<p>Neostigmine is a parasympathomimetic, specifically, a reversible cholinesterase inhibitor. By interfering with the breakdown of acetylcholine, Neostigmine indirectly stimulates both nicotinic and muscarinic receptors. It does cross the blood-brain barrier but only poorly. Neostigmine binds to the anionic site of cholinesterase. 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.</p>