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Encyclopedia > Synapse
Illustration of the major elements in a prototypical synapse. Synapses allow nerve cells to communicate with one another through axons and dendrites, converting electrical impulses into chemical signals.
Illustration of the major elements in a prototypical synapse. Synapses allow nerve cells to communicate with one another through axons and dendrites, converting electrical impulses into chemical signals.

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. A synapse between a motor neuron and a muscle cell is called a neuromuscular junction. Image File history File links Download high resolution version (853x564, 69 KB)Illustration of a archetypical synapse in the central nervous system. ... Image File history File links Download high resolution version (853x564, 69 KB)Illustration of a archetypical synapse in the central nervous system. ... Drawing by Santiago Ramón y Cajal of cells in the pigeon cerebellum. ... An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... The term Dendrite stems from the Greek word dendron (literally “tree”), and typically refers to the branched projections of a neuron that act to conduct the electrical stimulation received from other cells to and from the cell body, or soma of the neuron from which the dendrites project. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another cell. ... The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and also stops input from the senses, and initiates actions. ... A top-down view of skeletal muscle Muscle is contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. ... A gland is an organ in an animals body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). ... A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscles surface. ...


Synapses allow the neurons of the central nervous system to form interconnected neural circuits. 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. Drawing by Santiago Ramón y Cajal of cells in the pigeon cerebellum. ... A diagram showing the CNS: 1. ...


The human brain contains a huge number of synapses, with young children having about 1,000 trillion. This number declines with age, stabilizing by adulthood. Estimates for an adult vary from 100 to 500 trillion synapses. The human brain. ...


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". Sherrington is considered one of the fathers of neuroscience. ...

Contents


Anatomy

At a prototypical chemical synapse, such as those found at dendritic spines, 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 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, which is about 20 nm wide, is known as the synaptic cleft. Close up of the dendrite of a striatal medium spiny neuron. ... A biological membrane or biomembrane is a membrane which acts as a barrier within or around a cell. ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another cell. ... Schematic drawing of the effects of diffusion through a semipermeable membrane. ...


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 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. Immediately behind the post-synaptic membrane is an elaborate complex of interlinked proteins called the postsynaptic density. Proteins in the postsynaptic density serve myriad roles, from anchoring and trafficking neurotransmitter receptors into the plasma membrane, to anchoring various proteins which modulate the activity of the receptors. The postsynaptic cell need not be a neuron, and can also be gland or muscle cells. Transmembrane receptors are integral membrane proteins, which reside and operate typically within a cells plasma membrane, but also in the membranes of some subcellular compartments and organelles. ... An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... The term Dendrite stems from the Greek word dendron (literally “tree”), and typically refers to the branched projections of a neuron that act to conduct the electrical stimulation received from other cells to and from the cell body, or soma of the neuron from which the dendrites project. ... Schematic of cell adhesion The study of cell adhesion is part of cell biology. ... The postsynaptic density (PSD) is a cytoskeletal specialization at neuronal synapses that was originally identified as an electron-dense region at the membrane of a postsynaptic neuron, as viewed by electron microscopy. ... In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. ... A gland is an organ in an animals body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). ... A top-down view of skeletal muscle Muscle is contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. ...


