Epinephrine (also known as adrenaline, adrenalin, or β,3,4-trihydroxy-N-methylphenethylamine) is a hormone and a neurotransmitter. Epinephrine and norepinephrine are two separate but related hormones secreted by the medulla of the adrenal glands. They are also produced at the ends of sympathetic nerve fibres, where they serve as chemical mediators for conveying the nerve impulses to effector organs. The investigation of the pharmacology of epinephrine made a major contribution to the understanding of the autonomic system and the function of the sympathetic system. Epinephrine remains a useful medicine for several emergency indications. This is despite its non-specific action on adrenoceptors and the subsequent development of multiple selective medicines that target subtypes of the adrenoceptors. The word adrenaline is used in common parlance to denote increased activation of the sympathetic system associated with the energy and excitement of the fight-or-flight response. The influence of adrenaline is mainly limited to a metabolic effect and bronchodilation effect on organs devoid of direct sympathetic innervation.In chemical terms, epinephrine is one of a group of monoamines called the catecholamines. It is produced in some neurons of the central nervous system, and in the chromaffin cells of the adrenal medulla from the amino acids phenylalanine and tyrosine.