Pyridostigmine (PST)

Pyridostigmine is a parasympathomimetic and a reversible cholinesterase inhibitor. Since it is a quaternary amine, it is poorly absorbed in the gut and does not cross the blood–brain barrier, except possibly in stressful conditions. In a synapse, action potentials are conducted along motor nerves to their terminals where they initiate a Ca2+ influx and the release of acetylcholine (ACh). The ACh diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane, causing an influx of Na+ and an efflux of K+ ions, resulting in depolarization. If large enough, this depolarization results in an action potential. To prevent constant stimulation once the ACh is released, an enzyme called acetylcholinesterase is present in the endplate membrane close to the receptors on the post synaptic membrane, and quickly hydrolizes ACh.