HORMONES IN GENERAL

Endocrine glands secrete hormones directly into the blood, acting more slowly than nerves. Most hormones activate cells by a common mechanism, which uses adenyl cyclase to make cyclic AMP. Endocrine secretion: secretion of substances directly into the blood stream. Anything secreted this way is called a hormone. Hormones must have an effect on a target tissue. They are carried by the blood, so their targets can be in any location or in several places. Response over time: Endocrine glands usually act more slowly than the nervous system. Hormone levels may build up over time to control slow changes such as sexual development, growth, or metamorphosis. Embryological sources: Since the bloodstream distributes hormones, endocrine glands can be located anywhere and derived from any source. from endoderm: thyroid, parathyroids, intestinal lining, pancreas from mesoderm: adrenal cortex, ovary, testis from ectoderm: anterior and posterior pituitary, adrenal medulla Hormone activation: Most hormones activate their target cells by a common mechanism: The hormone binds to a specific receptor on the target cell surface. Hormone specificity depends on which target cells have receptors for which hormones and on how these target cells respond. The receptor activates the enzyme adenyl cyclase (=adenylate cyclase), which converts ATP into cyclic AMP (cAMP). The cyclic AMP acts as a "second messenger," carrying information from the cell membrane to the nucleus. The nucleus then responds by terning certain genes "on" or "off."

rev. June 2015