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Sunday, March 9, 2014
Why does myasthenia gravis only affect ocular muscles and other muscles for swallowing not other muscles?
It is still unclear why myasthenia seems to affect the neuromuscular junction at the extra-ocular and bulbar muscles more often then the other muscles of the body. One theory states that there are more neuromuscular junctions in these muscles because they are smaller and set for very specific, fine tasks. This means more sites to have acetylcholine blocked. Because of the added layer of complexity of the muscles needing more junctions to have finer control of movement, they are then more vulnerable to problems at the neuromuscular junction (aka myasthenia). Scientists have studied the normal extra-ocular NMJ and the normal skeletal muscle NMJ, and besides more nerve endings, they also have fewer junctional folds (less surface area for acetylcholine to react).
Other theories include: an asymmetry of acetylcholine release at these sites (an altered safety margin) or the acteylcholine receptor is slightly different between the sites. These have not been proven.
But make no mistake-- myasthenia can affect all the skeletal muscle in the body, even though it likes the eye muscles and pharyngeal muscles.
Robert W Neel, IV, MD
Assistant Professor of Neurology
College of Medicine
University of Cincinnati