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Lecture 14 The Endocrine System Overview of the Endocrine System Hormone Chemistry and Action o Hormones fall into three chemical classes: Steroids are lipids synthesized from cholesterol Monoamines are small molecules bearing an amino group They are synthesized from the amino acids tyrosine and tryptophan Peptide hormones are chains of about 3 to 200 amino acids The largest peptides are proteins o Some of these have carbohydrate chains bonded to them, making them glycoproteins o Hydrophilic and Hydrophobic hormones Some hormones are hydrophilic and mix freely with water, so they are easily transported with blood Steroid hormones and thyroid hormones are hydrophobic They must bind to a transport protein in the blood plasma to be carried to their target cells o Transport proteins also protect hormones from being broken down by enzymes attached hormones may circulate for hours to weeks free hormones may break down in a few minutes o Hormones and Target Cells Steroid and thyroid hormones diffuse into the cell and bind to receptors in the cytoplasm or nucleus Monoamines and peptides cannot enter the cell, but bind to receptors on the surface The hormone acts as a first messenger to the target cell and triggers the formation of a second messenger inside the cell o The second messenger then stimulates metabolic changes in the cell Target cells respond by Synthesizing new enzymes Activating or inhibiting preexisting hormones Changing metabolism Additional hormone terminology: Tropic or trophic hormones are hormones whose target organs are other endocrine glands o Gonadotropins are hormones whose target organs are the gonads Comparison of Nervous and Endocrine Systems o Nervous System Endocrine System -Communicates with electrical -Communicates with hormones impulses and neurotransmitters -Releases neurotransmitters at -Releases hormones into blood for synapses at specific target cells general distribution throughout body -Has relatively local, specific -Sometimes has very general, effects on target cells widespread effects on many organs -Reacts quickly to stimuli (in -Reacts more slowly to stimuli milliseconds) (often taking seconds to days) -Stops quickly when stimulus -May continue responding after stops stimulus stops -Adapts relatively quickly to -Adapts slowly, may continue continual stimulation responding for days to weeks of continual stimulation Despite differences, there are similarities o Some neurotransmitters and hormones produce identical effects on some cells Norepinephrine and glucagons stimulate the liver to break down glycogen and release glucose o The nervous and endocrine systems continually regulate each other as they coordinate the activities of organ systems o Some neurons trigger hormone secretion, and some hormones stimulate or inhibit neurons The Hypothalamus and Pituitary Gland Anatomy o Hypothalamus Shaped like a flattened funnel Forms the floor and walls of the third ventricle of the brain Regulates primitive functions ranging from water balance to sex drive Many functions are carried out by way of the pituitary gland o Pituitary Gland Also called (hypophysis) Housed in the sella turcica of the sphenoid bone Ovoid gland about 1.3 cm in diameter Composed of two structures Adenohypophysis (anterior lobe) o Anterior ¾ of the pituitary o 2 parts Pars distalis The most distal to the pituitary stalk Pars tuberalis Small mass of cells adhering to the anterior side of the stalk o Has no nervous connection to the hypothalamus It’s connected by a complex of blood vessels called the hypophyseal portal system Hormones are secreted by the hypothalamus and travel to the anterior pituitary The hormones either trigger or inhibit the release of pituitary hormones Neurohypophysis (posterior lobe) o Posterior ¼ of the pituitary o 3 parts Pars nervosa The most distal portion Infundibulum Portion that connects the pars nervosa to the hypothalamus Median eminence An extension of the hypothalamic floor o Not a true gland, but a mass of neuroglia and axons arising from certain hypothalamic neurons Hormones are synthesized in the hypothalamus, are transported to and stored in the posterior pituitary until a nerve signal from the hypothalmo-hypophyseal nerve tract triggers their release Hypothalamic Hormones o The hypothalamus produces nine hormones Seven are sent through the portal system to regulate the activities of the anterior pituitary In most cases, the name of the hypothalamic hormone indicates the pituitary hormone whose secretion it stimulates or inhibits o “Prolactin-releasing hormone” promotes prolactin secretion Gonadotropin-releasing hormones control both folliclestimulating hormones and luteinizing hormones Two are transported to the posterior pituitary and stored until neurons signal their release into the blood Oxytocin Antidiuretic hormone Anterior Pituitary Hormones o Follicle Stimulating Hormone Target cells are ovaries and testes In the ovaries: It stimulates the development of the eggs in the follicles that contain them It stimulates secretion