During myasthenia gravis, the body attacks the Ach receptors so they do not work; this could cause?
Since this is not my area of expertise, I searched Google under the key words "myasthenia gravis" to get these possible sources:
http://en.wikipedia.org/wiki/Myasthenia_gravis
http://www.ninds.nih.gov/disorders/myasthenia_gravis/detail_myasthenia_gravis.htm
http://www.mayoclinic.com/health/myasthenia-gravis/DS00375
http://www.neurologychannel.com/myastheniagravis/index.shtml
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.
I hope this helps. Thanks for asking.