The drug blocks the active site of acetylcholinesterase so the enzyme can no longer break down the acetylcholine molecules before they reach the postsynaptic membrane receptors. This allows for the threshold to be reached so a new impulse can be triggered in the next neuron. In myasthenia gravis there are too few acetylcholine receptors so with the acetylcholinesterase blocked, acetylcholine can bind to the few receptors and trigger a muscular contraction.