Myasthenia gravis (from
Greek μύς "muscle", "weakness", and "serious"; abbreviated
MG) is a
neuromuscular disease that leads to fluctuating
muscle weakness and
fatigue. In the most common cases, muscle weakness is caused by circulating
antibodies that block
acetylcholine receptors at the postsynaptic
neuromuscular junction, inhibiting the excitatory effects of the
neurotransmitter acetylcholine on nicotinic receptors at neuromuscular junctions. Alternatively, in a much rarer form, muscle weakness is caused by a genetic defect in some portion of the neuromuscular junction that is inherited at birth as opposed to developing through passive transmission from the mother's immune system at birth or through autoimmunity later in life.