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Revision as of 21:11, 26 April 2011

NMJunction.jpg

Neuromuscular Junctions

Introduction

A Neuromuscular junction (NMJ) is a connection between an axon of a motor neuron and the motor end plates on a muscle fibre. The NMJ can be found in both smooth and skeletal muscle fibres, with different purpose and mechanisms which may involve different receptors. Starting with historic researches, this group project establishes detailed information on how this junction works, which may lead to a skeletal muscle contraction, followed by a description for each of its engines involved during this process. Additionally, the NMJ is the site of some well-known diseases which will be briefly described in a table of NMJ’s disorders. Currently, the NMJ is a research topic in many areas such as pharmacology, pathology, physiology and medicine.

History

Thomas Willis

Historic researchers in NMJ:

In 1672, Myasthenia gravis (MG) was first described by Thomas Willis. It is an acquired autoimmune disease with antibodies against the nicotinic acetylcholine receptor (AChR) at the NMJ. [1]

In 1842, Claude Bernard concluded that the arrow poison curare acts at the NMJ to interrupt the stimulation of muscle by nerve impulses. [2]

In 1850, Claude Bernard demonstrated that the observed paralysis was mediated via the NMJ, although the precise mechanism was at that time unknown. [3]

In 1914, Sir Henry Dale, (in 1936, he was warded the Nobel Prize for Medicine and Physiology), investigating the pharmacological properties of ACh, distinguished two actions that were reproduced by the alkaloids, muscarine and nicotine. As the effects of muscarine mimicked the parasympathetic nervous system, he termed the receptors muscarinic, whereas those in autonomic ganglia and at the skeletal NMJ were termed nicotinic. [4]

In 1952 - 1954, the name end-plate potential is used since Fatt&Katz (1952) and Del Castillo & Katz (1954) studied the endplate potential (epp) at NMJ. They concluded that the ACh molecules were released in packets containing several thousand ACh molecules revealing the quantal nature of synaptic transmission.[5] [6]

Mechanism of action

The mechanism of action of NMJ involves many steps and engines which harmonically follow each other in which the ability of a muscle to contract or relax depend on. These steps are:

  • Nerve impulse reaches the motor nerve terminal
  • Specialized proteins forming ion channels in its cell membrane open quickly
  • Calcium enters into the axon terminal
  • Synaptic vesicles are filled with ACh which is attached to special site of it
  • Ca causes some of the vesicle membranes to fuse with the nerve terminal membrane
  • ACh content is released into synaptic cleft
  • ACh diffuses rapidly across the gap and binds to the ACh receptors (AChRs)
  • When this binding occur, small positively charged sodium (Na) ions enter the muscle
  • This lead to the depolarization across membrane
  • End-plate potential in turn opens the voltage-sensitive Na channels at the synaptic fold
  • An “all or nothing” action potential starts
  • Propagated along the muscle fibre in each direction
  • Initiation of a muscle contraction occur
  • Then, the AChR pore closes
  • ACh unbinds and broken down

To understand where this happens and the steps involved, please watch this video

Motor neuron

Motor Neuron
  • Efferent neurons
  • Originate in the ventral horn of the spinal cord
  • Synapse with muscle fibres
  • Carry information from the central nervous system to muscles
  • Facilitate muscle contraction
  • Somatic once are directly involved in the contraction of skeletal muscles
  • Excitatory
  • Influenced by input descending from the brain
  • Affected by a class of diseases known as motor neuron diseases
  • Amyotrophic lateral sclerosis is characterized by loss of lower neurons in spinal cord and brain stem and upper motor neurons that project in corticospinal tracts. Skeletal muscles innervated by the degenerated lower motor neurons show neurogenic atrophy.

