Neuromuscular junction: Difference between revisions
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===Neuromuscular junction=== |
===Neuromuscular junction=== |
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The '''neuromuscular junction''' differs from [[Synapse|synapses]] in the [[Central nervous system|central nervous system]]. Presynaptic motor axons are demyelinated and stop 30 nanometers from the [[Sarcolemma|sarcolemma]], the cell membrane of a muscle cell. This 30-nanometer space forms the [[Chemical synapse|synaptic cleft]] through which signaling molecules are released. The sarcolemma has [[Invagination|invaginations]] called postjunctional folds, which increase the surface area of the membrane exposed to the synaptic cleft.<ref name="Text">Nicholls, John G., A. Robert Martin, Paul A. Fuchs, David A. Brown, Matthew E. Diamond, and David A. Weisblat |
The '''neuromuscular junction''' differs from [[Synapse|synapses]] in the [[Central nervous system|central nervous system]]. Presynaptic motor axons are demyelinated and stop 30 nanometers from the [[Sarcolemma|sarcolemma]], the cell membrane of a muscle cell. This 30-nanometer space forms the [[Chemical synapse|synaptic cleft]] through which signaling molecules are released. The sarcolemma has [[Invagination|invaginations]] called postjunctional folds, which increase the surface area of the membrane exposed to the synaptic cleft.<ref name="Text">{{cite book |author=Nicholls, John G., |coathors=A. Robert Martin, Paul A. Fuchs, David A. Brown, Matthew E. Diamond, and David A. Weisblat |title=From Neuron to Brain |publisher=Sinauer Associates |location=Sunderland |year=2012 |edition=5th }}</ref> These postjunctional folds form what is referred to as the motor end-plate, which possesses [[Acetylcholine receptor|acetylcholine receptors]] (AChRs) at a density of 10,000 receptors/micrometer<sup>2</sup> in [[Skeletal striated muscle|skeletal muscle]].<ref name="Sine">{{cite journal |author=Sine SM |title=End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease |journal=Physiol. Rev. |volume=92 |issue=3 |pages=1189–234 |year=2012 |month=July |pmid=22811427 |pmc=3489064 |doi=10.1152/physrev.00015.2011 |url=http://physrev.physiology.org/cgi/pmidlookup?view=long&pmid=22811427}}</ref> The presynaptic axons form bulges called terminal boutons that project into the postjunctional folds of the sarcolemma. The presynaptic boutons have active zones that contain vesicles, quanta, full of acetylcholine molecules. These vesicles can fuse with the presynaptic membrane and release ACh molecules into the synaptic cleft via [[exocytosis]] after depolarization. <ref name="Text" /> |
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AChRs are localized by protein scaffolds at the postjunctional folds of the sarcolemma opposite the presynaptic terminals. [[Dystrophin]], a structural protein, connects the sarcomere, sarcolemma, and extracellular matrix components. [[Rapsyn]] is another protein that docks AChRs and structural proteins to the cytoskeleton. The [[receptor tyrosine kinase]] protein MuSK is a signaling protein involved in the development of the neuromuscular junction and is also held in place by rapsyn. <ref name="Text" /> |
AChRs are localized by protein scaffolds at the postjunctional folds of the sarcolemma opposite the presynaptic terminals. [[Dystrophin]], a structural protein, connects the sarcomere, sarcolemma, and extracellular matrix components. [[Rapsyn]] is another protein that docks AChRs and structural proteins to the cytoskeleton. The [[receptor tyrosine kinase]] protein MuSK is a signaling protein involved in the development of the neuromuscular junction and is also held in place by rapsyn. <ref name="Text" /> |
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Acetylcholine is a [[neurotransmitter]] synthesized in the human body from dietary [[choline]] and [[acetyl-CoA]] (ACoA), and it is involved in the stimulation of [[vertebrate]] muscle tissue. It is important to note that not all species use a [[cholinergic]] neuromuscular junction; in fact, crayfish have a [[glutamic acid|glutamatergic]] neuromuscular junction.<ref name="Text" /> The eukaryotic [[nicotinic acetylcholine receptor]] (AChR) is a heteropentamer, [[ligand-gated ion channel]]. Each subunit has a characteristic “cys-loop,” which is composed of a [[cysteine]] residue followed by 13 more [[amino acid]] residues and another cysteine residue. The two cysteine residues form a [[disulfide bond|disulfide linkage]] which results in the “cys-loop” receptor that is capable of binding acetylcholine and other ligands. These cys-loop receptors are found only in [[eukaryotes]], but [[prokaryotes]] possess ACh receptors with similar properties.<ref name="Sine" /> |
Acetylcholine is a [[neurotransmitter]] synthesized in the human body from dietary [[choline]] and [[acetyl-CoA]] (ACoA), and it is involved in the stimulation of [[vertebrate]] muscle tissue. It is important to note that not all species use a [[cholinergic]] neuromuscular junction; in fact, crayfish have a [[glutamic acid|glutamatergic]] neuromuscular junction.<ref name="Text" /> The eukaryotic [[nicotinic acetylcholine receptor]] (AChR) is a heteropentamer, [[ligand-gated ion channel]]. Each subunit has a characteristic “cys-loop,” which is composed of a [[cysteine]] residue followed by 13 more [[amino acid]] residues and another cysteine residue. The two cysteine residues form a [[disulfide bond|disulfide linkage]] which results in the “cys-loop” receptor that is capable of binding acetylcholine and other ligands. These cys-loop receptors are found only in [[eukaryotes]], but [[prokaryotes]] possess ACh receptors with similar properties.<ref name="Sine" /> |
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AChRs at the skeletal neuromuscular junction have two α, one β, one γ, and one δ subunits. When a single ACh ligand binds to one of the α subunits of the ACh receptor it induces a [[conformational change]] at the interface with the second AChR α subunit. This [[conformational change]] results in the increased affinity for a second ACh ligand in the second α subunit. AChRs therefore exhibit a sigmoidal dissociation curve due to this [[cooperative binding]] .<ref name="Sine" /> The presence of the inactive, intermediate receptor structure with a single-bound ligand keeps ACh in the synapse that might otherwise be lost by cholinesterase hydrolysis or diffusion. The persistence of these ACh ligands in the synapse can cause a prolonged post-synaptic response.<ref name="Scuka">Scuka |
AChRs at the skeletal neuromuscular junction have two α, one β, one γ, and one δ subunits. When a single ACh ligand binds to one of the α subunits of the ACh receptor it induces a [[conformational change]] at the interface with the second AChR α subunit. This [[conformational change]] results in the increased affinity for a second ACh ligand in the second α subunit. AChRs therefore exhibit a sigmoidal dissociation curve due to this [[cooperative binding]] .<ref name="Sine" /> The presence of the inactive, intermediate receptor structure with a single-bound ligand keeps ACh in the synapse that might otherwise be lost by cholinesterase hydrolysis or diffusion. The persistence of these ACh ligands in the synapse can cause a prolonged post-synaptic response.<ref name="Scuka">{{cite journal |author=Scuka M, Mozrzymas JW |title=Postsynaptic potentiation and desensitization at the vertebrate end-plate receptors |journal=Prog. Neurobiol. |volume=38 |issue=1 |pages=19–33 |year=1992 |pmid=1736323 |url=http://linkinghub.elsevier.com/retrieve/pii/0301-0082(92)90033-B}}</ref> |
