What's on this Page
How is the safety margin for neuromuscular transmission altered in Myesthenia Grevis?
In Myesthenia Grevis, antibodies decrease the number of functional AChRs. Because fewer AChRs are available for activation, the safety margin for neuromuscular transmission is lowered.
Fewer MEPPs are generated when ACh output falls and the EPP is lower.
With repeated activation of the nerve and further declines in ACh output, the EPP eventually falls below the threshold necessary to trigger depolarization and contraction of the muscle fiber (blocking of neuromuscular transmission).
With continued activation of the nerve, this happens at an increasing number of NMJs and many muscle fibers fail to activate, causing weakness.
With extrinsic repetitive electrical stimulation of the nerve at low frequencies, the size of the electrical response accompanying muscle contraction (the compound motor action potential or CMAP) decreases due to this same phenomenon.
After a period of rest, ACh content is restored, and these abnormalities may improve.
Sources
Ashizawa T, Oshima M, Ruan KH, et al.: Autoimmune recognition profile of the alpha chain of human acetylcholine receptor in myasthenia gravis. Adv Exp Med Biol 303:255-261, 1991.