Acetylcholine (ACh)

  • Cholinergic neurons
  • Skeletal muscle movement
  • Regulation of smooth muscle
  • One of the principle neurotransmitters of the peripheral nervous system
  • Released by a motor neuron at the NMJ
  • Bind and activate a receptor protein
  • A very effective deliverer of sodium ions, which stimulate muscle contractions and excites nerves
Acetylcholine Nicotine.jpg
  • The enzyme acetycholinesterase (AChE) hydrolyzes acetylcholine into acetic acid and choline
  • Choline travels back to be recycled into acetylcholine and start the process over again
  • Concentration of ACh remains higher if the AChE is inhibited
  • AChE inhibitors delay the degradation of acetylcholine
  • This inhibitors are used to reverse muscle relaxants and sometimes to treat Alzheimer's disease

Acetylcholine Receptors (AChR's)

Nicotinic and Muscarinic receptors are receptors for acetylcholine. The important difference between the two is their mode of action

  • Nicotinic receptors (nAChRs)

-Controls skeletal muscle contraction [7]

-Ionotrophic receptors

-located at synapses between two neurons and at synapses between neurons and skeletal muscle cells

  • Muscarinic receptors (mAChRs)

-Controls smooth muscle contraction[8]

-Metabotropic receptors (G-protein coupled receptors)

-located at the synapses of nerves with smooth or cardiac muscle

-Trigger a chain of chemical events referred to as signal transduction

-Excitation and inhibition response

-Response of mAChRs is slower

For more information on nAChRs and mAChRs see: Nicotinic and Muscarinic Acetylcholine Receptors

Motor End Plate

  • Specialised region of the sarcolemma
  • Highly folded
  • Holds a high concentration of AChRs
  • Also called Myoneural Junction [9]
  • Receive neurotransmitters in order to propagate an Action Potential
  • Responsible for the terminal tree like branching of a motor axon on a muscle fibre
  • Maintains muscle tone through stretch reflex

Important Structural Components

Important structural components of the NMJ
Component Location Description Picture
Presynaptic membrane The presynaptic membrane is the membrane of the neuron that faces the membrane of the muscle. In between these two membranes is the synaptic cleft In a NMJ, the presynaptic membrane is the axon terminal Presynaptic membrane.jpg
Synaptic cleft The space between the presynaptic membrane (axon terminal) and the postsynaptic membrane (motor end plate) Separates the presynaptic from the post synaptic cells, the electrical signal cannot cross this space from the neuron, but activates a chain of reactions that causes the muscle to contract. Synaptic cleft.jpg
Synaptic vesicles Found within the axon terminals of the neuron The synaptic vesicles are membranous organelles that contain the neurotransmitter Acetylcholine (Ach). They are released from the presynaptic membrane through exocytosis. Ach is released when it fuses with the postsynaptic membrane.
Synaptic vesicles.jpg
Ion gated channels and receptors Ion channels.jpg
Postsynaptic membrane Separated from presynaptic membrane by synaptic cleft In the NMJ, the postsynaptic membrane is the motor end plate. Myonuclei is located underneath this membrane where they locally synthesise AChRs and other postsynaptic proteins, MuSK provides a structural scaffold necessary to initiate aggregates of postsynaptic molecules, Responds via depolarization or hyperpolarization, Receives signal from the presynaptic cell, Characterised by the accumulation of nAChRs and voltage-gated sodium channels Postsynaptic membrane.jpg


Cellular organization of skeletal muscle:

The NMJ innervates muscle to contract, the following points outline the components of skeletal muscle:

  • Epimysium, an external sheath of dense connective tissue surrounds the entire skeletal muscle
  • Muscle fibers are arranged in regular bundles
  • The connective tissue, perimysium surrounds each bundle
  • Each fiber is surround by a delicate connective tissue called endomysium which is mainly composed of basal lamina and reticular fibers


Muscle fibers:

  • Characterized by the presence of several nuclei
  • These nuclei are located below the sarcolema
  • Contain a semifluid cytoplasm called sarcoplasm
  • Sarcoplasm is filled with mitochondria and myofibrils
  • Filaments (thin and thick) lie parallel to the long axis of the myofibrils
  • Sarcoplasmic reticulum ( SR- a saclike membranous network) surrounds each of the myofibrils
  • SR is associated with transverse tubules ( T tubules) which are connected with the sarcolema
  • T tubules help to transmit signals from the sarcolemma to the myofibrils


Proteins of muscle filaments:

  • Actin

- long filamentous polymers

- consists of two strands of globular monumers

- diameter 5.6nm

- twisted around each other

- double helical formation

- each monomer contain a binding site for myosin

  • Myosin

- large complex

- dissociated into two identical heavy chains and two pairs of light chain

- heavy chains are thin, rod-like molecules which are twisted together

- the heads of heavy chains have ATP-binding sites, ATPase activity and the ability to bind to actin

