BIO 311D 1st Edition Lecture 27 Outline of Last Lecture I. Neurons communicate with other cells at synapsesII. Generation of postsynaptic potentialsIII. Summation of postsynaptic potentialsIV. Generation of postsynaptic potentialsV. Modulating signals at synapsesVI. NeurotransmittersOutline of Current Lecture I. Hormones and the Endocrine SystemII. Intracellular CommunicationIII. Endocrine SignalingIV. Paracrine and Autocrine signalingV. Synaptic signalingVI. Endocrine Tissues and OrgansVII. Cellular Response PathwaysCurrent LectureHormones and the Endocrine SystemHormones and other signaling molecules bind to target receptors, triggering specific response pathways• Endocrine signaling is just one of several ways that information is transmitted between animal cellsAll hormones:A. are lipid soluble moleculesB. are protein moleculesC. elicit the same biological response from all of their target cellsD. are carried to target cells in the bloodE. Are produced by endocrine glandsIntercellular Communication• The ways that signals are transmitted between animal cells are classified by two criteria – The type of secreting cell– The route taken by the signal in reaching its targetEndocrine Signaling• Hormones secreted into extracellular fluids by endocrine cells reach their targets via the bloodstream• Endocrine signaling maintains homeostasis, mediates responses to stimuli, regulates growth and developmentParacrine and Autocrine Signaling• Local regulators are molecules that act over short distances, reaching target cells solely by diffusion• In paracrine signaling, the target cells lie near the secreting cells• In autocrine signaling, the target cell is also the secreting cellSynaptic and Neuroendocrine Signaling• In synaptic signaling, neurons form specialized junctions with target cells, called synapses• At synapses, neurons secrete molecules called neurotransmitters that diffuse short distances and bind to receptors on target cells• In neuroendocrine signaling, specialized neurosecretory cells secrete molecules called neurohormones that travel to target cells via the bloodstreamSignaling by Pheromones• Members of the same animal species sometimes communicate with pheromones, chemicals that are released into the environment• Pheromones serve many functions, including marking trails leading to food, defining territories, warning of predators, and attracting potential matesEndocrine Tissues and Organs• In some tissues, endocrine cells are grouped together in ductless organs called endocrine glands• Endocrine glands secrete hormones directly into surrounding fluid• These contrast with exocrine glands, which have ducts and which secrete substances onto body surfaces or into cavitiesChemical Classes of Hormones• Three major classes of molecules function as hormones in vertebrates– Polypeptides (proteins and peptides)– Amines derived from amino acids– Steroid hormones• Lipid-soluble hormones (steroid hormones) pass easily through cell membranes, while water-soluble hormones (polypeptides and amines) do not• The solubility of a hormone correlates with the location of receptors inside or on the surface of target cellsCellular Response Pathways• Water- and lipid-soluble hormones differ in their paths through a body• Water-soluble hormones are 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 cellsPathway for Water-Soluble Hormones• Binding of a hormone to its receptor initiates a signal transduction pathway leading to responses in the cytoplasm, enzyme activation, or a change in gene expression• The hormone epinephrine has multiple effects in mediating the body’s response to short-term stress• Epinephrine binds to receptors on the plasma membrane of liver cells• This triggers the release of messenger molecules that activate enzymes and result in the release of glucose into the bloodstreamPathway for Lipid-Soluble Hormones• The response to a lipid-soluble hormone is usually a change in gene expression• Steroids, thyroid hormones, and the hormonal form of vitamin D enter target cells and bind to protein receptors in the cytoplasm or nucleus• Protein-receptor complexes then act as transcription factors in the nucleus, regulating transcription of specific genesMultiple Effects of Hormones• The same hormone may have different effects on target cells that have– Different receptors for the hormone– Different signal transduction pathwaysSignaling by Local Regulators• Local regulators are secreted molecules that link neighboring cells or directly regulate the secreting cell• Types of local regulators– Cytokines and growth factors– Nitric oxide (NO)– Prostaglandins• In the immune system, prostaglandins promote fever and inflammation and intensify thesensation of pain• Prostaglandins help regulate aggregation of platelets, an early step in formation of bloodclotsCoordination of Neuroendocrine and Endocrine Signaling• The endocrine and nervous systems generally act coordinately to control reproduction and development• For example, in larvae of butterflies and moths, the signals that direct molting originate in the brain• In insects, molting and development are controlled by a combination of hormones• A brain hormone (PTTH) stimulates release of ecdysteroid from the prothoracic glands• Juvenile hormone promotes retention of larval characteristics• Ecdysone promotes molting (in the presence of juvenile hormone) and development (in the absence of juvenile hormone) of adult
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