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Myasthenia Gravis - Muscle weakness with a twist |
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Written by Lev Prasov
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Apr 15, 2006 at 04:53 AM |
Myasthenia gravis is an autoimmune disease that targets the neuromuscular junction, the area of communication between nerve and muscle cells. It leads to generalized muscle weakness, but can be effectively treated.
Have you ever been unable to open your eyes in the morning? The lids are just too heavy, and even if you get them partially open, they may still obstruct your vision. While most of us take this simple task for granted, patients with myasthenia gravis struggle with this task and countless others. Myasthenia Gravis is a disease that causes generalized weakness in voluntary skeletal muscles. The term literally means "grave muscle weakness" in Latin.
Figure I: Electron micrograph of the neuromuscular junction (NMJ). T marks the synaptic terminal of the incoming axon, while M marks the muscle cell that receives the signal from the nerve. The area between these two cells is termed the synaptic cleft. Acetylcholine receptors are found on the surface of the muscle cell. (Image courtesy of the National Institute of Health )
While myasthenia gravis targets the muscles, it is primarily a neurological disease. Myasthenia gravis is an autoimmune condition that targets the neuromuscular junction (NMJ), the point of communication between a nerve and a muscle cell. In this area (Figure I), packets of acetylcholine, a neurotransmitter, are released when the nerve fires. In turn, these molecules bind to acetylcholine receptors on the muscle cell, leading to a cascade of events that results in muscle contraction. In myasthenia gravis, this signaling is disrupted when T cells stimulate B cells to produce an antibody against the acetylcholine receptor. The antibody binds to the receptor, but does not lead to muscle contraction. The net effect is that there are less receptors available to bind acetylcholine, and thus much more stimulation is required to have a muscle contraction.
Due to the impaired muscle contraction, voluntary muscle weakness is the most commonly observed symptom. While many muscle groups are often involved in the disease, the muscles of facial expression, eye and eyelid movement, and swallowing are the most commonly affected. The weakness of these muscle groups can result in other clinical symptoms. The loss of eyelid movement can result in ptosis, a condition in which the eyelid droops, possibly obstructing vision. The weakness in the movement of the eyes can lead to diplopia (double vision), due to the inability of the eyes to focus on the same object. The trouble in swallowing may cause problems with eating, as well as speech. Weakness of muscles in the legs and body can lead to poor posture and difficulty in walking. In more severe forms of myasthenia gravis, the muscles of breathing can be affected, leaving patients out of breath.
Fortunately, myasthenia gravis can be controlled by various medications. One approach is to treat the disease with drugs that tend to increase the levels of acetylcholine at the neuromuscular junction. Though there are less receptors available to bind acetylcholine (as they are tied up by the antibody), increasing the amount of acetylcholine tends to restore the sensitivity of the muscle to nerve impulses. The drugs of choice for this are cholinesterase inhibitors, which function by stopping the breakdown of acetylcholine at the NMJ. Another way to treat myasthenia gravis is with immunosuppressive drugs. After stopping the immune response, antibody production tapers and eventually new acetylcholine receptors are made available on the muscle cell.
It is thought that myasthenia gravis could be the result of abnormalities of the thymus, the site where T cells that respond to self-peptides are eliminated. As a result, surgical removal of the thymus can improve the condition. This procedure can improves symptoms in more than half of the patients, and can even cure some of them. While myasthenia gravis can have a severe affect on the quality of life of a patient, most patients will not have major problems if they seek treatment.
For more information on Myasthenia Gravis, check out the following resources:
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Last Updated ( Sep 22, 2007 at 08:10 PM )
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