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The pre and postsynaptic cells at the neuromuscular junction also contain [[muscarinic acetylcholine receptors]] that are [[metabotropic receptor|metabotropic receptors]] coupled with [[G-proteins]]. Muscarinic receptors can be found on the presynaptic cell, which act via negative feedback by binding acetylcholine molecules released by the presynaptic cell, thereby inhibiting it from releasing more neurotransmitters through a signaling cascade. G-protein subunits can inactivate the calcium channels necessary for neurotransmitter release into the synaptic cleft. Muscarinic AChRs on the sarcolemma of the postsynaptic cell bind ACh, which activates a G-protein that opens Kir3.1 potassium channels, thereby hyperpolarizing the muscle fiber.<ref name="Text" /> |
The pre and postsynaptic cells at the neuromuscular junction also contain [[muscarinic acetylcholine receptors]] that are [[metabotropic receptor|metabotropic receptors]] coupled with [[G-proteins]]. Muscarinic receptors can be found on the presynaptic cell, which act via negative feedback by binding acetylcholine molecules released by the presynaptic cell, thereby inhibiting it from releasing more neurotransmitters through a signaling cascade. G-protein subunits can inactivate the calcium channels necessary for neurotransmitter release into the synaptic cleft. Muscarinic AChRs on the sarcolemma of the postsynaptic cell bind ACh, which activates a G-protein that opens Kir3.1 potassium channels, thereby hyperpolarizing the muscle fiber.<ref name="Text" /> |
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==Development of the neuromuscular junction== |
==Development of the neuromuscular junction== |
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The development of the neuromuscular junction requires signaling from both the motor neuron's terminal and the muscle cell's [[central region]]. During development, muscle cells produce acetylcholine receptors (AChRs) and express them in the central regions in a process called prepatterning. [[Agrin]], a heparin [[proteoglycan]], and MuSK kinase are thought to help stabilize the accumulation of AChR in the central regions of the myocyte. MuSK is a receptor [[tyrosine kinase]] |
The development of the neuromuscular junction requires signaling from both the motor neuron's terminal and the muscle cell's [[central region]]. During development, muscle cells produce acetylcholine receptors (AChRs) and express them in the central regions in a process called prepatterning. [[Agrin]], a heparin [[proteoglycan]], and MuSK kinase are thought to help stabilize the accumulation of AChR in the central regions of the myocyte. MuSK is a receptor [[tyrosine kinase]] — meaning that it induces cellular signaling by causing the release of [[phosphate]] molecules to particular [[tyrosine]]s on itself, and on proteins which bind the [[cytoplasm]]ic [[protein domain|domain]] of the receptor.<ref>{{cite journal | author = Valenzuela D, Stitt T, DiStefano P, Rojas E, Mattsson K, Compton D, Nuñez L, Park J, Stark J, Gies D | title = Receptor tyrosine sinase specific for the skeletal muscle lineage: expression in embryonic muscle, at the neuromuscular junction, and after injury | journal = Neuron | volume = 15 | issue = 3 | pages = 573–84 | year = 1995 | pmid = 7546737 | doi = 10.1016/0896-6273(95)90146-9}}</ref>Upon activation by its ligand agrin, MuSK signals via two proteins called "[[Dok-7]]" and "[[rapsyn]]", to induce "clustering" of acetylcholine receptors.<ref>{{cite journal | author = Glass D, Bowen D, Stitt T, Radziejewski C, Bruno J, Ryan T, Gies D, Shah S, Mattsson K, Burden S, DiStefano P, Valenzuela D, DeChiara T, Yancopoulos G | title = Agrin acts via a MuSK receptor complex | journal = Cell | volume = 85 | issue = 4 | pages = 513–23 | year = 1996 | pmid = 8653787 | doi = 10.1016/S0092-8674(00)81252-0}}</ref> ACh release by developing motor neurons produces postsynaptic potentials in the muscle cell that positively reinforces the localization and the stabilization of the developing neuromuscular junction. <ref>{{cite journal |author=Witzemann V |title=Development of the neuromuscular junction |journal=Cell Tissue Res. |volume=326 |issue=2 |pages=263–71 |year=2006 |month=November |pmid=16819627 |doi=10.1007/s00441-006-0237-x }}</ref> |
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===Knockout studies=== |
===Knockout studies=== |
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These findings were demonstrated in part by mouse "[[transgenic|knockout]]" studies. In mice which are deficient for either agrin or MuSK, the neuromuscular junction does not form. Further, mice deficient in [[Dok-7]] did not form either acetylcholine receptor clusters or neuromuscular synapses.<ref>{{cite journal | author = Okada K, Inoue A, Okada M, Murata Y, Kakuta S, Jigami T, Kubo S, Shiraishi H, Eguchi K, Motomura M, Akiyama T, Iwakura Y, Higuchi O, Yamanashi Y | title = The muscle protein Dok-7 is essential for neuromuscular synaptogenesis | journal = Science | volume = 312 | issue = 5781 | pages = 1802–5 | year = 2006 | pmid = 16794080 | doi = 10.1126/science.1127142 |
These findings were demonstrated in part by mouse "[[transgenic|knockout]]" studies. In mice which are deficient for either agrin or MuSK, the neuromuscular junction does not form. Further, mice deficient in [[Dok-7]] did not form either acetylcholine receptor clusters or neuromuscular synapses.<ref>{{cite journal | author = Okada K, Inoue A, Okada M, Murata Y, Kakuta S, Jigami T, Kubo S, Shiraishi H, Eguchi K, Motomura M, Akiyama T, Iwakura Y, Higuchi O, Yamanashi Y | title = The muscle protein Dok-7 is essential for neuromuscular synaptogenesis | journal = Science | volume = 312 | issue = 5781 | pages = 1802–5 | year = 2006 | pmid = 16794080 | doi = 10.1126/science.1127142 |url=http://www.sciencemag.org/cgi/content/abstract/312/5781/1802}}</ref> |
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Many other proteins also comprise the NMJ, and are required to maintain its integrity.<ref>{{cite journal | author = Strochlic L, Cartaud A, Cartaud J | title = The synaptic muscle-specific kinase (MuSK) complex: new partners, new functions | journal = Bioessays | volume = 27 | issue = 11 | pages = 1129–35 | year = 2005 | pmid = 16237673 | doi = 10.1002/bies.20305}}</ref> |
Many other proteins also comprise the NMJ, and are required to maintain its integrity.<ref>{{cite journal | author = Strochlic L, Cartaud A, Cartaud J | title = The synaptic muscle-specific kinase (MuSK) complex: new partners, new functions | journal = Bioessays | volume = 27 | issue = 11 | pages = 1129–35 | year = 2005 | pmid = 16237673 | doi = 10.1002/bies.20305}}</ref> |
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#This depolarization spreads across the surface of the muscle fiber and continues the [[excitation-contraction coupling]] to contract the muscle. |
#This depolarization spreads across the surface of the muscle fiber and continues the [[excitation-contraction coupling]] to contract the muscle. |
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#The action of acetylcholine is terminated when the [[enzyme]] [[acetylcholinesterase]] degrades part of the neurotransmitter (producing [[choline]] and an [[acetate]] group) and the rest of it diffuses away. |
