field being added, some print resources especially, but also online references are better than none. Those sources would likely help with good material to further improve the article anyway. - User:Taxman 23:00, Oct 26, 2004 (UTC) :I added two of the most common neuroscience textbooks (Kandel and Bear) as references. If anyone who contributed to this article had more specific references in mind, please add them. User:Sayeth 17:43, Nov 1, 2004 (UTC) ::Would you say you are very confident that the material in those books agrees with what is here? Otherwise that is potentially dishonest to list them as references. - User:Taxman 19:11, Nov 1, 2004 (UTC) :::Yes, both books state what is now the established dogma of synaptic transmission and action potential propagation, which is repeated in the wikipedia articles. In all likelihood, at least some of the authors of these articles used the Kandel or Brown textbooks as their source, since these are the primary textbooks for graduate and undergraduate courses in neuroscience. If the articles delved into more the esoteric mechanics of these neural functions, then there might be some conflict with the books, but as it stands now, they agree. User:Sayeth 17:03, Nov 2, 2004 (UTC) ::::Ok, sounds good to me. Thanks for helping and being willing to explain. - User:Taxman 18:35, Nov 2, 2004 (UTC) I used Kandel & Schwartz in graduate school and think it's a perfect reference. == Dimension of a synapse ? == Which is the average size of a synapse ? It is probably smaller than a micron since the neurons size is between 5 and 150 microns, but I would like to know their real size. : It's hard to tell exactly what dimensions you're speaking of, but I'd bet you're asking about the distance between pre- and postsynaptic neurons. The gap is called the synaptic cleft and it is typically 20-30 nanometer across. Here's a [http://synapses.mcg.edu/anatomy/chemical/cleft.htm nice visual]. --User:Diberri | User talk:Diberri 22:09, Apr 13, 2005 (UTC)
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