- the light chains are associated with the head

  • Troponin (Tn)
Tropomyosin.jpg

- Tn complex bind to tropomyosin’s surface

- complex of three subunits

1. TnT attaches to tropomyosin

2. TnC binds Ca ions

3. TnI inhibits the actin-myosin interaction

  • Tropomyosin (Tm)

- long thin molecule

- contains two polypeptide chains

- Tm molecules are bound head to tail

- forming a polymer that run over the actin subunits

- binds to actin and acts as a molecular barrier

- blocks myosin-binding sites in relaxed muscle

- thus, prevents the crossbridge cycle from occurring

- myosin-binding sites are exposed when Ca ions are released

- several Tm’s isoforms

Development of the neuromuscular junction

During week 9, i.e. the beginning of the fetal period, the first neuromuscular junctions appear on the newly created myotubes (muscle fibers form from the fusion of myoblasts into multi-nucleated fibers).

The development of NMJ requires reciprocal signals from nerve and muscle. It can be divided into the following stages:

- Formation of clusters of acetylcholine (ACh) concentrated in the central regions of the myofibers. This is induced by kinase releasing activity of the muscle specific MuSK protein.

- ACh released by branching nerve endings regulates and refines the localization and stabilization of nerve-muscle contact sites.

- Synaptic contacts are stabilized by agrin released from the branching nerve. Agrin enhances the MuSK activity and ACh accumulation.

- Motor axons branch onto specific regions (endplates) of individual muscle fibers.

Common neuromuscular junction disorders

Neuromuscular junction disorders are due to impaired transmission of impulses at the neuromuscular junction. This may result from disorders that affect receptor function, pre- or postsynaptic membrane function, or acetylcholinesterase activity. The majority of diseases in this category are associated with autoimmune, toxic, or inherited conditions.


Neuromuscular junction disorders
Disorder Description Clinical Manifestation Treatment Picture
Myasthenia gravis Autoimmune neuromuscular disorder caused when antibodies block post-synaptic acetylcholine receptors in neuromuscular junctions. When the passage acetylcholine across the synapse is inhibited, muscles are unable to function normally and as a result they are quick to fatigue. There are two forms of clinical manifestations of myasthenia gravis: ocular and generalized.In about 10-40% cases, weakness is restricted to the ocular muscles. The rest experience a fluctuating degree and variable combination of weakness in ocular, bulbar, limb, and respiratory muscles. Common symptoms include breakthing difficulty, chewing or swallowing difficulty, fatigue, facial paralysis, double vision and eyelid drooping. Immunosuppressants Myasthenia.jpg
Lambert–Eaton Myasthenic Syndrome (LEMS) Autoimmune disorder caused by antibody blockage voltage-gated calcium channels (VGCCs) in the presynaptic cell, resulting in abnormal release of acetylcholine at the neuromuscular junction. Most occurrences follow cancer although LEMS may be diagnosed first. Symptoms resemble those of Myasthenia gravis, most notably muscle weakness in limbs. Azathioprene, steroids, and/or immunoglobin as immune system suppressants. Pyridostigmine, dyaminopyridine to enhance acetylcholine release.
X-ray of a small cell lung cancer (SCLC) that often precedes LEMS
Botulism A disease caused by the excrement of the bacteria Clostridium Botulinum. This excrement, known as Botulinum toxin, is an extremely lethal neurotoxin. It works by blocking the release of acetylcholine at neuromuscular junctions, resulting paralysis and ultimately respiratory failure. The bacteria may enter the body through wounds, or improperly canned or preserved food. Infant botulism occurs when living bacteria or its spores are eaten and grow within the baby's gastrointestinal tract. However, recently developed medical techniques use the toxin beneficially. Careful injections help to reduce various areas of excess muscle activity, and facial injections inhibit the formation of wrinkles - a minimally invasive form of plastic surgery that has been popularized by celebrities (the toxin is thereby more commonly known as Botox). Clinical manifestation of muscle weakness without fever is characteristic of botulism. It starts with facial muscles weakness, which then spreads to the arms and legs. Symptoms include diplopia or ptosis, dysphagia and dyspnoea. In addition, patient may experience dry mouth and GI related symptoms e.g. vomiting and constipation especially in infants. For infants, Botulism Immune Globulin Intravenous-Human (BIG-IV) also known as BabyBIG. Antitoxins (Trivalent, Heptavalent)
Botulinum toxin in a presynaptic cell
Congenital myasthenic syndrome (CMS) Inherited disorder where not enough acetylcholine crosses the neuromuscular synapse with onset at or shortly after birth or in early childhood. This can be pre-synaptic, where not enough acetycholine is produced or released in the first place, or post-synaptic, where acetylcholine receptors malfunction and do not stay open long enough. Symptoms are similar to those expressed through Myasthenia gravis. Treatments vary depending on the type of CMS. Cholinesterase inhibitors, quinine, and fluoxetine are some effective clinical treatments.
Black widow spider venom Causes a complete depletion of acetylcholine:there is initial spasms and then paralysis. Redbackspider.jpg
Cobratoxins Cobratoxins bind specifically to the acetylcholine receptor.
Aminoglycosides and excess Magnesium These interfere with calcium-mediated release of quanta.