#The action of acetylcholine is terminated when the [[enzyme]] [[acetylcholinesterase]] degrades part of the neurotransmitter (producing [[choline]] and an [[acetate]] group) and the rest of it diffuses away. |
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#The choline produced by the action of acetylcholinesterase is recycled — it is transported, through [[reuptake]], back into the presynaptic terminal, where it is used to synthesize new acetylcholine molecules.<ref name="Purves" >{{cite book | author = Dale Purves, George J. Augustine, David Fitzpatrick, William C. Hall, Anthony-Samuel LaMantia, James O. McNamara, and Leonard E. White | title = Neuroscience |
#The choline produced by the action of acetylcholinesterase is recycled — it is transported, through [[reuptake]], back into the presynaptic terminal, where it is used to synthesize new acetylcholine molecules.<ref name="Purves" >{{cite book | author = Dale Purves, George J. Augustine, David Fitzpatrick, William C. Hall, Anthony-Samuel LaMantia, James O. McNamara, and Leonard E. White | title = Neuroscience |edition=4th | publisher = Sinauer Associates | pages = 121–2 | year = 2008 | isbn = 978-0-87893-697-7}}</ref> |
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==Diseases== |
==Diseases== |
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===Autoimmune=== |
===Autoimmune=== |
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====Myasthenia gravis==== |
====Myasthenia gravis==== |
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[[Myasthenia gravis]] is an autoimmune disorder where the body makes antibodies against either the acetylcholine receptor (AchR) (in 80% of cases), or against postsynaptic muscle-specific kinase (MuSK) ( |
[[Myasthenia gravis]] is an autoimmune disorder where the body makes antibodies against either the acetylcholine receptor (AchR) (in 80% of cases), or against postsynaptic muscle-specific kinase (MuSK) (0–10% of cases). In seronegative myasthenia gravis [[low density lipoprotein]] receptor-related protein 4 is targeted by [[IgG1]], which acts as a competitive inhibitor of its ligand, preventing the ligand from binding its receptor. It is not known if seronegative myasthenia gravis will respond to standard therapies.<ref>{{cite journal |author=Finsterer J, Papić L, Auer-Grumbach M |title=Motor neuron, nerve, and neuromuscular junction disease |journal=Curr. Opin. Neurol. |volume=24 |issue=5 |pages=469–74 |year=2011 |month=October |pmid=21825986 |doi=10.1097/WCO.0b013e32834a9448 |url=http://meta.wkhealth.com/pt/pt-core/template-journal/lwwgateway/media/landingpage.htm?issn=1350-7540&volume=24&issue=5&spage=469}}</ref> |
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=====Neonatal MG===== |
=====Neonatal MG===== |
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Neonatal MG is an autoimmune disorder that affects 1 in 8 children born to mothers who have been diagnosed with Myasthenia gravis (MG). MG can be transferred from the mother to the fetus by the movement of AChR antibodies through the [[placenta]]. Signs of this disease at birth include weakness, which responds to anticholinesterase medications, as well as fetal akinesia, or the lack of fetal movement. This form of the disease is transient, lasting for about three months. However, in some cases, neonatal MG can lead to other health effects, such as arthrogryposis and even fetal death. These conditions are thought to be initiated when maternal AChR antibodies are directed to the [[fetal]] AChR and can last until the 33rd week of [[gestation]], when the γ subunit of AChR is replaced by the ε subunit.<ref name="Newsom">Newsom-Davis |
Neonatal MG is an autoimmune disorder that affects 1 in 8 children born to mothers who have been diagnosed with Myasthenia gravis (MG). MG can be transferred from the mother to the fetus by the movement of AChR antibodies through the [[placenta]]. Signs of this disease at birth include weakness, which responds to anticholinesterase medications, as well as fetal akinesia, or the lack of fetal movement. This form of the disease is transient, lasting for about three months. However, in some cases, neonatal MG can lead to other health effects, such as arthrogryposis and even fetal death. These conditions are thought to be initiated when maternal AChR antibodies are directed to the [[fetal]] AChR and can last until the 33rd week of [[gestation]], when the γ subunit of AChR is replaced by the ε subunit.<ref name="Newsom">{{cite journal |author=Newsom-Davis J |title=The emerging diversity of neuromuscular junction disorders |journal=Acta Myol |volume=26 |issue=1 |pages=5–10 |year=2007 |month=July |pmid=17915563 |pmc=2949330 }}</ref> |
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====Lambert-Eaton myasthenic syndrome==== |
====Lambert-Eaton myasthenic syndrome==== |
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[[Lambert-Eaton myasthenic syndrome]] (LEMS) is an autoimmune disorder that affects the presynaptic portion of the neuromuscular junction. This rare disease can be marked by a unique triad of symptoms: proximal muscle weakness, [[autonomic dysfunction]], and areflexia.<ref name="Luigetti">Luigetti |
[[Lambert-Eaton myasthenic syndrome]] (LEMS) is an autoimmune disorder that affects the presynaptic portion of the neuromuscular junction. This rare disease can be marked by a unique triad of symptoms: proximal muscle weakness, [[autonomic dysfunction]], and areflexia.<ref name="Luigetti">{{cite journal |author=Luigetti M, Modoni A, Lo Monaco M |title=Low rate repetitive nerve stimulation in Lambert-Eaton myasthenic syndrome: Peculiar characteristics of decremental pattern from a single-centre experience |journal=Clin Neurophysiol |volume= |issue= |pages= |year=2012 |month=October |pmid=23036181 |doi=10.1016/j.clinph.2012.08.026 |url=http://linkinghub.elsevier.com/retrieve/pii/S1388-2457(12)00625-6}}</ref> Proximal muscle weakness is a product of [[pathogenic]] autoantibodies directed against P/Q-type voltage-gated calcium channels, which in turn leads to a reduction of acetylcholine release from motor nerve terminals on the presynaptic cell. Examples of autonomic dysfunction caused by LEMS includes [[dry mouth]], which is most common, [[constipation]], and [[erectile dysfunction]] in men. Less common dysfunctions include [[dry eyes]] and altered [[perspiration]]. [[Areflexia]] is a condition in which tendon reflexes are reduced and it may subside temporarily after a period of exercise.<ref name="Titulaer">{{cite journal |author=Titulaer MJ, Lang B, Verschuuren JJ |title=Lambert-Eaton myasthenic syndrome: from clinical characteristics to therapeutic strategies |journal=Lancet Neurol |volume=10 |issue=12 |pages=1098–107 |year=2011 |month=December |pmid=22094130 |doi=10.1016/S1474-4422(11)70245-9 |url=http://linkinghub.elsevier.com/retrieve/pii/S1474-4422(11)70245-9}}</ref> |
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50–60% of the patients that are diagnosed with LEMS also have present an associated [[tumor]], which is typically [[small-cell lung carcinoma]] (SCLC). This type of tumor also expresses [[voltage-gated calcium channels]].<ref name="Titulaer" /> Oftentimes, LEMS also occurs alongside myasthenia gravis.<ref name="Luigetti" /> |
