Myasthenia Gravis. Myasthenia gravis (MG) is a neuromuscular disorder that causes weakness in the skeletal muscles, which are the muscles that the body uses for movement.It occurs when communication between nerve cells and muscles becomes impaired.yasthenia Gravis. Myasthenia gravis (MG) is a neuromuscular disorder that causes weakness in the skeletal muscles, which are the muscles that the body uses for movement.