Chapter 18: Endocrine System Flashcards
Which substance acts as a neurotransmitter and a hormone?
Norepinephrine. Released as a NT from sympathetic postganglionic neurons. Released as a hormone from chromatin cells of adrenal medullae.
Exocrine glands.
Secrete their products into ducts that carry the secretions into body cavities, the lumen of an organ, or to the outer surface of the body. Sudoriferous (sweat), sebaceous (oil), mucous and digestive glands.
Endocrine glands.
Secrete their products into the interstitial fluid surrounding the secretory cells and then into blood capillaries which carries the hormones to target cells throughout the body. Pituitary, thyroid, parathyroid, adrenal and pineal glands.
Functions of hormones.
Regulate chemical composition, volume of internal environment, metabolism, energy balance, contraction of smooth and cardiac muscle fibres, glandular secretions, immune system, growth, development, reproductive system, circadian rhythms.
A target cell has how many receptors for a specific hormone?
2000-100000.
Down regulation.
If a hormone is present in excess, then the number of target cell receptors will decrease to make the cell less sensitive to the hormone.
Up regulation.
If a hormone is deficient, then the number of target cell receptors will increase to make the cell more sensitive to the hormone.
Circulating hormones.
Most endocrine hormones. Pass from secretory cells to interstitial fluid to blood. Linger in blood and have longer-lasting effects. Inactivated by liver and excreted by kidneys.
What might happen in kidney or liver failure?
Build-up of hormones in the blood.
Local hormones.
Act locally on neighbouring cells or on the same cell that secreted them without entering the blood. Paracrines and autocrines. Inactivated quickly.
Lipid-soluble hormones.
Steroid hormones, thyroid hormones, nitric oxide.
Water-soluble hormones.
Amine hormones, peptide and protein hormones, eicosanoid hormones.
Steroid hormones.
Derived from cholesterol.
Thyroid hormones.
Synthesized by attaching iodine to tyrosine.
Nitric oxide.
A hormone and a NT. Synthesis is catalyzed by nitric oxide synthase.
Amine hormones.
Synthesized by decarboxylating certain amino acids. Catecholamines are made by modifying tyrosine. Histamine is made by modifying histidine. Serotonin and melatonin are made by modifying tryptophan.
Peptide and protein hormones.
Amino acid polymers. Small: 3-49 amino acids. Large: 50-200 amino acids.
Eicosanoid hormones.
Derived from arachidonic acid. Prostaglandins. Leukotrienes.
How do water-soluble hormones circulate in the blood?
Circulate freely in the watery blood plasma.
How do lipid-soluble hormones circulate in the blood?
Transport proteins.
Functions of transport proteins.
Transport lipid soluble hormones in blood. Retard passage of small hormones through the filtering mechanism in the kidneys to slow the rate of hormone loss in the urine. Provide a ready reserve of hormone in the bloodstream.
What happens to the 0.1-10% of lipid-soluble hormones not bound to transport proteins?
The free fraction diffuse out of capillaries, bind to receptors and trigger responses.
Where are the receptors for lipid-soluble hormones?
Inside the target cell.
Where are the receptors for water-soluble hormones?
Plasma membrane of the target cell.
Describe the action of a lipid-soluble hormone.
Hormone diffuses from blood –> interstitial fluid –> lipid bilayer of PM –> target cell –> binds and activates receptors –> alters gene expression –> DNA transcribes –> mRNA forms –> mRNA leave nucleus –> mRNA enters cytosol –> direct synthesis of a new protein on the ribosomes –> new proteins alter cell activity.
Describe the action of a water-soluble hormone.
Hormones diffuses from blood –> interstitial fluid –> binds to integral transmembrane protein receptor on PM –> activates G-protein –> activates adenylyl cyclase –> converts ATP to cAMP in cytosol –> activates protein kinases to phosphorylate cellular proteins –> reactions –> physiological responses –> phosphodiesterase eventually inactivates cAMP –> response is turned off.
What does the responsiveness of a target cell to a hormone depend on?
The hormone concentration in the blood, the abundance of the target cell’s hormone receptors, and influences exerted by other hormones.
The actions of some hormones on target cells require:
A simultaneous or recent exposure to a second hormone, which has a permissive effect. Sometimes the permissive hormone increases the number of receptors for the other hormone, and sometimes it promotes the synthesis of an enzyme required for the expression of the other hormone’s effects.
Synergistic effect.
When the effect of two hormones acting together is greater than the sum of their individual effects. The hormones activate pathways that lead to the formation of the same types of second messengers to amplify the cellular response.
Antagonistic effect.
When one hormone opposes the actions of another by activating the opposite cellular responses, or decreasing the number of receptors for the other hormone.
Hormone secretion is regulated by signals from:
Nervous system, chemical changes in the blood, and other hormones. Most hormone regulatory systems work via negative feedback, but some work via positive feedback.
Hypothalamus.
Major link between nervous and endocrine systems. Hypothalamus cells synthesize at least 9 different hormones.
Pituitary gland.
Controlled by hypothalamus. Secretes 7 different hormones. Pea-shaped. Lies in hypophyseal fossa of sella turcica of sphenoid bone. Attaches to hypothalamus by infundibulum.
Pars intermedia.
Third region of the pituitary gland. Atrophies during human fetal development. Ceases to exist as a separate lobe in adults.