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Treatment for LEMS consists of using 3,4-diaminopyridine as a first measure, which serves to increase the compound muscle action potential as well as muscle strength by lengthening the time that voltage-gated calcium channels remain open after blocking voltage-gated potassium channels, or by targeting the β-subunit of VGCC directly. Further treatment includes the use of [[prednisone]] and [[azathioprine]] in the event that 3,4-diaminopyridine does not aid in treatment.<ref name="Titulaer" /> |
Treatment for LEMS consists of using 3,4-diaminopyridine as a first measure, which serves to increase the compound muscle action potential as well as muscle strength by lengthening the time that voltage-gated calcium channels remain open after blocking voltage-gated potassium channels, or by targeting the β-subunit of VGCC directly. Further treatment includes the use of [[prednisone]] and [[azathioprine]] in the event that 3,4-diaminopyridine does not aid in treatment.<ref name="Titulaer" /> |
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====Congenital myasthenic syndromes==== |
====Congenital myasthenic syndromes==== |
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[[Congenital myasthenic syndrome]]s (CMS) are diseases incurred due to mutations, typically [[recessive]], in at least 10 genes that affect presynaptic, synaptic, and postsynaptic proteins in the neuromuscular junction. Such mutations usually arise in the ε-subunit of AChR<ref name="Newsom" />, thereby affecting the kinetics and expression of the receptor itself. Single nucleotide substitutions or deletions may cause loss of function in the subunit. Other [[mutation]]s, such as those affecting [[acetylcholinesterase]] and [[acetyltransferase]], can also cause the expression of CMS, with the latter being associated specifically with episodic [[apnea]].<ref> |
[[Congenital myasthenic syndrome]]s (CMS) are diseases incurred due to mutations, typically [[recessive]], in at least 10 genes that affect presynaptic, synaptic, and postsynaptic proteins in the neuromuscular junction. Such mutations usually arise in the ε-subunit of AChR<ref name="Newsom" />, thereby affecting the kinetics and expression of the receptor itself. Single nucleotide substitutions or deletions may cause loss of function in the subunit. Other [[mutation]]s, such as those affecting [[acetylcholinesterase]] and [[acetyltransferase]], can also cause the expression of CMS, with the latter being associated specifically with episodic [[apnea]].<ref>{{cite journal |author=Harper CM |title=Congenital myasthenic syndromes |journal=Semin Neurol |volume=24 |issue=1 |pages=111–23 |year=2004 |month=March |pmid=15229798 |doi=10.1055/s-2004-829592 |url=http://www.thieme-connect.com/DOI/DOI?10.1055/s-2004-829592}}</ref> These syndromes can present themselves at different times within the life of an individual. They may arise during the fetal phase, causing fetal [[akinesia]], or the perinatal period, during which certain conditions, such as [[arthrogryposis]], [[:wikt:ptosis|ptosis]], [[hypotonia]], [[ophthalmoplegia]], and feeding or breathing difficulties, may be observed. They could also activate during adolescence or adult years, causing the individual to develop[slow-channel syndrome.<ref name="Newsom" /> |
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====Neuromyotonia==== |
====Neuromyotonia==== |
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===Genetic=== |
===Genetic=== |
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====Bulbospinal muscular atrophy==== |
====Bulbospinal muscular atrophy==== |
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[[Spinal_and_bulbar_muscular_atrophy|Bulbospinal muscular atrophy]], also known as Kennedy’s disease, is a rare [[recessive]] trinucleotide, polyglutamine disorder that is linked to the [[X chromosome]]. Because of its linkage to the X chromosome, it is typically transmitted through females. However, Kennedy’s disease is only present in adult males as the onset of the disease is typically later in life. This disease is specifically caused by the expansion of a CAG-tandem repeat in exon 1 found on the androgen-receptor (AR) gene on [[chromosome]] Xq11-12. Poly-Q-expanded AR accumulates in the nuclei of cells, where it begins to fragment. After fragmentation, degradation of the cell begins, leading to a loss of both motor neurons and [[dorsal root ganglia]].<ref name="Finsterer">Finsterer |
[[Spinal_and_bulbar_muscular_atrophy|Bulbospinal muscular atrophy]], also known as Kennedy’s disease, is a rare [[recessive]] trinucleotide, polyglutamine disorder that is linked to the [[X chromosome]]. Because of its linkage to the X chromosome, it is typically transmitted through females. However, Kennedy’s disease is only present in adult males as the onset of the disease is typically later in life. This disease is specifically caused by the expansion of a CAG-tandem repeat in exon 1 found on the androgen-receptor (AR) gene on [[chromosome]] Xq11-12. Poly-Q-expanded AR accumulates in the nuclei of cells, where it begins to fragment. After fragmentation, degradation of the cell begins, leading to a loss of both motor neurons and [[dorsal root ganglia]].<ref name="Finsterer">{{cite journal |author=Finsterer J |title=Perspectives of Kennedy's disease |journal=J. Neurol. Sci. |volume=298 |issue=1-2 |pages=1–10 |year=2010 |month=November |pmid=20846673 |doi=10.1016/j.jns.2010.08.025 |url=http://linkinghub.elsevier.com/retrieve/pii/S0022-510X(10)00392-8}}</ref> |
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Symptoms of Kennedy’s disease include weakness and wasting of the facial, [[bulbar]] and extremity muscles, as well as sensory and endocrinological disturbances, such as [[gynecomastia]] and reduced [[fertility]]. Other symptoms include elevated [[testosterone]] and other sexual hormone levels, development of hyper-CK-emia, abnormal conduction through motor and sensory nerves, and [[neuropathic]] or in rare cases [[myopathic]] alterations on biopsies of muscle cells.<ref name="Finsterer" /> |
Symptoms of Kennedy’s disease include weakness and wasting of the facial, [[bulbar]] and extremity muscles, as well as sensory and endocrinological disturbances, such as [[gynecomastia]] and reduced [[fertility]]. Other symptoms include elevated [[testosterone]] and other sexual hormone levels, development of hyper-CK-emia, abnormal conduction through motor and sensory nerves, and [[neuropathic]] or in rare cases [[myopathic]] alterations on biopsies of muscle cells.<ref name="Finsterer" /> |
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====Duchenne muscular dystrophy==== |
====Duchenne muscular dystrophy==== |
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[[Duchenne muscular dystrophy]] is an X-linked genetic disorder that results in the absence of the structural protein [[dystrophin]] at the neuromuscular junction. It affects 1 in 3, |
[[Duchenne muscular dystrophy]] is an X-linked genetic disorder that results in the absence of the structural protein [[dystrophin]] at the neuromuscular junction. It affects 1 in 3,600–6,000 males and frequently causes death by the age of 30. The absence of dystrophin causes muscle [[degeneration]], and patients present with the following symptoms: abnormal [[gait]], [[hypertrophy]] in the calf muscles, and elevated [[creatine kinase]]. If left untreated, patients may suffer from [[respiratory distress]], which can lead to death.<ref name="Beytia">{{cite journal |author=Beytía Mde L, Vry J, Kirschner J |title=Drug treatment of Duchenne muscular dystrophy: available evidence and perspectives |journal=Acta Myol |volume=31 |issue=1 |pages=4–8 |year=2012 |month=May |pmid=22655510 |pmc=3440798 }}</ref> |