of ovarian hormones In the testes, it stimulates the production of sperm o Luteinizing Hormone In females, it stimulates ovulation (the release of the egg) It’s named for the fact that after ovulation, the follicle is called the corpus luteum. It also stimulates the corpus luteum secrete progesterone which is important to pregnancy In males, it stimulates the testes to produce testosterone o Thyroid-stimulating Hormone It stimulates the growth of the thyroid gland and the secretion of the thyroid hormone o Adrenocorticotropic Hormone It helps regulate the body’s response to stress It stimulates the adrenal cortex (outer layer of the adrenal gland) to secrete glucocorticoids, which are important in glucose, fat, and protein metabolism o Prolactin After a woman gives birth, it stimulates the mammary glands to secrete milk In males, it makes the testes more sensitive to luteinizing hormone, which enhances secretion of testosterone o Growth Hormone It promotes mitosis and cellular differentiation, thus promoting widespread tissue growth Posterior Pituitary Hormones o Antidiuretic Hormone It increases water retention by the kidneys, reduces urine volume, and helps prevent dehydration o Oxytocin In childbirth, it stimulates labor contractions In lactating mothers, it stimulates the flow of milk from the mammary glands to the nipple It surges in both sexes during sexual arousal and orgasm and may play a role in emotional bonding Other Endocrine Glands The Pineal Gland o Found attached to the roof of the third ventricle beneath the posterior end of the corpus callosum o It produces melotonin Melatonin secretion rises in the dark and occurs at a low level in daylight Its secretion fluctuates seasonally with changes in day length Melatonin levels are elevated in seasonal affective disorder and premenstrual syndrome The Thymus o Gland in the mediastinum superior to the heart, behind the sternal manubrium o It is a site of maturation of T lymphocytes, which are critical for immune defense o It secretes several hormones that stimulate the development of other lymphatic organs and regulate the development and activity of the T lymphocytes The Thyroid Gland o Gland composed to two lobes that lie adjacent to the trachea, immediately below the larynx o The two lobes are joined together by a bridge of tissue called the isthmus, which crosses in front of the trachea o The gland is composed mostly of sacs called thyroid follicles Thyroid follicles are lined by simple cuboidal epithelium of follicular cells Follicular cells secrete thyroid hormone o Technically, thyroid hormone consists of 2 hormones, thyroxine and triiodothyronine Thyroid hormone increases the concentration and activities enzymes that make ATP and the activity of sodiumpotassium pumps o This influences the rate at which the body consumes calories o Between thyroid follicles are C cells (parafollicular cells) C cells secrete calcitonin Calcitonin inhibits the bone-resorbing activity of osteoclasts This lowers blood calcium levels The Parathyroid Glands o These are small ovoid glands that adhere to the posterior side of the thyroid gland o Chief cells of the parathyroid secrete parathyroid hormone Parathyroid hormone raises blood calcium It promotes intestinal calcium absorption It inhibits urinary calcium excretion It stimulates osteoclasts to resorb bone The Adrenal Glands o Attached to the top medial surfaces of the kidneys o Hormones of the adrenal medulla Epinephrine and norepinephrine Mimic the arousing effects of the sympathetic nervous system o Hormones of the Adrenal Cortex Mineralocorticoids Steroids that control electrolyte balance Aldosterone is a hormone that influence blood volume, and blood pressure by influencing sodium and potassium in the kidneys Glucocorticoids Stimulate fat and protein catabolism and the release of fatty acids and glucose in the blood Help the body adapt to stress and repair damaged tissues Sex steroids These are weak androgens and smaller amounts of estrogens Androgens stimulate the sex drive and growth of pubic and axillary hair in both sexes The Pancreatic Islets o Most of the pancreas is an exocrine digestive gland o Scattered among those cells are pancreatic islets which secrete hormones Alpha cells secrete glucagons Glucagon secretion rises between meals when blood glucose falls In the liver, it stimulates the breakdown of glycogen and the synthesis of glucose, and the release of glucose into the blood In adipose tissue, it stimulates fat catabolism Beta cells secrete insulin Insulin secretion rises during and immediately after a meal It stimulates cells to absorb nutrients and store or metabolize them It promotes the synthesis of glycogen, fat, and protein The Gonads o Both exocrine and endocrine glands o Exocrine products are eggs or sperm o Endocrine products are gonadal hormones Granulosa cells of the ovaries produce estrogen Interstitial cells of the testes produce testosterone Endocrine Cells in Other Organs Heart, skin, liver, kidneys, stomach, small intestine, and placenta also contain cells that produce hormones