Current associated research

Future research

Current research has shown that the use of embryonic stem cells to replace damaged tissue can be beneficial in various neurodegenerative diseases. These stem cells integrate and form connections with host cells. The problem with some stem cell treatments is the capability of these cells to form long axons from the spinal cord to the muscle cell and forming a junction with the muscle. Further research into the signaling mechanisms of the NMJ may find additional mechanisms by which transplanted cells may be of therapeutic benefit. [10]

External links

Glossary

  • Ionotrophic receptors in the NMJ are receptors that allow ions to pass through them when they bind to acetylcholine
  • Signal Transduction is the process by which an extracellular signaling molecule activates a membrane receptor that in turn alters intracellular molecules creating a response

References

  1. Farrugia M.E. 2002, "Myasthenia Gravis", JR Coll Physicians Edinb, vol 34, pp. 14-18.
  2. Leake, C. D. An Historical Account of Pharmacology to the Twentieth Century; Charles C. Thomas: Springfield, IL, 1975.
  3. Sneader, W. Drug Discovery: The Evolution of Modern Medicines; Wiley: New York, 1985
  4. Feldberg WS. Henry Hallett Dale, 1875-1968. Biogr Mem Fellows R Soc. 1970;16:77–174.
  5. Fatt P & Katz B (1951). An analysis of the end-plate potential recorded with an intra-cellular electrode. J Physiol 115, 320–370
  6. del Castillo J & Katz B (1954). Quantal components of the end-plate potential. J Physiol 124, 560–573
  7. Guyton AC, Hall JE: Textbook of Medical Physiology (2006) Elsevier Saunders, Philadelphia. P.87
  8. Guyton AC, Hall JE: Textbook of Medical Physiology (2006) Elsevier Saunders, Philadelphia. P.87
  9. Junqueira C.L. & Caraneiro J., 2005. Basic Histology, text & atlas, 11th edition. McGraw-Hill Companies.
  10. Stem cell-derived neurotrophic support for the neuromuscular junction in spinal muscular atrophy PMID20955113

Coordinator Comment to all Groups

I will add a general comment that will be the same to all groups under this heading.

Referencing Extension Problem

--Mark Hill 13:16, 3 May 2011 (EST) As mentioned in the lecture, I am aware of the referencing extension problem on your project pages. I have the following temporary solution, of removing the extension, so that groups can continue to add content to their project pages. I am also giving everyone a 1 week extension before the peer assessment.

This should only be done if your project page is not allowing you to save changes!

A. The Easy Way....

The following 4 steps can be done on the webpage or select all content in edit mode, copy and paste into a text editor. All steps must be completed before you attempt to save.

  1. In page edit mode, find all <pubmed> reference tags.
  2. Replace this tag with [http://www.ncbi.nlm.nih.gov/pubmed/ Note, there should be no spaces between the internet address and the pmid number.
  3. Now find all </pubmed> reference tags.
  4. Replace this second tag with ]

This will generate a numbered reference list that we can later fix up.


B. The Better Looking Result....

Whatever is between the <ref> </ref></pubmed> tags is what will appear in your reference list, so you can format the reference and link to appear in your reference list.

2011 Projects: Synaptic Junctions | Gap Junctions | Tight Junctions | Desmosomes | Adherens Junctions | Neuromuscular Junction