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==Toxins== |
==Toxins== |
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===Botox=== |
===Botox=== |
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Botulinum toxin, also known as [[Botox]], acts as an inhibitor of the release of acetylcholine (ACh) at the neuromuscular junction by interfering with SNARE proteins.<ref name="Text" /> By doing so, it induces a transient paralysis localized to the striated muscle that it has affected. The inhibition of the ACh release does not set in until roughly two weeks after the injection is made. Three months after the inhibition occurs, neuronal activity begins to regain partial function, and six months, complete neuronal function is regained.<ref>Papapetropoulos |
Botulinum toxin, also known as [[Botox]], acts as an inhibitor of the release of acetylcholine (ACh) at the neuromuscular junction by interfering with SNARE proteins.<ref name="Text" /> By doing so, it induces a transient paralysis localized to the striated muscle that it has affected. The inhibition of the ACh release does not set in until roughly two weeks after the injection is made. Three months after the inhibition occurs, neuronal activity begins to regain partial function, and six months, complete neuronal function is regained.<ref>{{cite journal |author=Papapetropoulos S, Singer C |title=Botulinum toxin in movement disorders |journal=Semin Neurol |volume=27 |issue=2 |pages=183–94 |year=2007 |month=April |pmid=17390263 |doi=10.1055/s-2007-971171 |url=http://www.thieme-connect.com/DOI/DOI?10.1055/s-2007-971171}}</ref> |
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===Snake Venom=== |
===Snake Venom=== |
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Presynaptic neurotoxins, commonly known as β-neurotoxins, affect the presynaptic regions of the neuromuscular junction. The majority of these neurotoxins act by inhibiting the release of neurotransmitters, such as acetylcholine, into the synapse between neurons. However, some of these toxins have also been known to enhance neurotransmitter release. Those that inhibit neurotransmitter release create a [[neuromuscular blockade]] that prevents signaling molecules from reaching their postsynaptic target receptors. In doing so, the victim of the snake bite suffers from profound weakness. Such neurotoxins do not respond well to anti-venoms. After one hour of inoculation of these toxins, including notexin and [[taipoxin]], many of the affected nerve terminals show signs of irreversible physical damage, leaving them devoid of any [[synaptic vesicle]]s. |
Presynaptic neurotoxins, commonly known as β-neurotoxins, affect the presynaptic regions of the neuromuscular junction. The majority of these neurotoxins act by inhibiting the release of neurotransmitters, such as acetylcholine, into the synapse between neurons. However, some of these toxins have also been known to enhance neurotransmitter release. Those that inhibit neurotransmitter release create a [[neuromuscular blockade]] that prevents signaling molecules from reaching their postsynaptic target receptors. In doing so, the victim of the snake bite suffers from profound weakness. Such neurotoxins do not respond well to anti-venoms. After one hour of inoculation of these toxins, including notexin and [[taipoxin]], many of the affected nerve terminals show signs of irreversible physical damage, leaving them devoid of any [[synaptic vesicle]]s. |
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Postsynaptic neurotoxins, otherwise known as α-neurotoxins, act oppositely to the presynaptic neurotoxins in that they bind to the postsynaptic acetylcholine receptors. This prevents interaction between the acetylcholine released by the presynaptic terminal and the receptors on the postsynaptic cell. In effect, the opening of sodium channels associated with these acetylcholine receptors is prohibited, resulting in a neuromuscular blockade, similar to the effects seen due to presynaptic neurotoxins. This causes paralysis in the muscles involved in the affected junctions. Unlike presynaptic neurotoxins, postsynaptic toxins are more easily affected by anti-venoms, which accelerate the dissociation of the toxin from the receptors, ultimately causing a reversal of paralysis. These neurotoxins experimentally and qualitatively aid in the study of acetylcholine receptor [[density]] and [[turnover]], as well as in studies observing the direction of [[antibodies]] toward the affected acetylcholine receptors in patients diagnosed with [[myasthenia gravis]].<ref> |
Postsynaptic neurotoxins, otherwise known as α-neurotoxins, act oppositely to the presynaptic neurotoxins in that they bind to the postsynaptic acetylcholine receptors. This prevents interaction between the acetylcholine released by the presynaptic terminal and the receptors on the postsynaptic cell. In effect, the opening of sodium channels associated with these acetylcholine receptors is prohibited, resulting in a neuromuscular blockade, similar to the effects seen due to presynaptic neurotoxins. This causes paralysis in the muscles involved in the affected junctions. Unlike presynaptic neurotoxins, postsynaptic toxins are more easily affected by anti-venoms, which accelerate the dissociation of the toxin from the receptors, ultimately causing a reversal of paralysis. These neurotoxins experimentally and qualitatively aid in the study of acetylcholine receptor [[density]] and [[turnover]], as well as in studies observing the direction of [[antibodies]] toward the affected acetylcholine receptors in patients diagnosed with [[myasthenia gravis]].<ref>{{cite journal |author=Lewis RL, Gutmann L |title=Snake venoms and the neuromuscular junction |journal=Semin Neurol |volume=24 |issue=2 |pages=175–9 |year=2004 |month=June |pmid=15257514 |doi=10.1055/s-2004-830904 |url=http://www.thieme-connect.com/DOI/DOI?10.1055/s-2004-830904}}</ref> |
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==See also== |
==See also== |
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Revision as of 10:48, 14 November 2012
| Neuromuscular junction | |
|---|---|
.jpg) Electron micrograph showing a cross section through the neuromuscular junction. T is the axon terminal, M is the muscle fiber. The arrow shows junctional folds with basal lamina. Postsynaptic densities are visible on the tips between the folds. Scale is 0.3 µm. Source: NIMH | |
 Detailed view of a neuromuscular junction: 1. Presynaptic terminal 2. Sarcolemma 3. Synaptic vesicle 4. Nicotinic acetylcholine receptor 5. Mitochondrion | |
| Details | |
| Identifiers | |
| Latin | synapsis neuromuscularis; junctio neuromuscularis |
| MeSH | D009469 |
| TH | H2.00.06.1.02001 |
| FMA | 61803 |
| Anatomical terminology | |
The neuromuscular junction is synapse or junction located at the motor end plate where motor neurons synapse on muscle fibers, also known as a muscle cells. As an action potential reaches the end of a motor neuron, voltage gated calcium channels open allowing calcium to enter the neuron. Calcium facilitates vesicle binding and neurotransmitter release from the motor neuron into the synaptic cleft. In vertebrates, motor neurons releases acetylcholine (ACh), a small molecule neurotransmitter, which diffuses through the synapse and binds its receptor on the microfiber, opening ligand gated sodium channels, depolarizing the muscle fiber and causing a muscle contraction.