Signaling 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, also known as exocytosis: Within the pre-synaptic nerve terminal, vesicles containing neurotransmitter sit "docked" and ready at the synaptic membrane. The arriving action potential produces an influx of calcium ions through voltage-dependent, calcium-selective ion channels. Calcium ions then trigger a biochemical cascade which results in vesicles fusing with the presynaptic-membrane and release their contents to the synaptic cleft. Vesicle fusion is driven by the action of a set of proteins in the presynaptic terminal known as SNAREs. 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 resulting change in voltage is called a postsynaptic potential. In general, the result is excitatory, in the case of depolarizing currents, or inhibitory in the case of hyperpolarizing currents. Whether a synapse is excitatory or inhibitory depends on what type(s) of ion channel conduct the post-synaptic current display(s), which in turn is a function of the type of receptors and neurotransmitter employed at the synapse. A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... Exocytosis is the process by which a cell is able to get rid of large molecules or materials including wastes through its membrane. ... In cell biology, a vesicle is a relatively small and enclosed compartment, separated from the cytosol by at least one lipid bilayer. ... In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay signals within a cell. ... Another, unrelated ion channeling process is part of ion implantation. ... SNARE proteins (an acronym derived from soluble NSF attachment receptor) are a large protein superfamily consisting of more than 60 members in yeast and mammalian cells. ... In membrane biophysics sometimes used interchangeably with cell potential, but applicable to any lipid bilayer or membrane. ... Postsynaptic potentials are changes in the membrane potential of the neuron that receives information at a synapse. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... Hyperpolarization has several meanings: In biology, hyperpolarization occurs when a cells membrane potential dips below its resting level. ...


Electrical synapses

Main article: electrical synapse

An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. At gap junctions, cells approach within about 3.5 nm of each other (Kandel et al., 2000, p. 179), a much shorter distance than the 20 to 40 nm distance that separates cells at chemical synapses (Hormuzdi et al., 2004). As opposed to chemical synapses, the postsynaptic potential in electrical synapses is not caused by the opening of ion channels by chemical transmitters, but by direct electrical coupling between both neurons. Electrical synapses are therefore faster and more reliable than chemical synapses. Electrical synapses are found throughout the nervous system, yet are less common than chemical synapses. An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. ... In science and engineering, conductors are materials that contain movable charges of electricity. ... Drawing by Santiago Ramón y Cajal of cells in the pigeon cerebellum. ... Cells in culture, stained for keratin (red) and DNA (green). ... A gap junction is a junction between certain animal/plant cell-types that allows different molecules and ions to pass freely between cells. ...


Modulation of synaptic transmission

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 "re-uptake" 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 can tune 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. For example, a class of drugs known as selective serotonin re-uptake inhibitors or SSRIs affect certain synapses by inhibiting the re-uptake of the neurotransmitter serotonin. In contrast, one important excitatory neurotransmitter, acetylcholine, does not undergo re-uptake, but instead is removed from the synapse by the action of the enzyme acetylcholinesterase. Reuptake is the reabsorption of a neurotransmitter by the molecular transporter of a pre-synaptic neuron after it has performed its function of transmitting a neural impulse. ... Desensitization is a method to reduce or eliminate an organisms negative reaction to a substance or stimulus. ... Phosphorylation is the addition of a phosphate (PO4) group to a protein or a small molecule. ... Selective serotonin reuptake inhibitors (SSRIs) are a class of antidepressants for treating depression, anxiety disorders and some personality disorders. ... Serotonin (5-hydroxytryptamine, or 5-HT) is a monoamine neurotransmitter synthesised in serotonergic neurons in the central nervous system and enterochromaffin cells in the gastrointestinal tract. ... The skeletal structure of acetylcholine The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... In biochemistry, cholinesterase is a term which refers to one of the two enzymes (EC 3. ...


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 post-synaptic potential ("EPSP") will not reach the threshold for action potential initiation. In the brain, however, each neuron typically forms synapses with 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. This process is known as summation. 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 are thought to figure similarly in neural circuits. A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... In neuroscience, an excitatory postsynaptic potential (EPSP) is a temporary increase in postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... Chemical structure of GABA Gamma-aminobutyric acid (GABA) is a neurotransmitter in widely divergent species. ... Inhibitory Postsynaptic Potential is commonly abbreviated to Impulses are transmitted from neuron to neuron by the release of a chemical transmitter across synaptic clefts from the synaptic vesicles along the axon to the postsynaptic receptors of another neuron. ... Sir John Carew Eccles (January 27, 1903 – May 2, 1997) was an Australian neurophysiologist who won the 1963 Nobel Prize in Physiology or Medicine for his work on the synapse. ... List of Nobel Prize laureates in Physiology or Medicine from 1901 to the present day. ... Assorted transistors The transistor is a solid state semiconductor device that can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. ... A computer is a machine designed for manipulating data according to a list of instructions known as a program. ...


