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Chapter 10

Chap 10 Endocrine System.docx

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PSY 322
Cory Crane

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Chapter 10: The Endocrine System 10.1 Principles of Chemical Communication Chemical messengers: allow cells to communicate with each other to regulate body activities • Most are produced by a specific collection of cells or by a gland • Gland: organ consisting of epithelial cells that specialize in secretion • Secretion: the controlled release of chemicals from a cell • 4 types based on the source of the chemical messenger and its mode of transport in the body: o Autocrine: stimulates the cell that originally secreted it  Released by specialized cell  “Auto”  self  Ex: those secreted by white blood cells during an infection  Several white blood cells can stimulate their own replication so that the total number of white blood cells increases rapidly o Paracrine: act locally on nearby cells  Secreted by one cell type into the extracellular fluid and affect surrounding cells • Ex: histamine: released by certain white blood cells during allergic reactions o Stimulates vasodilation in nearby blood vessel o Neurotransmitter: secreted by neurons that activate an adjacent cell, whether its another neuron, a muscle cell, or a glandular cell  Secreted into a synaptic cleft, rather than into the bloodstream • Paracrine agents, therefore considered as a separate category o Endocrine: secreted into the bloodstream by certain glands and cells, which together constitute the endocrine system  Released by specialized cells to be picked up my cardiovascular cells  Affect cells that are distant from their source (hormones)  Not all cells respond to hormones • No receptor  tissue doesn’t respond 10.2 Functions of the Endocrine System 10 main regulatory functions: 1. Metabolism a. The sum of the chemical changes that occur in tissues 2. Control of food intake & digestion a. Regulates the level of satiety (fullness) & breakdown of food into individual nutrients 3. Tissue development 4. Ion regulation a. Regulates ion concentration in the blood 5. Water balance a. By controlling solutes in the blood 6. Heart rate and blood pressure regulation a. Helps prepare the body for physical activity 7. Control of blood glucose and other nutrients in the blood 8. Reproduction 9. Uterine contractions and milk release a. Regulates uterine contractions during delivery b. Stimulates milk release from the breasts in lactating females 10. Immune system regulation a. Helps control the production and functions of immune cells 10.3 Characteristics of the Endocrine System Endocrine system: composed of endocrine glands and specialized endocrine cells located throughout the body Endocrine glands: secrete minute amounts of messengers called hormones into the bloodstream, rather than into a duct • NO DUCTS, “duct-less” Hormones: substance secreted by endocrine tissues into the blood that acts on a target tissue to produce a specific response • No receptor, no effect • Good for “general effect” Target tissue (Effectors): tissue on which a hormone acts • Hormone receptors • Produces a coordinated response Endocrine: ductless gland that secretes internally, usually into the circulatory system Exocrine glands: have ducts that carry out their secretions to the outside of the body or into a hollow organ • Examples of exocrine secretions: o Saliva o Breast milk o Digestive enzymes Endocrinology: the study of the endocrine system Figure 10.1 Major Endocrine Glands and Their Locations 10.4 Hormones 2 categories of hormones; distinction on their chemical behavior: 1. Lipid-soluble hormones a. Nonpolar b. Small size c. Low solubility in aqueous fluids d. Transport travel in the bloodstream attached to binding proteins, proteins that transport the hormones i. As a result, the rate at which lipid-soluble hormones are degraded or eliminated from the circulation is greatly reduced and their life spans range from a few days to as long as several weeks e. Without binding proteins: i. They’d quickly diffuse out of capillaries and be degraded by enzymes of the liver and lungs or be removed from the body of the kidneys f. Circulating hydrolytic enzymes can also metabolize free lipid- soluble hormones g. Breakdown products are then excreted in the urine or the bile h. Includes: i. Steroid hormones ii. Thyroid hormones iii. Fatty acid derivative hormones (ex: certain eicosanoids) i. Examples: i. Synthetic estrogen and progesterone-like hormones in birth control pills & steroids that reduce the severity of inflammation j. Protein hormone: cannot diffuse across the wall of an intestine i. Therefore, must be broken down into individual amino acids beforehand ii. Therefore, must be injected, not taken orally iii. Ex: insulin k. Lipid-soluble hormones: can diffuse across the wall of an intestine 2. Water-soluble hormones a. Polar molecules b. Quite large c. Can dissolve in blood, therefore many circulate as free hormones d. Free hormones: most of them dissolve directly into the blood and are delivered to their target tissue without attaching to a binding protein e. Transport: i. Tend to diffuse form the blood into tissue spaces more slowly ii. Capillaries of organs that are regulated by protein hormones are usually very porous (fenestrated) iii. Other water-soluble hormones can be quite small 1. Require attachment to a larger protein to avoid being filtered out of the blood f. Includes: i. Protein hormones ii. Peptide hormones iii. Most amino acid derivative hormones g. Have relatively short half-lives because they are rapidly degraded by enzyme, called proteases, within the bloodstream h. The kidneys then remove the hormone breakdown products from the blood Degradation of Hormones: All hormones are destroyed either: • In the circulation • At their target cells o Destroy water-soluble hormones when the hormones are internalized via endocytosis o Once hormones are in target cell, lysosomal enzymes degrade them o Often the target cell recycles the amino acids of peptide and protein hormones and used them to synthesize new proteins • Half-life  affected by many factors -Destruction and elimination of hormones limit the length of time they are active -When hormones are secreted that remain functional for only short periods, body processes tend to change quickly • Some water soluble hormones are more stable in the circulation than others o Protein and peptide hormones have a carbohydrate attached to them, or their terminal ends are modifies  These modifications protect them from protease activity to a greater extent  Some water-soluble hormones also attach to binding proteins and therefore circulate in the plasma longer than free water-soluble hormones do Steroid hormones: those derived from cholesterol Thyroid hormones: derived from the amino acid tyrosine Other hormones: categorized as amino acid derivatives, peptides, or proteins 10.5 Control of Hormone Secretion 3 types of stimuli regulate hormone release: • Humoral • Neural • Hormonal No matter what stimulus releases the hormone, the blood level or most hormones fluctuates within a homeostatic range through negative- feedback mechanisms. In a few instances, positive-feedback mechanisms also regulate blood hormone levels Humoral stimuli: blood born molecules that directly stimulate the release of some hormones • Circulate in the blood • “Humoral”  refers to body fluids, including blood • Sensitive to the blood levels of a particular substance like glucose, calcium, or sodium • Ex: if a runner has just finished a long race during hot weather, he may not produce urine for up to 12 hours after the race because his elevated concentration of blood solutes stimulates the release of a water- conservation hormone called antidiuretic hormone (ADH). Figure 10.2 Humoral Regulation of Hormone Secretion When the blood level of a particular molecule changes (calcium), the hormone (PTH) is released in response to the molecule’s concentration Neural stimuli: of endocrine glands • Following action potentials, neurons release a neurotransmitter into the synapse with the cells that produce the hormone • In some cases the neurotransmitter stimulates the cells to increase hormone secretion Figure 10.3 Control of Hormone Secretion by Direct Neural Innervation • In response to stimuli, such as stress or exercise, the sympathetic division of the autonomic nervous system stimulates the adrenal gland to secrete epinephrine and norepinephrine, helping the body to respond to the stimulus • Responses include o Elevated heart rate o Increased blood flow through the exercising muscles • When the stimulus is no longer present o The neural stimulation declines and the secretion of epinephrine and norepinephrine decreases Releasing hormone: hormone that is released from neurons in the hypothalamus and flows through the hypothalamic-pituitary portal system to the anterior pituitary gland; regulates the secretion of