Structure
Neuromuscular junction
The neuromuscular junction differs from synapses in the central nervous system. Presynaptic motor axons are demyelinated and stop 30 nanometers from the sarcolemma, the cell membrane of a muscle cell. This 30-nanometer space forms the synaptic cleft through which signaling molecules are released. The sarcolemma has invaginations called postjunctional folds, which increase the surface area of the membrane exposed to the synaptic cleft.[1] These postjunctional folds form what is referred to as the motor end-plate, which possesses acetylcholine receptors (AChRs) at a density of 10,000 receptors/micrometer2 in skeletal muscle.[2] The presynaptic axons form bulges called terminal boutons that project into the postjunctional folds of the sarcolemma. The presynaptic boutons have active zones that contain vesicles, quanta, full of acetylcholine molecules. These vesicles can fuse with the presynaptic membrane and release ACh molecules into the synaptic cleft via exocytosis after depolarization. [1] AChRs are localized by protein scaffolds at the postjunctional folds of the sarcolemma opposite the presynaptic terminals. Dystrophin, a structural protein, connects the sarcomere, sarcolemma, and extracellular matrix components. Rapsyn is another protein that docks AChRs and structural proteins to the cytoskeleton. The receptor tyrosine kinase protein MuSK is a signaling protein involved in the development of the neuromuscular junction and is also held in place by rapsyn. [1]
Acetylcholine receptor
Acetylcholine is a neurotransmitter synthesized in the human body from dietary choline and acetyl-CoA (ACoA), and it is involved in the stimulation of vertebrate muscle tissue. It is important to note that not all species use a cholinergic neuromuscular junction; in fact, crayfish have a glutamatergic neuromuscular junction.[1] The eukaryotic nicotinic acetylcholine receptor (AChR) is a heteropentamer, ligand-gated ion channel. Each subunit has a characteristic “cys-loop,” which is composed of a cysteine residue followed by 13 more amino acid residues and another cysteine residue. The two cysteine residues form a disulfide linkage which results in the “cys-loop” receptor that is capable of binding acetylcholine and other ligands. These cys-loop receptors are found only in eukaryotes, but prokaryotes possess ACh receptors with similar properties.[2]
AChRs at the skeletal neuromuscular junction have two α, one β, one γ, and one δ subunits. When a single ACh ligand binds to one of the α subunits of the ACh receptor it induces a conformational change at the interface with the second AChR α subunit. This conformational change results in the increased affinity for a second ACh ligand in the second α subunit. AChRs therefore exhibit a sigmoidal dissociation curve due to this cooperative binding .[2] The presence of the inactive, intermediate receptor structure with a single-bound ligand keeps ACh in the synapse that might otherwise be lost by cholinesterase hydrolysis or diffusion. The persistence of these ACh ligands in the synapse can cause a prolonged post-synaptic response.[3]
The pre and postsynaptic cells at the neuromuscular junction also contain muscarinic acetylcholine receptors that are metabotropic receptors coupled with G-proteins. Muscarinic receptors can be found on the presynaptic cell, which act via negative feedback by binding acetylcholine molecules released by the presynaptic cell, thereby inhibiting it from releasing more neurotransmitters through a signaling cascade. G-protein subunits can inactivate the calcium channels necessary for neurotransmitter release into the synaptic cleft. Muscarinic AChRs on the sarcolemma of the postsynaptic cell bind ACh, which activates a G-protein that opens Kir3.1 potassium channels, thereby hyperpolarizing the muscle fiber.[1]
Research methods
del Castillo and Katz used ionophoresis to determine the location and density of nicotinic acetylcholine receptors (AChRs) at the neuromuscular junction. With this technique, a microelectrode is placed inside the motor end-plate of the muscle fiber, and a micropipette filled with acetylcholine (ACh) is placed directly in front of the endplate in the synaptic cleft. A positive voltage is applied to the tip of the micropipette, which causes a burst of positively charged ACh molecules to be released from the pipette. These ligands flow into the space representing the synaptic cleft and bind to AChRs. The intracellular microelectrode monitors the amplitude of the depolarization of the motor end plate in response to ACh binding to nicotinic (ionotropic) receptors. Katz and de Castillo showed that the amplitude of the depolarization (excitatory postsynaptic potential) depended on the proximity of the micropipette releasing the ACh ions to the endplate. The farther the micropipette was from the motor endplate, the smaller the depolarization was in the muscle fiber. This allowed the researchers to determine that the nicotinic receptors were localized to the motor end-plate in high density.[1][2]
Toxins are also used to determine the location of acetylcholine receptors at the neuromuscular junction. α-bungarotoxin is a toxin found in the snake species Bungarus multicinctus that acts as an ACh antagonist and binds to AChRs irreversibly. By coupling assayable enzymes such as horseradish peroxidase (HRP) or fluorescent proteins to the α-bungarotoxin, AChRs on the motor end plate can be visualized and quantified.[1]
Development of the neuromuscular junction
The development of the neuromuscular junction requires signaling from both the motor neuron's terminal and the muscle cell's central region. During development, muscle cells produce acetylcholine receptors (AChRs) and express them in the central regions in a process called prepatterning. Agrin, a heparin proteoglycan, and MuSK kinase are thought to help stabilize the accumulation of AChR in the central regions of the myocyte. MuSK is a receptor tyrosine kinase — meaning that it induces cellular signaling by causing the release of phosphate molecules to particular tyrosines on itself, and on proteins which bind the cytoplasmic domain of the receptor.[4]Upon activation by its ligand agrin, MuSK signals via two proteins called "Dok-7" and "rapsyn", to induce "clustering" of acetylcholine receptors.[5] ACh release by developing motor neurons produces postsynaptic potentials in the muscle cell that positively reinforces the localization and the stabilization of the developing neuromuscular junction. [6]
Knockout studies
These findings were demonstrated in part by mouse "knockout" studies. In mice which are deficient for either agrin or MuSK, the neuromuscular junction does not form. Further, mice deficient in Dok-7 did not form either acetylcholine receptor clusters or neuromuscular synapses.[7]
Many other proteins also comprise the NMJ, and are required to maintain its integrity.[8]
Mechanism of action
The neuromuscular junction is the location where the neuron activates muscle to contract. This is a step in the excitation-contraction coupling of vertebrate skeletal muscle.
- Upon the arrival of an action potential at the presynaptic neuron terminal, voltage-dependent calcium channels open and Ca2+ ions flow from the extracellular fluid into the presynaptic neuron's cytosol.
- This influx of Ca2+ causes neurotransmitter-containing vesicles to dock and fuse to the presynaptic neuron's cell membrane through SNARE proteins.
- Fusion of the vesicular membrane with the presynaptic cell membrane results in the emptying of the vesicle's contents (acetylcholine) into the synaptic cleft, a process known as exocytosis.
- Acetylcholine diffuses into the synaptic cleft and binds to the nicotinic acetylcholine receptors bound to the motor end plate.
- These receptors are ligand-gated ion channels, and when they bind acetylcholine, they open, allowing sodium ions to flow in and potassium ions to flow out of the muscle's cytosol.
- Because of the differences in electrochemical gradients across the plasma membrane, more sodium moves in than potassium out, producing a local depolarization of the motor end plate known as an end-plate potential (EPP).
- This depolarization spreads across the surface of the muscle fiber and continues the excitation-contraction coupling to contract the muscle.
- The action of acetylcholine is terminated when the enzyme acetylcholinesterase degrades part of the neurotransmitter (producing choline and an acetate group) and the rest of it diffuses away.
