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Thursday, March 1, 2012

The GUIDE: "Three M's" - Part 2 Myasthenia Gravis

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Welcome to Part 2, and the second of the "Three M's" - Myasthenia Gravis.

Like Multiple Sclerosis (MS), Myasthenia Gravis (MG) is an autoimmune disease - meaning that the body's immune system attacks its own cells, thus causing the disease.  It tends to affect younger women and older men.  Unlike MS, in which the target is the insulation of neurons, the target in MG is the junction between neurons and muscles.

Copyright Alila Sao Mai, 2012
Used under license from Shutterstock.com

The diagram at the right shows a typical junction between two neurons.  The "axon" of one neurons acts as the "sending" terminal for information.  Within this "presynaptic" neuron, information is represented as the pattern, frequency and spread of electrical action potentials, much like Morse Code, FM radio signals or computer serial communications.  However, since  the electrical signals cannot be conducted directly to target neurons (without loss) the signal must be converted to a chemical one at the "synapse" where the sending and receiving neurons touch.  Each action potential entering the axon terminal results in release of a small amount of chemical neurotransmitter.  That neurotransmitter diffuses across to the "receptors" on the "post-synaptic" receiving neuron.  The receptors are proteins in the cell membrane that allow charged ions into the postsynaptic neuron.  Thus when the neurotransmitter activates the receptors, it can cause a copy of the electrochemical action potential in the receiving neuron, allowing transmission to the next neuron, and the next...

At the muscle, a similar synapse can be found in which a motor neuron releases neurotransmitter (always acetylcholine) onto receptors on the postsynaptic muscle cell.  We call this type of synapse a "neuromuscular junction."  The receptors on the muscle cells serve two functions - (1) to allow muscle action potentials to occur, and (2) to allow calcium into the muscle cells in response to positive voltages.  The muscle action potential ensures that a small amount of positive electrical voltage (about 70 milliVolts) spreads all throughout the muscle.  The positive voltage opens ion channels for calcium, allowing it into the muscle cells.  Within the cells, calcium is responsible for the actual contraction of the muscle.  [I highly recommend the video at left (along with the New Zealand accent) for a pretty decent explanation of muscle contraction.]

In Myasthenia Gravis, the autoimmune reaction results from the body making antibodies to the acetylcholine receptors on the muscle.  Symptoms of MG can be similar to MS or muscular dystrophy, except that the symptoms are confined to the muscles.  Tests of muscle conduction (the ability to transmit electrical impulses throughout the muscle) and nerve conduction will be normal, but the transfer of signals from nerve to muscle will be delayed or impaired.  Aside from simple muscle weakness, MG can result in breathing, swallowing or chewing difficulty, paralysis, fatigue, difficulty holding the head or body upright, or problems with double vision, focusing the eyes or drooping eyelid (typically the first symptom noticed).  The doctor may also run tests for presence of the acetylcholine receptor antibodies.

Like MS, the course of the disease can be slowed with immune suppressants, and specific support of the muscle function can be provided by drugs that prolong acetylcholine action at the neuromuscular junction (such as neostigmine or physostigmine which inhibit the breakdown of acetylcholine).  Caution must be exercised, however, since too much acetylcholine causes what medical students learn as "S.L.U.D.E. syndrome" (Salivation - drooling, Lacrimation - tearing of the eyes, Urination - full bladder or loss of bladder control, Defecation - loss of sphincter control or diarrhea, Emesis - nausea and vomiting), thus acetylcholine support therapies must be very carefully monitored.

MG can cause medical crisis when the muscle that control breathing are involved.  Typical therapy involves "plasmapheresis" in which the blood is filtered to remove the excess antibody to the acetylcholine receptor.  Unfortunately, plasmapheresis removes all antibodies, so the patient must avoid risk of infection.  There is some indication that the thymus, an immune-system gland in the lower neck, may be involved in MG, and surgical thymectomy or removal of the thymus can provide some relief.

Like MS, there is no cure for MG.  Once the synapses and neuromuscular junctions are damaged, they can become scarred and will not recover.  However immunosuppressant therapy, thymectomy, acetylcholine therapies and lifestyle changes can provide a normal lifespan with close control of symptoms. For those wanting more specific information, such as diagnosis and treatment, as always, I defer to the excellent work at PubMed Health:  http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001731/.

In the next segment, we will discuss the remaining of the "Three M's" - Muscular Dystrophy - which is a catch-all term for many neuromuscular diseases, then conclude this topic with a discussion of Amyotrophic Lateral Sclerosis - ALS or Lou Gehrig's Disease.

Until next time!

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