hormones from the cells of the anterior pituitary gland Hormonal stimuli: occurs when a hormone is secreted that stimulates the secretion of other hormones • Examples: o Hormones from the anterior pituitary gland (tropic hormones) o Tropic hormones: many are part of a complex process in which a releasing hormone from the hypothalamus stimulates the release of a tropic hormone from the pituitary gland  Then travels to a third endocrine gland, stimulating the release of a third hormone Figure 10.4 Hormonal Regulation of Hormone Secretion Inhibition of hormone release involves all 3 types of stimuli: 1. Humoral stimuli: exists a companion hormone whose release is inhibited by the same humoral stimulus a. Usually, the companions hormone’s effects oppose those of the secreted hormone counteract the secreted hormone’s action 2. Neural stimuli: neurons inhibit targets just as often as stimulating them a. If neurotransmitter is inhibitory, target endocrine gland does not secrete its hormone b. Inhibiting hormones: hormones from the hypothalamus that prevent the secretion of tropic hormones from the pituitary gland 3. Hormonal stimuli: some hormones prevent the secretion of other hormones 2 major mechanisms that maintain hormone levels in the blood in a homeostatic range 1. Negative feedback: a. Hormones secretion is inhibited by the hormone itself once blood levels have reached a certain point and there is adequate hormone to activate the target cell b. The hormone may inhibit the action of other, stimulatory hormones to prevent the secretion of the hormone in question c. “Self-limiting system” 2. Positive feedback: a. Some hormones, when stimulated by a tropic hormone, promote the synthesis and secretion of the tropic hormone in addition to stimulating their target cell b. In turn, this stimulates further secretion of the original hormone c. “Self-propagating system” Figure 10.5 Negative and Positive Feedback 10.6 Hormone Receptors and Mechanisms of Action Receptors: Protein molecule on the cell surface or within the cytoplasm that binds to a specific factor • One of the sensory nerve endings in the skin, deep tissues, viscera, and special sense organs Receptor site: the shape and chemical characteristics of each receptor site allow only a specific type of hormone to bind to it Specificity: tendency for each type of hormone to bind to one type of receptor and not to others Figure 10.6 Target Tissue Specificity and Response Figure 10.7 General Comparison of Nuclear and Membrane-bound Receptors Lipid-soluble and water-soluble hormones bind to their own classes of receptors Lipid-soluble hormones bind to nuclear receptors: • Lipid-soluble hormones = relatively small • They diffuse through the plasma membrane and bind to nuclear receptors • Nuclear receptors = usually found in the cell nucleus, sometimes in the cytoplasm • When hormones bind to nuclear receptors, the hormone-receptor complex interacts with DNA in the nucleus or with cellular enzymes to regulate the transcription of particular genes in the target tissue • Slower than water-soluble hormones • Long lag between when hormone is released and effect can be seen Water-soluble hormones bind to membrane-bound receptors: • Water-soluble hormone = large molecules • Cannot pass through plasma membrane • Instead, they interact with membrane-bound receptors • Membrane-bound receptors: proteins that extend across the plasma membrane, with their hormone-binding sites exposed on the plasma membrane’s outer surface • When a hormone binds to a receptor on the outside of the plasma membrane, the hormone-receptor complex initiates a response inside the cell • Includes: o Proteins, peptides, and some amino acid derivatives, such as epinephrine and norepinephrine Hormone-response elements: finger like projections that recognize and bind to specific nucleotide sequences of DNA on receptors Transcription factor: formed by a combination of a receptor and the hormone Figure 10.8 Nuclear Receptor Molecule Membrane-bound receptors have peptide chains that are anchored in the phospholipid bilayer of the plasma membrane. • Membrane-bound receptors activate responses in two ways: o (1) Some receptors alter the activity of G proteins at the inner surface of the plasma membrane o (2) Other receptors directly alter the activity of intracellular enzymes. These intracellular pathways elicit specific responses in cells, including the production of intracellular mediators.  Some intracellular mediators are called second messengers.  