- The choline produced by the action of acetylcholinesterase is recycled — it is transported, through reuptake, back into the presynaptic terminal, where it is used to synthesize new acetylcholine molecules.[9]
Diseases
Any disorder that compromises the synaptic transmission between a motor neuron and a muscle cell is categorized under the umbrella term neuromuscular diseases. These disorders can be inherited or acquired. These disorders vary in their severity and their mortality. In general, most tend to be caused by mutations or autoimmune disorders. Autoimmune disorders, in the case of neuromuscular diseases, tend to be humoral mediated, B cell mediated, and result in an antibody against a motor neuron or muscle fiber protein that interferes with synaptic transmission or signaling.
Autoimmune
Myasthenia gravis
Myasthenia gravis is an autoimmune disorder where the body makes antibodies against either the acetylcholine receptor (AchR) (in 80% of cases), or against postsynaptic muscle-specific kinase (MuSK) (0–10% of cases). In seronegative myasthenia gravis low density lipoprotein receptor-related protein 4 is targeted by IgG1, which acts as a competitive inhibitor of its ligand, preventing the ligand from binding its receptor. It is not known if seronegative myasthenia gravis will respond to standard therapies.[10]
Neonatal MG
Neonatal MG is an autoimmune disorder that affects 1 in 8 children born to mothers who have been diagnosed with Myasthenia gravis (MG). MG can be transferred from the mother to the fetus by the movement of AChR antibodies through the placenta. Signs of this disease at birth include weakness, which responds to anticholinesterase medications, as well as fetal akinesia, or the lack of fetal movement. This form of the disease is transient, lasting for about three months. However, in some cases, neonatal MG can lead to other health effects, such as arthrogryposis and even fetal death. These conditions are thought to be initiated when maternal AChR antibodies are directed to the fetal AChR and can last until the 33rd week of gestation, when the γ subunit of AChR is replaced by the ε subunit.[11]
Lambert-Eaton myasthenic syndrome
Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder that affects the presynaptic portion of the neuromuscular junction. This rare disease can be marked by a unique triad of symptoms: proximal muscle weakness, autonomic dysfunction, and areflexia.[12] Proximal muscle weakness is a product of pathogenic autoantibodies directed against P/Q-type voltage-gated calcium channels, which in turn leads to a reduction of acetylcholine release from motor nerve terminals on the presynaptic cell. Examples of autonomic dysfunction caused by LEMS includes dry mouth, which is most common, constipation, and erectile dysfunction in men. Less common dysfunctions include dry eyes and altered perspiration. Areflexia is a condition in which tendon reflexes are reduced and it may subside temporarily after a period of exercise.[13]
50–60% of the patients that are diagnosed with LEMS also have present an associated tumor, which is typically small-cell lung carcinoma (SCLC). This type of tumor also expresses voltage-gated calcium channels.[13] Oftentimes, LEMS also occurs alongside myasthenia gravis.[12]
Treatment for LEMS consists of using 3,4-diaminopyridine as a first measure, which serves to increase the compound muscle action potential as well as muscle strength by lengthening the time that voltage-gated calcium channels remain open after blocking voltage-gated potassium channels, or by targeting the β-subunit of VGCC directly. Further treatment includes the use of prednisone and azathioprine in the event that 3,4-diaminopyridine does not aid in treatment.[13]
Congenital myasthenic syndromes
Congenital myasthenic syndromes (CMS) are diseases incurred due to mutations, typically recessive, in at least 10 genes that affect presynaptic, synaptic, and postsynaptic proteins in the neuromuscular junction. Such mutations usually arise in the ε-subunit of AChR[11], thereby affecting the kinetics and expression of the receptor itself. Single nucleotide substitutions or deletions may cause loss of function in the subunit. Other mutations, such as those affecting acetylcholinesterase and acetyltransferase, can also cause the expression of CMS, with the latter being associated specifically with episodic apnea.[14] These syndromes can present themselves at different times within the life of an individual. They may arise during the fetal phase, causing fetal akinesia, or the perinatal period, during which certain conditions, such as arthrogryposis, ptosis, hypotonia, ophthalmoplegia, and feeding or breathing difficulties, may be observed. They could also activate during adolescence or adult years, causing the individual to develop[slow-channel syndrome.[11]
Neuromyotonia
Neuromyotonia (NMT), otherwise known as Isaac’s syndrome, is unlike many other diseases present at the neuromuscular junction. Rather than causing muscle weakness, NMT leads to the hyperexcitation of motor nerves. NMT causes this hyperexcitation by producing longer depolarizations through down-regulation of voltage-gated potassium channels, which in effect would cause greater neurotransmitter release and therefore repetitive firing. This increase in rate of firing leads to more active transmission and as a result, a greater muscular activity in the affected individual. NMT is also believed to be of autoimmune origin due to its associations with autoimmune symptoms in the individual affected.[11]
Genetic
Bulbospinal muscular atrophy
Bulbospinal muscular atrophy, also known as Kennedy’s disease, is a rare recessive trinucleotide, polyglutamine disorder that is linked to the X chromosome. Because of its linkage to the X chromosome, it is typically transmitted through females. However, Kennedy’s disease is only present in adult males as the onset of the disease is typically later in life. This disease is specifically caused by the expansion of a CAG-tandem repeat in exon 1 found on the androgen-receptor (AR) gene on chromosome Xq11-12. Poly-Q-expanded AR accumulates in the nuclei of cells, where it begins to fragment. After fragmentation, degradation of the cell begins, leading to a loss of both motor neurons and dorsal root ganglia.[15]
Symptoms of Kennedy’s disease include weakness and wasting of the facial, bulbar and extremity muscles, as well as sensory and endocrinological disturbances, such as gynecomastia and reduced fertility. Other symptoms include elevated testosterone and other sexual hormone levels, development of hyper-CK-emia, abnormal conduction through motor and sensory nerves, and neuropathic or in rare cases myopathic alterations on biopsies of muscle cells.[15]
Duchenne muscular dystrophy
Duchenne muscular dystrophy is an X-linked genetic disorder that results in the absence of the structural protein dystrophin at the neuromuscular junction. It affects 1 in 3,600–6,000 males and frequently causes death by the age of 30. The absence of dystrophin causes muscle degeneration, and patients present with the following symptoms: abnormal gait, hypertrophy in the calf muscles, and elevated creatine kinase. If left untreated, patients may suffer from respiratory distress, which can lead to death.[16]
Toxins
Botox
Botulinum toxin, also known as Botox, acts as an inhibitor of the release of acetylcholine (ACh) at the neuromuscular junction by interfering with SNARE proteins.[1] By doing so, it induces a transient paralysis localized to the striated muscle that it has affected. The inhibition of the ACh release does not set in until roughly two weeks after the injection is made. Three months after the inhibition occurs, neuronal activity begins to regain partial function, and six months, complete neuronal function is regained.[17]
Snake Venom
Snake venoms can act as toxins within the neuromuscular junction by inducing the weakness and [[paralysis] that occurs after a venomous snake bite. There are two types of neurotoxins that are present within snake venoms: presynaptic and postsynaptic neurotoxins.
