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Encyclopedia > Synaptic cleft
Synapses allow nerve cells to communicate with one another through axons and dendrites, converting electrical signals into chemical ones.
Synapses allow nerve cells to communicate with one another through axons and dendrites, converting electrical signals into chemical 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. Source: http://teens. ... Neurons (also spelled neurones or called nerve cells) are the primary cells of the nervous system. ... An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, which conducts electrical impulses away from the neurons cell body or soma. ... In biology, a dendrite is a slender, typically branched projection of a nerve cell, or neuron, which conducts the electrical stimulation received from other cells to the body or soma of the cell from which it projects. ... Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between two neurons: the presynaptic neuron and the postsynaptic neuron. ... The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and processes input from the senses, and initiates actions. ... A top-down view of skeletal muscle Muscle is a contractile form of tissue. ... 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). ...


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. Neurons (also spelled neurones or called nerve cells) are the primary cells of the nervous system. ...


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 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 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. A dendritic spine is a mushroom-shaped bud that protrudes from a dendrite and forms one half of a synapse, especially in synapses of the cortex. ... 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 two neurons: the presynaptic neuron and the postsynaptic neuron. ... Diffusion is the spontaneous spreading of something such as particles, heat, or momentum. ...


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. 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, which conducts electrical impulses away from the neurons cell body or soma. ... In biology, a dendrite is a slender, typically branched projection of a nerve cell, or neuron, which conducts the electrical stimulation received from other cells to the body or soma of the cell from which it projects. ... Schematic of cell adhesion The study of cell adhesion is part of cell biology. ...


Note: There also exists a less elaborate form of junction called an electrical synapse. An electical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed by proteins known as gap junctions. ...


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, 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, 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 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, which in turn is a function of the type of receptors and neurotransmitter employed at the synapse. Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ... Exocytosis is the process of a biological cell releasing substances into the extracellular fluid (its environment). ... 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 a signal within a cell. ... Another, unrelated ion channeling process is part of ion implantation. ... In membrane biophysics sometimes used interchangeably with cell potential, but applicable to any lipid bilayer or membrane. ... 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. ...


Synaptic strength

Synaptic strength is the amount of current, or, more strictly, the change in transmembrane potential of the synapse. These changes in the propensity of neurons to fire together via a given synapse are induced by both sensory experience (i.e., sights, sounds), and internally generated self-stimulation (i.e., thoughts, dreams). The variability of synaptic strength is often referred to as synaptic plasticity. Changes in transmembrane potential can be short-term and without permanent structural change in the neurons themselves, lasting seconds to minutes - or long-term and potent (long-term potentiation, or LTP), in which repeated or continuous electrical stimulation can result in second messenger neurotransmitters being released into the neuron's nucleus, initiating protein synthesis, resulting in alteration of the structure of the neuron itself. In electricity, current refers to electric current, which is the flow of electric charge. ... Dreaming is the subjective experience of imaginary images, sounds/voices, thoughts or sensations during sleep, usually involuntarily. ... The term synaptic plasticity refers to the variability of the strength of a signal transmitted through a synapse. ... In neuroscience, long-term potentiation (LTP) is the strengthening (or potentiation) of the connection between two nerve cells which lasts for an extended period of time (minutes to hours in vitro and hours to days and months in vivo). ... In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay a signal within a cell. ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a presynaptic and a postsynaptic neuron. ... Plural: nuclei In chemistry and physics, the nucleus (atomic nucleus) is the collection of protons and neutrons in the center of an atom that carries the bulk of the atoms mass and positive charge. ... In biochemistry, protein synthesis refers to the creation of a macromolecule from amino acids. ...


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 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. 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. ... Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave 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 Currently there is more information available under the heading synapse. ... 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. ... 1963 was a common year starting on Tuesday (link will take you to calendar). ... Through hole transistors (tape measure marked in centimeters) The transistor is a solid state semiconductor device which can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. ... The tower of a personal computer (specifically a Power Mac G5). ...


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 "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 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 SSRIs 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. 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. ... 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 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. ...


By analogy to true synapses described above, the interface between an antigen presenting cell and lymphocyte is sometimes called an immunological synapse. An antigen is a molecule that stimulates the production of antibodies. ...


References

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  • 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

 
 

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