The organs of the endocrine system produce hormones. A hormone is a messenger molecule, made in small quantities by endocrine cells/glands and released into the blood circulation to coordinate cellular activities in distant tissues. Hormone molecules elicit a response in the target cells by attaching to a receptor protein on the cell’s plasma membrane (or inside the cell) that is specific for that molecule. The hypothalamus is a region of the brain that controls the endocrine system and integrates the activities of the nervous and endocrine systems in three ways. (1) The hypothalamus produces and secretes regulatory hormones that control the secretion of six or seven hormones from the endocrine cells in the anterior pituitary gland. In turn, the hormones, released by the anterior pituitary, stimulate their target endocrine organs to produce and release hormones which go on to influence the behaviour of the body cells which have receptors for these hormones. (2) The hypothalamus produces ADH and oxytocin which are transported to the posterior pituitary to be stored and released into the blood following appropriate stimuli. (3) The hypothalamus contains “autonomic centres” that exert neural control over endocrine cells of the medullae of the adrenal glands. When activated, the adrenal medullae release hormones into the bloodstream.