Presynaptic neurotoxins, commonly known as β-neurotoxins, affect the presynaptic regions of the neuromuscular junction. The majority of these neurotoxins act by inhibiting the release of neurotransmitters, such as acetylcholine, into the synapse between neurons. However, some of these toxins have also been known to enhance neurotransmitter release. Those that inhibit neurotransmitter release create a neuromuscular blockade that prevents signaling molecules from reaching their postsynaptic target receptors. In doing so, the victim of the snake bite suffers from profound weakness. Such neurotoxins do not respond well to anti-venoms. After one hour of inoculation of these toxins, including notexin and taipoxin, many of the affected nerve terminals show signs of irreversible physical damage, leaving them devoid of any synaptic vesicles.
Postsynaptic neurotoxins, otherwise known as α-neurotoxins, act oppositely to the presynaptic neurotoxins in that they bind to the postsynaptic acetylcholine receptors. This prevents interaction between the acetylcholine released by the presynaptic terminal and the receptors on the postsynaptic cell. In effect, the opening of sodium channels associated with these acetylcholine receptors is prohibited, resulting in a neuromuscular blockade, similar to the effects seen due to presynaptic neurotoxins. This causes paralysis in the muscles involved in the affected junctions. Unlike presynaptic neurotoxins, postsynaptic toxins are more easily affected by anti-venoms, which accelerate the dissociation of the toxin from the receptors, ultimately causing a reversal of paralysis. These neurotoxins experimentally and qualitatively aid in the study of acetylcholine receptor density and turnover, as well as in studies observing the direction of antibodies toward the affected acetylcholine receptors in patients diagnosed with myasthenia gravis.[18]
See also
External links
- Histology image: 21501lca – Histology Learning System at Boston University
Further reading
- Kandel, ER (2000). Principles of Neural Science (4th ed.). New York: McGraw-Hill. ISBN 0-8385-7701-6.
{{cite book}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - Nicholls, J.G. (2001). From Neuron to Brain (4th ed.). Sunderland, MA.: Sinauer Associates. ISBN 0-87893-439-1.
{{cite book}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - Engel, A.G. (2004). Myology (3rd ed.). New York: McGraw Hill Professional. ISBN 0-07-137180-X.
References
- ^ a b c d e f g h Nicholls, John G., (2012). From Neuron to Brain (5th ed.). Sunderland: Sinauer Associates.
{{cite book}}: Unknown parameter|coathors=ignored (help)CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link) - ^ a b c d Sine SM (2012). "End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease". Physiol. Rev. 92 (3): 1189–234. doi:10.1152/physrev.00015.2011. PMC 3489064. PMID 22811427.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ Scuka M, Mozrzymas JW (1992). "Postsynaptic potentiation and desensitization at the vertebrate end-plate receptors". Prog. Neurobiol. 38 (1): 19–33. PMID 1736323.
- ^ Valenzuela D, Stitt T, DiStefano P, Rojas E, Mattsson K, Compton D, Nuñez L, Park J, Stark J, Gies D (1995). "Receptor tyrosine sinase specific for the skeletal muscle lineage: expression in embryonic muscle, at the neuromuscular junction, and after injury". Neuron. 15 (3): 573–84. doi:10.1016/0896-6273(95)90146-9. PMID 7546737.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Glass D, Bowen D, Stitt T, Radziejewski C, Bruno J, Ryan T, Gies D, Shah S, Mattsson K, Burden S, DiStefano P, Valenzuela D, DeChiara T, Yancopoulos G (1996). "Agrin acts via a MuSK receptor complex". Cell. 85 (4): 513–23. doi:10.1016/S0092-8674(00)81252-0. PMID 8653787.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Witzemann V (2006). "Development of the neuromuscular junction". Cell Tissue Res. 326 (2): 263–71. doi:10.1007/s00441-006-0237-x. PMID 16819627.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ Okada K, Inoue A, Okada M, Murata Y, Kakuta S, Jigami T, Kubo S, Shiraishi H, Eguchi K, Motomura M, Akiyama T, Iwakura Y, Higuchi O, Yamanashi Y (2006). "The muscle protein Dok-7 is essential for neuromuscular synaptogenesis". Science. 312 (5781): 1802–5. doi:10.1126/science.1127142. PMID 16794080.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Strochlic L, Cartaud A, Cartaud J (2005). "The synaptic muscle-specific kinase (MuSK) complex: new partners, new functions". Bioessays. 27 (11): 1129–35. doi:10.1002/bies.20305. PMID 16237673.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Dale Purves, George J. Augustine, David Fitzpatrick, William C. Hall, Anthony-Samuel LaMantia, James O. McNamara, and Leonard E. White (2008). Neuroscience (4th ed.). Sinauer Associates. pp. 121–2. ISBN 978-0-87893-697-7.
{{cite book}}: CS1 maint: multiple names: authors list (link) - ^ Finsterer J, Papić L, Auer-Grumbach M (2011). "Motor neuron, nerve, and neuromuscular junction disease". Curr. Opin. Neurol. 24 (5): 469–74. doi:10.1097/WCO.0b013e32834a9448. PMID 21825986.
{{cite journal}}: Unknown parameter|month=ignored (help)CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link) - ^ a b c d Newsom-Davis J (2007). "The emerging diversity of neuromuscular junction disorders". Acta Myol. 26 (1): 5–10. PMC 2949330. PMID 17915563.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ a b Luigetti M, Modoni A, Lo Monaco M (2012). "Low rate repetitive nerve stimulation in Lambert-Eaton myasthenic syndrome: Peculiar characteristics of decremental pattern from a single-centre experience". Clin Neurophysiol. doi:10.1016/j.clinph.2012.08.026. PMID 23036181.
{{cite journal}}: Unknown parameter|month=ignored (help)CS1 maint: multiple names: authors list (link) - ^ a b c Titulaer MJ, Lang B, Verschuuren JJ (2011). "Lambert-Eaton myasthenic syndrome: from clinical characteristics to therapeutic strategies". Lancet Neurol. 10 (12): 1098–107. doi:10.1016/S1474-4422(11)70245-9. PMID 22094130.
{{cite journal}}: Unknown parameter|month=ignored (help)CS1 maint: multiple names: authors list (link) - ^ Harper CM (2004). "Congenital myasthenic syndromes". Semin Neurol. 24 (1): 111–23. doi:10.1055/s-2004-829592. PMID 15229798.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ a b Finsterer J (2010). "Perspectives of Kennedy's disease". J. Neurol. Sci. 298 (1–2): 1–10. doi:10.1016/j.jns.2010.08.025. PMID 20846673.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ Beytía Mde L, Vry J, Kirschner J (2012). "Drug treatment of Duchenne muscular dystrophy: available evidence and perspectives". Acta Myol. 31 (1): 4–8. PMC 3440798. PMID 22655510.
{{cite journal}}: Unknown parameter|month=ignored (help)CS1 maint: multiple names: authors list (link) - ^ Papapetropoulos S, Singer C (2007). "Botulinum toxin in movement disorders". Semin Neurol. 27 (2): 183–94. doi:10.1055/s-2007-971171. PMID 17390263.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ Lewis RL, Gutmann L (2004). "Snake venoms and the neuromuscular junction". Semin Neurol. 24 (2): 175–9. doi:10.1055/s-2004-830904. PMID 15257514.
{{cite journal}}: Unknown parameter|month=ignored (help)