Key Concepts Animals use at least six major types of chemical signals Hormones are chemical signals that are present in tiny concentrations and travel throughout the body to affect target cells The information carried by hormones helps animals develop as embryos undergo sexual maturation respond to environmental change and achieve homeostasis The production of a hormone is tightly regulated by input from the nervous system and by other hormones 2011 Pearson Education Inc Key Concepts Some hormones bind to receptors inside target cells and change gene expression Other hormones bind to receptors at the cell surface and lead to changes in protein activation 2011 Pearson Education Inc Introduction A group of organs and cells called the endocrine system produces chemical signals and secretes them into the bloodstream These signals are called hormones They circulate through the blood or other bodily fluids and have a relatively long lasting effect on distant target cells In combination animal nervous systems and endocrine systems process and respond to information about the environment 2011 Pearson Education Inc Cell to Cell Signaling An Overview Animal chemical signals are present in extremely low concentrations but can have enormous effects on their target cells Unlike action potentials which are electrical impulses that have a short term effect on a single cell or on a small population of adjacent cells the messages that chemical signals carry have a relatively long lasting effect In combination electrical and chemical signals allow animals to coordinate the activities of cells throughout the body 2011 Pearson Education Inc Major Categories of Chemical Signals There are six major categories of chemical signals in animals 1 2 3 4 5 6 Autocrine signals Paracrine signals Endocrine signals Neural signals Neuroendocrine signals Pheromones These six classes of chemical messenger do not coincide with six structurally distinct classes of molecules A single chemical messenger can be assigned to more than one category of signal based on its mode of action 2011 Pearson Education Inc 2011 Pearson Education Inc Autocrine and Paracrine Signals Autocrine signals act on the same cell that secretes them Cytokines are the best studied autocrine signals Most cytokines amplify the response of a cell to a stimulus Paracrine signals diffuse locally and act on neighboring target cells Like some cytokines the cell cell signals insulin glucagon and somatostatin cross categories acting as paracrine signals but also as hormones 2011 Pearson Education Inc Endocrine and Neural Signals Endocrine signals are hormones and are produced and secreted by specialized cells or discrete organs called glands Hormones are carried between distant cells by blood or other body fluids Neural signals are neurotransmitters that diffuse a short distance from a presynaptic cell to a postsynaptic cell where it binds to a membrane receptor and results in a change in the membrane potential 2011 Pearson Education Inc Neuroendocrine Signals and Pheromones Neuroendocrine signals are released from neurons but are considered hormones because they are carried by blood or other body fluids and act on distant cells Pheromones are released into the environment and act on a different individual In many species they help coordinate reproduction in males and females or function in attracting mates 2011 Pearson Education Inc Hormone Signaling Pathways Like plant hormones some animal hormones are also sent directly from endocrine cells to effector cells in response to a stimulus But more frequently hormonal signaling in animals involves additional steps In many or most cases information about external or internal conditions is gathered by receptors and then integrated by neurons in the central nervous system CNS prior to the production of a hormonal signal 2011 Pearson Education Inc Hormone Signaling Pathways Hormones act via three pathways all of which are regulated by negative feedback 1 The endocrine pathway sends hormones directly from endocrine cells to effector cells 2 The neuroendocrine pathway releases neuroendocrine signals that act directly on effector cells 3 In the neuroendocrine to endocrine pathway neuroendocrine signals stimulate cells in the endocrine system which respond by producing an endocrine signal that acts on effector cells 2011 Pearson Education Inc 2011 Pearson Education Inc Hormone Signaling Pathways The nervous system and endocrine system are tightly integrated Endocrine signals are released in response to electrical signals in turn endocrine signals modulate the electrical signals transmitted by the nervous system In animal cell cell signaling feedback inhibition reduces production and or secretion of the hormone 2011 Pearson Education Inc What Makes Up the Endocrine System Organs that secrete hormones into the bloodstream are called endocrine glands The tissues and organs that make up the endocrine system vary widely among animals For example Neurons that manufacture and secrete hormones are particularly important in insects where they regulate molting metamorphosis and other processes Salmon have an unusual gland that secretes a hormone responsible for regulating calcium ion concentration 2011 Pearson Education Inc What Makes Up the Endocrine System The major human endocrine glands are as follows The hypothalamus is a region deep within the brain The pituitary gland with distinct anterior and posterior regions sits just below the hypothalamus The thyroid gland is situated in the neck The four parathyroid glands are embedded in the thyroid gland The two kidneys lie in the posterior part of the abdominal cavity The two adrenal glands sit atop the kidneys 2011 Pearson Education Inc What Makes Up the Endocrine System The endocrine component of the pancreas is located in the anterior part of the abdominal cavity The paired ovaries in females or testes in males are in or suspended below the pelvic cavity respectively Cells in many other organs and tissues secrete hormones even though they are not organized into discrete glands Not all glands in the body are part of the endocrine system Exocrine glands deliver their secretions through ducts into a space other than the circulatory system 2011 Pearson Education Inc 2011 Pearson Education Inc Chemical Characteristics of Hormones Most animal hormones belong to one of three chemical families 1 Polypeptides chains of amino acids linked by peptide bonds 2 Amino acid derivatives