Intracellular mediator: a chemical produced inside a cell once a hormone or another chemical messenger binds to certain membrane-bound receptors. • The intracellular mediator then activates specific cellular processes inside the cell in response to the hormone. In some cases, this coordinated set of events is referred to as a second-messenger system. G Proteins: many membrane-bound receptors produce responses through the action of G proteins. G Proteins have 3 subunits from largest to smallest 1. Alpha 2. Beta 3. Gamma • Inactive state: a GDP molecule is bound to the subunit of each G protein • Active state: GTP is bound to the a subunit, alters activity of other enzymes Figure 10.9 Membrane-Bound Receptors Activating G Proteins Activated a subunits of G proteins can alter the activity of enzymes inside the cell • Adenylate cyclase: an enzyme that converts ATP to cAMP • Cyclic AMP: functions as a 2 messenger, an intracellular mediator that carries out cellular metabolic processes in response to hormonal activation • Protein kinases: enzymes that, in turn, regulate the activity of other enzymes • Phosphodiesterase: enzyme in the cytoplasm that breaks down cAMP to AMP o Once the cAMP levels drop, the enzymes in the cell are no longer stimulated Figure 10.10 Membrane-Bound Receptors Activating G Proteins to Increase the Synthesis of cAMP Signal amplification: when each receptor produces thousands of second messengers, leading to a cascade effect and ultimately amplification of the hormonal signal • A single hormone activates many second messengers, each of which activated enzymes that produce an enormous amount of final product nd • Efficiency of this 2 -messenger amplification is virtually unparalleled in the body and can be thought of as an “army of molecules” launching an offensive • “In war, the general gives the signal to attack, and thousands of soldiers carry out the order” o With amplification, one hormone has an army of molecules working simultaneously to produce the final products Figure 10.11 Cascade Effect 10.7 Endocrine Glands and Their Hormones Pituitary gland: • “Master gland” • Controlled by the hypothalamus • Small endocrine gland • Rests in a depression of the sphenoid bone inferior to the hypothalamus of the brain • Housed by the sella turcica • Sella turcica: provides a nice pouch for the pituitary gland so it doesn’t get squished by the rest of the gland • Secretes hormones that influence the function of several other glands and tissues • Divided into 2 parts: o Anterior Pituitary (adenohypophysis): portion of the pituitary gland derived from the oral epithelium  Made up of epithelial cells derived from the embryonic oral cavity  Connected to hypothalamus through blood vessels o Posterior Pituitary (neurohypophysis): posterior portion of pituitary gland that secretes oxytocin and antidiuretic hormone  Extension of neurons that reside in the hypothalamus  Consists of processes of nerve cells that have their bodies located in the hypothalamus  Cell bodies send action potentials down the axons down the infundibulum into the posterior pituitary  Directly connected to hypothalamus Hypothalamus: important autonomic nervous system and endocrine control center of the brain • Located inferior to the thalamus Infundibulum: funnel-shaped structure or passage Hypothalamic pituitary portal vasculature (HPP): blood vessel • We go from capillary bed to capillary bed without passing through the heart o Releasing hormones are carried down by the HPP o Come out of the capillaries o Then, induce release of other hormones Anterior pituitary -Stimuli from nervous system trigger neurons to release factors -Hormones get released into blood vessels & travels down the HPP -Gets carried down into anterior pituitary -Hormones come out of circulation and affect cells in anterior pituitary -Cells release other hormones into circulation to be carried out to other tissues Figure 10.12 Pituitary Gland, Its Hormones, and Their Target Tissues  Pituitary gland controls the function of many other glands  Hypothalamus controls the pituitary gland In 2 ways: 1. Hormonal control: Neurons of the hypothalamus produce and secrete neurohormones that act on cells of the anterior pituitary gland a. Act as either releasing hormones or inhibiting hormones i. Releasing hormones: stimulate the production and secretion of a specific hormone by the anterior pituitary • Affects specific target endocrine cells
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