Chapter 18 The Endocrine System I Intercellular Communication p 591 Objectives 1 Explain the importance of intercellular communication and describe the mechanisms involved 2 Compare the modes of intercellular communication used by the endocrine and nervous systems 3 Discuss the functional significance of the differences between the two systems The nervous system cannot regulate such long term processes as growth development or reproduction which involve or affect metabolic activities in virtually every cell and tissue Regulation is provided by the endocrine system which uses chemical messengers to relay information and instructions between cells Cellular activities are coordinated by the exchange of ions and molecules between adjacent cells across gap junctions This direct communication occurs between two cells of the same type and the cells must be in extensive physical contact The two cells communicate so closely that they function as a single entity Direct communication is highly specialized and relatively rare Most communication between cells involves the release and receipt of chemical messages Paracrine communication involves the use of chemical signals to transfer information from cell to cell within a single tissue Endocrine communication is carried out by endocrine cells releasing chemicals called hormones into the bloodstream which alters the metabolic activities of many tissues and organs simultaneously Hormones exert their effects by modifying the activities of target cells specific cells that possess the receptors needed to bind and read the hormonal message A hormone may 1 stimulate the synthesis of an enzyme or a structural protein not already present in the cytoplasm by activating appropriate genes in the cell nucleus 2 increase or decrease the rate of synthesis of a particular enzyme or other protein by changing the rate of transcription or translation 3 turn an existing enzyme or membrane channel on or off by changing its shape or structure The endocrine system is totally unable to handle situations requiring split second responses Crisis management is the job of the nervous system Synaptic communication release of a neurotransmitter at a synapse very close to target cells that bear the appropriate receptors is ideal for crisis management Table 18 1 The endocrine system and nervous system are similarly organized Both systems 1 rely on the release of chemicals that bind to specific receptors on their target cells 2 share many chemical messengers for example norepinephrine and epinephrine are called hormones when released into the bloodstream but neurotransmitters when released across synapses 3 are regulated primarily by negative feedback control mechanisms 4 share a common goal to preserve homeostasis by coordinating and regulating the activities of other cells tissues organs and systems II An Overview of the Endocrine System p 593 Objectives 1 Compare the cellular components of the endocrine system with those of other tissues and systems 2 Compare the major structural classes of hormones 3 Explain the general mechanisms of hormonal action 4 Describe how endocrine organs are controlled The endocrine system includes all the cells and endocrine tissues of the body that produce hormones or paracrine factors Endocrine cells are glandular secretory cells that release their secretions into the extracellular fluid Exocrine cells secrete their products onto epithelial surfaces generally by way of ducts Figure 18 1 Classes of Hormones p 593 Hormones can be divided into three groups amino acid derivatives peptide hormones and lipid derivatives Figure 18 2 Amino Acid Derivatives p 595 Amino acid derivatives are relatively small molecules that are structurally related to amino acids This group of hormones sometimes known as the biogenic amines is synthesized from the amino acids tyrosine and tryptophan Tyrosine derivatives include 1 thyroid hormones produced by the thyroid gland 2 compounds epinephrine E norepinephrine NE and dopamine which are sometimes called catecholamines The primary hormone derivative of tryptophan is melatonin produced by the pineal gland Peptide Hormones p 595 Peptide hormones are chains of amino acids They are synthesized as prohormones inactive molecules that are converted to active hormones either before or after they are secreted Peptide hormones are divided into two groups 1 Group 1 Glycoproteins more than 200 amino acids long with carbohydrate side chains Glycoproteins include thyroid stimulating hormone TSH luteinizing hormone LH and follicle stimulating hormone FSH from the anterior lobe of the pituitary gland as well as several hormones produced in other organs 2 Group 2 large and diverse including hormones from short polypeptide chains such as antidiuretic hormone ADH and oxytocin to small proteins such as growth hormone GH and prolactin PRL This group includes all the hormones secreted by the hypothalamus heart thymus digestive tract pancreas and posterior lobe of the pituitary gland as well as most of the hormones secreted by the anterior lobe of the pituitary gland Lipid Derivatives p 595 There are two classes of lipid derivatives 1 Eicosanoids derived from arachidonic acid 2 Steroid hormones derived from cholesterol Eicosanoids are small molecules with a five carbon ring at one end These compounds are important paracrine factors that coordinate cellular activities and affect enzymatic processes in extracellular fluids Leukotrienes are eicosanoids released by activated white blood cells or leukocytes Leukotrienes are important in coordinating tissue responses to injury or disease Prostaglandins a second group of eicosanoids are produced in most tissues of the body Prostaglandins are involved primarily in coordinating local cellular activities In some tissues prostaglandins are converted to thromboxanes and prostacyclins which also have strong paracrine effects Steroid hormones are lipids structurally similar to cholesterol They are released by male and female reproductive organs androgens by the testes estrogens and progestins by the ovaries the adrenal glands corticosteroids and the kidneys calcitriol In the blood steroid hormones are bound to specific transport proteins in the plasma They remain in circulation longer than do secreted peptide hormones The liver gradually absorbs these steroids and converts them to a soluble form that can be excreted in the bile or urine Secretion and Distribution of Hormones p 595 Hormones may