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University of Guelph
BIOL 1080
Jim Kirkland

Lecture 5 Reading Pg. 110-114 Chapter 3a(i) - all body functions require communication between cells - in some instances, cells are physically linked by gap junctions however in most instances cells communicate through chemical messengers Gap Junctions - gap junctions link adjacent cells and are formed by plasma membrane proteins called connexions(these form structures called connexons) - these connexons form channels that allow ions and small molecules to pass directly from one cell to another - the movement of these ions through gap junctions electrically couples the cells so that an electrical signals in one cell are directly transmitted to neighbouring cells - the movement of these ions also metabolically couples the cells so that one cell can provide necessary nutrients to other cells - gap junctions are sometimes found in some glands and between some neurons in the brain and retina Chemical Messengers - chemical messengers are also known as ligands, which are molecules that bind to proteins reversibly - communication between cells using chemical messengers occurs when one cell releases a chemical into the interstitial fluid, and the target cell responds to the messenger - the receptors on the target cell specifically recognize and bind the messenger Lecture 10 Reading – chapter 3a(iv) pg. 162-173 The Endocrine System - Endocrine Glands – release hormones into the fluid outside cells. The hormones then diffuse into the bloodstream to be transported throughout the body - Endocrine System – consists of endocrine glands and of organs that contain some endocrine tissue…these organs have other functions besides hormone secretion - main function is to regulate and coordinate other body systems and maintain homeostasis - three organs with endocrine tissue: hypothalamus, thymus and pancreas Hormones as Chemical Messengers - Hormones – chemical messengers of the endocrine system - Lipid-soluble Hormones – a group of hormones that dissolve in fat derived from cholesterol, including steroid hormones (ovaries, testes and adrenal glands are the main organs that secrete steroid hormones) - lipid soluble hormones move easily through any cell’s plasma membrane because it is a lipid bilayer - once inside a target cell, a steroid hormone combines with receptor molecules in the cytoplasm…this hormone-receptor complex then moves into the nucleus of the cell where it attaches to DNA and activates certain genes (this leads to the target cell to synthesize specific proteins) - Water-soluble Hormones – a group of hormones that cannot pass through the lipid bilayer of the plasma membrane and therefore cannot enter target cells themselves (ex. Protein or peptide) - First Messenger – binds to a receptor on the plasma membrane of the target cell which activates a molecule called the second messenger - Second Messenger – molecules within the cell that influence the activity of enzymes and ultimately the activity of the cell to produce the effect of the hormone (ex. cAMP) Feedback Mechanisms and Secretion of Hormones - Negative Feedback Mechanisms – homeostatic mechanism in which the outcome of a process feeds back to the system which shuts the process down - Positive Feedback Mechanisms – homeostatic mechanism in which the outcome of a process feeds back to the system and stimulate the process to continue (ex. Uterine contractions) Interactions Between Hormones - Antagonistic – when the effect of one hormone opposes that of another hormone - Synergistic – the response of a tissue to a combination of two hormones is much greater than its response to either individual hormone - Permissive – one hormone must be present for any other hormone to exert its effects Pituitary Gland - is the size of a pea and is suspended from the base of the brain by a short stalk - stalk connects the pituitary gland to the hypothalamus - Hypothalamus – the area of the brain that regulates physiological responses like body temperature, sleeping, and water balance - pituitary gland consists of two lobes: anterior and posterior - Releasing Hormones – substances produced in the hypothalamus and are released by the anterior lobe - Inhibiting Hormones – hormones that inhibit secretion by the anterior lobe Anterior Lobe - anterior lobe secretes six major hormones including Growth Hormone (GH) - Growth Hormone – its primary function is to stimulate growth through increases in cell size and rates of cell division…target cells of this hormone are quite diverse - two hormones in the hypothalamus regulate the synthesis and release of GH…Growth hormone- releasing hormone (GHRH) stimulates the release of GH - Growth hormone-inhibiting hormone (GHIH) inhibits the release of GH - also secretes Thyroid-stimulating hormone (TSH) which acts on the thyroid gland and stimulates synthesis and release of thyroid hormones - adrenocorticotropic hormone (ACTH) controls the synthesis and secretion of glucocorticoid hormones from the outer cortex of the adrenal glands - follicle stimulating hormone (FSH) promotes development of egg cells and secretion of the hormone estrogen from the ovaries in females - luteinizing hormone (LH) causes ovulation in females Posterior Lobe - does not produce any hormones of its own but rather secretes hormones produced by the hypothalamus - antidiuretic hormone (ADH) conserves water by decreasing urine output by prompting the kidneys to remove water from the fluid destined to become urine - Oxytocin (OT) stimulates uterine contractions during childbirth as well as milk ejection from the mammary glands - thyroid gland is lined with parafollicular cells and these secrete the hormone calcitonin (CT)…this helps regulate the concentration of calcium in the blood to ensure proper functioning of muscle cells and neurons - calcium ions bind to the protein troponin which leads to changes in other muscle proteins and eventually causes muscle contraction - when the level of calcium in the blood is high, CT stimulates the absorption of calcium by bone and inhibits the breakdown of bone which thereby lowers the level of calcium in the blood - CT also lowers blood calcium by stimulating an initial increase in the excretion of calcium in the urine…when the level of calcium in the blood is low, the parathyroid glands are prompted to release parathyroid hormone (PTH) - low levels of calcium in the blood stimulate the release of PTH from the parathyroid glands, which causes calcium to move from the bone and urine into the blood - PTH exerts its effects by stimulating bone-destroying cells (called osteoclasts) that release calcium brome bone into the blood and by stimulating the removal of calcium from the urine and its return to the blood, and by stimulating the rate at which calcium is absorbed into the blood from the gastrointestinal tract - glucocorticoids are hormones secreted by the adrenal cortex that affect glucose levels - glucocorticoids act on the liver to promote the conversion on fat and protein to intermediate substances that are ultimately converted into glucose - they also act on adipose tissue to prompt the breakdown of fats to fatty acids that are released into the bloodstream - glucocorticoids also inhibit the inflammatory response which can be beneficial when the body is faced with swelling and intense irritation associated with skin rashes such as that caused by poison ivy…this is done by slowing the movement of white blood cells to the site of injury and also by reducing the likelihood that other cells will release chemicals that promote inflammation - epinephrine and norepinephrine are produced in the adrenal medulla - these hormones are essential in the fight or flight response – the reaction by the body’s sympathetic nervous system to emergencies - in response to these emergencies, heart rate, respiratory rate, and blood glucose levels rise - pancreas is located in the abdomen just behind the stomach…islets of Langerhans have three types of cells that produce glucagon (increases the level of blood sugar), insulin (decreases blood sugar levels), and somatostatin (also secreted by digestive tract) - insulin shock – consequences associated with severe depletion of blood glucose…can often result in death unless sugar levels are increased Chapter 3a(v) First Line of Defense – skin and mucous membranes help keep foreign substances from entering the body…physical barriers – skin cells, filled with keratin which waterproofs the skin and makes it resistant to the disruptive toxins/enzymes of invaders…chemical barriers – sweat and oil wash away microbes Second Line of Defense – defensive cells and proteins, inflammation and fever - this line of defense is employed if the first line fails - defensive cells include phagocytes which engulf pathogens, damaged tissue, or dead cells by the process of phagocytosis…phagocytes are classified as white blood cells - macrophages attack and consume pretty much anything that is unrecognized by the body including viruses, bacteria, damaged tissue - natural killer (NK) cells – roam the body in search of abnormal cells and quickly organizes their death - interferons – small proteins secreted by virally infected cells before they die…interferons act to
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