Ch 45

• Hormones


  The body's long-distance regulators
• Regulator or monitor


  Setpoint, Change in factor, Controlling factor released, Hormonal release, Circulation, Target cell, Receptor, Signal transduction, Balance restored
• Endocrine system


  Chemical signaling by hormones
• Nervous system


  A network of specialized cells—neurons—that transmit signals along dedicated pathways
• The nervous and endocrine systems often overlap in function
• Hormones and other signaling molecules bind to target receptors, triggering specific response pathways
• Animals use chemical signals to communicate in diverse ways
• Endocrine signaling


  Endocrine cells secrete hormones into extracellular fluids, hormones picked up by the circulatory system, maintains homeostasis, mediates responses to stimuli, and regulates growth and development
• Paracrine signaling


  Local regulators are molecules that act over short distances, reaching target cells solely by diffusion, the target cells lie near the secreting cells
• Autocrine signaling


  Local regulators are molecules that act over short distances, reaching target cells solely by diffusion, the target cell is also the secreting cell
• Paracrine and autocrine signaling play roles in processes such as blood pressure regulation, nervous system function, and reproduction
• Prostaglandins


  Local regulators that mediate paracrine and autocrine signaling, function in the immune system and blood clotting
• Synaptic signaling


  Neurons form specialized junctions with target cells called synapses, secreted molecules called neurotransmitters diffuse short distances and bind to receptors on target cells
• Neuroendocrine signaling


  Specialized neurosecretory cells secrete neurohormones that diffuse from nerve endings into the bloodstream
• Members of some animal species may communicate with pheromones, chemicals that are released into the environment marking trails leading to food, defining territories, warning of predators, and attracting potential mates
• Local regulators


  Modified fatty acids, polypeptides, and gases like nitric oxide
• When the level of oxygen in blood falls, nitric oxide activates an enzyme that results in vasodilation, increasing blood flow to tissues
• Classes of hormones


  • Polypeptides, steroids, and amines
• Polypeptides and most amines are water-soluble, steroid hormones and other largely nonpolar hormones are lipid-soluble
• Water-soluble hormones


  Secreted by exocytosis, travel freely in the bloodstream, and bind to cell-surface receptors
• Lipid-soluble hormones


  Diffuse across cell membranes, travel in the bloodstream bound to transport proteins, and diffuse through the membrane of target cells, bind to receptors in the cytoplasm or nucleus of the target cells
• Binding of a water-soluble hormone to its receptor initiates a signal transduction pathway leading to responses in the cytoskeleton, enzyme activation, or a change in gene expression (sometimes)
• The hormone epinephrine (or adrenaline) regulates many organs in response to stressful situations, epinephrine binds to receptors on the plasma membrane of liver cells, triggering the release of messenger molecules that activate enzymes and result in the release of glucose into the bloodstream
• The response to a lipid-soluble hormone is usually a change in gene expression, when a steroid hormone binds to its cytosolic receptor, a hormone-receptor complex forms that moves into the nucleus, there, the receptor part of the complex acts as a transcriptional regulator of specific target genes
• In female birds and frogs, estradiol, a form of estrogen, binds to a cytoplasmic receptor in liver cells, the estradiol-bound receptor activates transcription of genes needed to produce egg yolk
• Steroid hormones


  • Androgens (testes primarily), Estrogens (ovaries), Progesterone (ovaries), Cortisol (adrenal cortex), Aldosterone (adrenal cortex)
• Multiple Responses to a Single Hormone

  One hormone may have different effects on target cells that have Different receptors for the hormone Different signal transduction pathways
• Epinephrine


  Has multiple effects that form the basis of the "fight-or-flight" response
• Endocrine cells


  Often grouped in ductless organs called endocrine glands, such as the thyroid and parathyroid glands and testes or ovaries
• Exocrine glands


  Have ducts to carry secreted substances onto body surfaces or into body cavities
• Simple Endocrine Pathways


  Hormones are released from an endocrine cell, travel through the bloodstream, and interact with specific receptors within a target cell to cause a physiological response
• Simple endocrine pathway


  STIMULUS (Low pH in duodenum) -> S cells of duodenum (Endocrine cell) -> Hormone (Secretin) -> Circulation throughout body -> Target cells (Pancreatic cells) -> RESPONSE (Bicarbonate release)
• Homeostasis regulating blood glucose

  Increased glucose uptake into cells -> Blood glucose level decreases -> Decreased glycogenolysis and gluconeogenesis (Liver) -> Insulin level increases, Glucagon level decreases
• Homeostasis regulating blood glucose

  Reduced glucose uptake into cells -> Blood glucose level increases -> Increased glycogenolysis and gluconeogenesis (Liver) -> Glucagon level increases, Insulin level decreases
• Simple Neuroendocrine Pathways


  The stimulus is received by a sensory neuron, which stimulates a neurosecretory cell. The neurosecretory cell secretes a neurohormone, which enters the bloodstream and travels to target cells.
• Simple neuroendocrine pathway


  STIMULUS (Suckling) -> Neurosecretory cell (Hypothalamus) -> Hormone (Oxytocin) -> Circulation throughout body -> Target cells (Smooth muscle in mammary glands) -> RESPONSE (Milk release)
• Negative feedback


  Inhibits a response by reducing the initial stimulus, thus preventing excessive pathway activity
• Positive feedback


  Reinforces a stimulus to produce an even greater response
• In a wide range of animals, endocrine organs in the brain integrate function of the endocrine system with that of the nervous system
• Invertebrate endocrine pathway controlling molting

  Neurosecretory cells in the larval brain produce PTTH -> PTTH directs the release of ecdysteroid in the prothoracic gland -> Bursts of ecdysteroid trigger each successive molt as well as metamorphosis -> Metamorphosis is not triggered until the level of juvenile hormone (JH) drops