Synaptic strength

The strength of a synapse is defined by the change in transmembrane potential resulting from activation of the postsynaptic neurotransmitter receptors. This change in voltage is known as a post-synaptic potential, and is a direct result of ionic currents flowing through the post-synaptic receptor-channels. Changes in synaptic strength can be short-term and without permanent structural changes in the neurons themselves, lasting seconds to minutes - or long-term (long-term potentiation, or LTP), in which repeated or continuous synaptic activation can result in second messenger molecules initiating protein synthesis in the neuron's nucleus, resulting in alteration of the structure of the synapse itself. Learning and memory are believed to result from long-term changes in synaptic strength, via a mechanism known as synaptic plasticity. In electricity, current refers to electric current, which is the flow of electric charge. ... An example of long-term potentiation (LTP). ... In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay signals within a cell. ... Biological and artificial methods for creation of proteins differ significantly. ... In cell biology, the nucleus (from Latin nucleus or nuculeus, kernel) is found in all eukaryotic cells and contains the nuclear genes which form most of the cells genetic material. ... In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength. ...


Immunological synapses

By analogy to true synapses described above, the interface between an antigen-presenting cell and lymphocyte is sometimes called an immunological synapse. An antigen presenting cell (APC) is a cell that displays foreign antigen complexed with MHC on its surface. ... Lymphocyte (stained) A lymphocyte is a type of white blood cell involved in the human bodys immune system. ...


See also

Postsynaptic potentials are changes in the membrane potential of the neuron that receives information at a synapse. ... In neuroscience, an excitatory postsynaptic potential (EPSP) is a temporary increase in postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell. ... Inhibitory Postsynaptic Potential is commonly abbreviated to Impulses are transmitted from neuron to neuron by the release of a chemical transmitter across synaptic clefts from the synaptic vesicles along the axon to the postsynaptic receptors of another neuron. ... A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscles surface. ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another cell. ... In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. ... Exocytosis is the process by which a cell is able to get rid of large molecules or materials including wastes through its membrane. ...

References

    • Mark Bear, Mark F. Bear, Barry W. Connors, Michael A. Paradiso (2001). Neuroscience: Exploring the Brain. Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0781739446.
    • Hormuzdi SG, Filippov MA, Mitropoulou G, Monyer H, Bruzzone R. Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks. Biochim Biophys Acta. 2004 Mar 23;1662(1-2):113-37. PMID 15033583
    • Eric R. Kandel, James H. Schwartz, Thomas M. Jessell (2000). Principles of Neural Science, 4th edition, New York: McGraw-Hill. ISBN 0838577016.
    • J.G. Nicholls, A.R. Martin, B.G. Wallace, and P.A. Fuchs (2001). From Neuron to Brain, 4th edition, Sinauer Associates location=Sunderland, MA. ISBN 0878924391.
    • Karp, Gerald (2002), Cell and Molecular Biology - Fourth Edition, John Wiley & Sons, Inc..

    Eric Richard Kandel (born November 7, 1929) is a psychiatrist, a neuroscientist and professor of biochemistry and biophysics at Columbia University. ... Principles of Nerual Science cover First published in 1981, Principles of Neural Science is a neuroscience textbook edited by Eric R. Kandel, James Schwartz, and Thomas Jessell. ...

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      Results from FactBites:
     
    Synapse - definition of Synapse in Encyclopedia (965 words)
    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.
    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.
    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.
    synapse - Columbia Encyclopedia article about synapse (282 words)
    synapse (sĭn`ăps), junction between various signal-transmitter cells, either between two neurons or between a neuron and a muscle or gland.
    An electric synapse, unlike a chemical one, uses channels known as gap junctions to permit direct transmission of signals between neurons.
    Such synapses are found in the human body, within many organs and in the glial cells of the nervous system.
      More results at FactBites »

     
     

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