Anterior pituitary.
Adenohypophysis. Larger region of the pituitary gland. Composed of epithelial tissue. Pars distalis (larger part). Pars tuberalis (forms a sheath around the infundibulum).
What are the anterior pituitary cells, and what do they secrete?
Somatotrophs (growth hormone), thryotrophs (TSH), gonadotrophs (FSH and LH), lactotrophs (prolactin), corticotrophs (ACTH, MSH).
What is MSH?
Melanocyte-stimulating hormone. Increases skin pigmentation in amphibians by stimulating the dispersion of melanin granules in melanocytes. Its exact role in humans is unknown, but the presence of MSH receptors in the brain suggests it may influence brain activity. Continued administration of MSH for days produces skin darkening.
Hypothalamic hormones reach the anterior pituitary through:
A portal system. Blood will flow from a capillary network –> portal vein –> second capillary network –> return to heart.
The name of the portal system indicates:
The location of the second capillary network in the system.
Hypophyseal portal system.
Blood flows from capillaries in the hypothalamus –> portal veins –> capillaries of anterior pituitary.
Which arteries bring blood to the hypothalamus?
Superior hypophyseal arteries, which are branches of internal carotid arteries.
Primary plexus.
First capillary network in portal system.
Secondary plexus.
Second capillary network in portal system.
Control of anterior pituitary secretions.
Neurosecretory cells synthesize hypothalamic releasing and inhibiting hormones –> package into vesicles –> exocytosis when stimulated –> diffuse into blood of primary plexus of hypophyseal portal system –> hypophyseal portal veins –> secondary plexus –> diffuse out of blood –> anterior pituitary cells secrete hormones into secondary plexus –> hormones drain into hypophyseal veins –> general circulation –> target tissues.
Which anterior pituitary cells work via negative feedback?
Thyrotrophs, corticotrophs, gonadotrophs.
Growth hormone.
Most abundant anterior pituitary hormone. Regulates growth (indirectly) and metabolism (directly). Exerts its effects indirectly through insulin-like growth factors (IGFs), which are secreted by liver, skeletal muscle, cartilage and bone cells.
Describe the difference between IGFs synthesized by the liver, and IGFs synthesized by skeletal muscle, cartilage and bone.
Liver: enter blood as hormones that circulate to target cells. Other: act locally as autocrines or paracrines.
Three functions of IGFs.
Increase growth of bones and soft tissues, enhance lipolysis, and decrease glucose uptake.
How do IGFs increase growth of bones and soft tissues?
Bones: IGFs stimulate osteoblasts, promote cell division at epiphyseal plate, and enhance synthesis of the proteins needed to build bone matrix. Soft tissue: IGFs cause cells to grow by increasing uptake of amino acids into cells, accelerating protein synthesis, and decreasing the breakdown of proteins and the use of amino acids for ATP production.
How do IGFs influence CHO metabolism?
By decreasing glucose uptake, which decreases the use of glucose for ATP production by most body cells. This allows for the availability of glucose for ATP production in neurons when glucose levels are low. Liver cells are also stimulated to release glucose into the blood.
How is growth hormone released?
Somatotrophs release bursts of GH every few hours.
Describe the release of GH.
GHRH is secreted from hypothalamus –> enters hypophyseal portal system –> anterior pituitary –> somatotrophs secrete GH –> GH acts directly on cells to promote metabolic reactions –> elevated levels of GH and IGFs inhibit release of GHRH and GH –> GHIH is secreted from hypothalamus –> enters hypophyseal portal system –> anterior pituitary –> prevents somatotrophs from secreting GH.
Thyroid stimulating hormone.
Stimulates T3 and T4 synthesis and secretion. Is stimulated by thyroid releasing hormone from the hypothalamus.
Follicle stimulating hormone.
Females: FSH initiates development of ovarian follicles, and stimulates follicular cells to secrete estrogens. Males: FSH stimulates sperm production in testes. Is stimulated by gonadotropin releasing hormone from the hypothalamus.
What suppresses GnRH and FSH release?
Estrogens (females) and testosterone (males) through negative feedback.
Luteinizing hormone.
Females: LH triggers ovulation, formation of corpus luteum in ovary, and the secretion of progesterone by the corpus luteum. Males: LH stimulates testes cells to secrete testosterone. Is stimulated by gonadotropin releasing hormone from the hypothalamus.
How do FSH and LH work together?
Stimulate secretion of estrogens by ovarian cells. Estrogens and progesterone prepare uterus for implantation, and prepare the mammary glands for milk secretion.
Prolactin.
Initiates milk production by mammary glands after the glands have been primed by estrogens, progesterone, glucocorticoids, GH, T4, and insulin.
Which substance regulates the ejection of milk from the mammary glands?
Oxytocin.
Prolactin inhibitory hormone.
Dopamine. In females, PH inhibits the release of prolactin from the anterior pituitary. Each month before menstruation, the secretion of PIH decreases and the blood level of prolactin increases, but not enough to stimulate milk production. As menstruation begins, PIH is again secreted and prolactin decreases.
Does prolactin increase or decrease during pregnancy?
Increase. Stimulated by PRH from hypothalamus.
Sucking action of an infant causes:
A reduction in hypothalamic secretion of PIH.
Hypersecretion of prolactin causes:
Females: galactorrhea and amenorrhea. Males: erectile dysfunction.