Nicotine is biologically active. It competes with acetylcholine for binding to specific membrane receptors, so-called nicotinic cholinergic receptors (nAChRs). They are widely expressed in the nervous system and skeletal muscle. Nicotinic receptors are also present in many cell types, e.g. epithelial, blood, and cancer cells. When nicotine binds to the nAChRs, the conformation of the receptor subunits changes, opening the receptor channel gate. This allows the influx of sodium and calcium ions into the cell, leading to membrane depolarization.