Misplaced Pages

This is an old revision of this page, as edited by SchreiberBike (talk | contribs) at 07:29, 8 November 2012 (You can help. Repaired link to the disambiguation page, Ptosis.). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Neuromuscular junction
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[edit on Wikidata]

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. 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/micrometer in skeletal muscle. 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. 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.

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. 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.

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 . 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.

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.

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.

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.

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.Upon activation by its ligand agrin, MuSK signals via two proteins called "Dok-7" and "rapsyn", to induce "clustering" of acetylcholine receptors. 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.

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.

Many other proteins also comprise the NMJ, and are required to maintain its integrity.

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.

1. Upon the arrival of an action potential at the presynaptic neuron terminal, voltage-dependent calcium channels open and Ca ions flow from the extracellular fluid into the presynaptic neuron's cytosol.
2. This influx of Ca causes neurotransmitter-containing vesicles to dock and fuse to the presynaptic neuron's cell membrane through SNARE proteins.
3. 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.
4. Acetylcholine diffuses into the synaptic cleft and binds to the nicotinic acetylcholine receptors bound to the motor end plate.
5. 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.
6. 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).
7. This depolarization spreads across the surface of the muscle fiber and continues the excitation-contraction coupling to contract the muscle.
8. 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.
9. 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.

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.

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.

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. 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.

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. Oftentimes, LEMS also occurs alongside myasthenia gravis.

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.

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, 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. 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.

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.

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.

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.

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.

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. 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.

Snake Venom

Snake venoms can act as toxins within the neuromuscular junction by inducing the weakness and 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.

See also

• Synapse
• Skeletal muscle
• Nicotinic acetylcholine receptor
• Neuroeffector junction

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

1. ^ Nicholls, John G., A. Robert Martin, Paul A. Fuchs, David A. Brown, Matthew E. Diamond, and David A. Weisblat. From Neuron to Brain. 5th ed. Sunderland: Sinauer Associates, 2012. Print.
2. ^ Sine, Steven M. "End-Plate Acetylcholine Receptor: Structure, Mechanism, Pharmacology, and Disease." Physiological Reviews 92.3 (2012): 1189-234. Print.
3. Scuka, M., and J. Mozrzymas. "Postsynaptic Potentiation and Desensitization at the Vertebrate End-plate Receptors." Progress in Neurobiology 38.1 (1992): 19-33. Print.
4. 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)
5. 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)
6. Witzemann, Veit. "Development of the Neuromuscular Junction." Cell Tissue Res. 326.2 (2006): 263–271. Web. 3 Nov. 2012.
7. 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) link
8. 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)
9. 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)
10. Finsterer, Josef. "Motor Neuron, Nerve, and Neuromuscular Junction Disease." Current opinion in neurology. 24.5 (2011): 469-474. Print.
11. ^ Newsom-Davis, J. "The emerging diversity of neuromuscular junction disorders." Acta Myologica. 26. (2007): 5-10. Print.
12. ^ Luigetti, Marco, Anna Modoni, and Mauro Lo Monaco. "Low rate repetitive nerve stimulation in Lambert-Eaton myasthenic syndrome: Peculiar characteristics of decremental pattern from a single-centre experience." Clinical Neurophysiology. (2012).
13. ^ Titulaer, Maarten J, Bethan Lang, and Jan J G M Verschuuren. "Lambert–Eaton myasthenic syndrome: from clinical characteristics to therapeutic strategies.” The Lancet Neurology. 10. (2011): 1098-1107.
14. Harper, C. Michel. "Congenital Myasthenic Syndromes." Seminars in Neurology. 24.1 (2004): 111-123.
15. ^ Finsterer, J. "Perspectives on Kennedy’s disease." Journal of the Neurological Sciences. 298. (2010): 1-10. Print.
16. Beytia, Maria De Los Angeles, Julia Vry, and Janbernd Kirschner. "Drug Treatment of Duchenne Muscular Dystrophy: Available Evidence and Perspectives." Acta Myologica 31.1 (2012): 4-8. Print.
17. Papapetropoulos, Spiridon, and Carlos Singer. "Botulinum Toxin in Movement Disorders." Seminars in Neurology. 27.2 (2007): 183-194.
18. Lewis, Robert L, and Ludwig Gutmann. "Snake Venoms and the Neuromuscular Junction." Seminars in Neurology. 24.2 (2004): 175-179.

• Somatic motor system
• Skeletal muscle
• Neurophysiology