Amines are organic derivatives of an inorganic compound–ammonia, with one or more hydrogens being replaced with alkyl or aryl groups. They are classified as primary, secondary, or tertiary amines based on the degree of substitution at the nitrogen atom. If the nitrogen atom is bonded to one or more alkyl carbons, the amine is classified as an aliphatic amine. If one of the bonded carbons is part of an aryl group, the amine is an aromatic amine. Amines in which nitrogen is part of a ring structure are called heterocyclic amines. If the ring is aromatic, such amines are classified as heterocyclic aromatic amines. The nitrogen atom can also bear four organyl groups. Such amines carry a formal positive charge and exist as quaternary ammonium salts. Amines occur widely in nature and have various physiological effects. For example, cocaine, a tertiary amine obtained from coca leaves, is a central nervous system stimulant.