The Endocrine System Chapter 10 Endocrine System Control

The Endocrine System Chapter 10

Endocrine System § “Control system” along with nervous system § Helps maintain homeostasis § Allows for communication among cells

Comparison of Nervous & Endocrine Nervous system § quick response (milliseconds) § relatively short duration of effect § works via electrical impulses (action potentials) and direct release of chemicals (neurotransmitters) across interstitial fluid to adjacent cells § affects specific sets of cells (neuron, muscle, gland) resulting in specific responses (eg. muscle contraction or glandular secretion) Endocrine system § slower response (minutes to hours) § potentially long duration of effects (hours to days) § works via chemical signals (“hormones”) which are released through interstitial fluid into blood capillaries § affects multiple cells throughout the body that have specific hormone receptors (“target cells”) resulting in change of metabolic activities of cells (eg. affects protein synthesis)

Overview of Endocrine System § Endocrine system includes all endocrine cells, tissues, and endocrine glands (organs) of the body § Many different organs have some endocrine cells/tissues within their structure, but the organ’s primary function is not endocrine (ie: heart, kidney, digestive organs, pancreas hypothalamus, gonads, thymus) § Some organs are primarily endocrine organs (endocrine glands) (ie. pituitary, thyroid, parathyroid, adrenal, pineal)

Secretion, Distribution & Effect of Hormones § Hormones are released on demand – only when needed § Hormone release is primarily regulated by negative feedback mechanisms § changes in ECF (ISF/plasma) concentration/composition § changes in circulating levels of hormone itself or regulating hormones § neural stimuli § Hormones secreted by endocrine cells into interstitial fluid, and distributed through bloodstream § Only cells that posses receptors for that particular hormone can be affected (“target cell/target organ”) § Effect of hormones is on metabolic activity of cells, via activation/inactivation of specific genes, affecting protein synthesis and/or ATP production

Secretion, Distribution & Effect of Hormones

Structure of Hormones classified based on their chemical structure: § Amino acid derivatives – eg. epinephrine, norepinephrine, thyroid hormones, melatonin § Peptide/protein hormones – eg. Antidiuretic hormone (ADH), oxytocin, growth hormone, hypothalamic hormones, pancreatic hormones, thymic hormones § Lipid derivatives – include steroid hormones (estrogens, testosterone, adrenal hormones)

Mechanism of Hormonal Action Depends on whether hormone can cross cell membrane & enter cell: § Epinephrine, norepinephrine, & peptide hormones are not lipid soluble & therefore will bind to receptors on cell membrane (membrane receptors). § Steroid hormones & thyroid hormones can enter cell & therefore will bind to receptors within the cell (intracellular receptors)

Mechanism of Hormonal Action § Hormones that bind to membrane receptors work indirectly, via “second messengers” § Most common second messenger is cyclic AMP (c. AMP)

Mechanism of Hormonal Action Hormones that can enter the cell and bind to intracellular receptors can directly affect the structure or function of the cell, via affect on protein synthesis &/or energy production

Pituitary Gland (Hypophysis) § “master gland” § Sits within sella turcica § Attached to hypothalamus via infundibulum §Two regions – anterior pituitary & posterior pituitary § Regulated by hypothalamus § Secretes 9 hormones

Anterior Pituitary Gland Comprised of endocrine cells that produce & secrete 7 hormones: § Adrenocorticotropic hormone (ACTH) * § Thyroid stimulating hormone (TSH) * § Growth Hormone (GH) * § Follicle stimulating hormone (FSH) * § Luteinizing hormone (LH) * § Prolactin (PRL) § Melanocyte-stimulating hormone (MSH) § Controlled by regulatory hormones (releasing hormones &/or inhibiting hormones) from hypothalamus §Hypothalamic regulatory hormones reach anterior pituitary via “hypophyseal portal system”

Effects of Anterior Pituitary Hormones § Adrenocorticotropic hormone (ACTH) Targets adrenal cortex to secrete glucocorticoids (cortisol, cortisone, corticosterone), which affect glucose metabolism and response to stress § Thyroid stimulating hormone (TSH) Targets thyroid gland to secrete thyroid hormones (thyroxine (T 4), triiodothyronine (T 3)), which affect metabolism, growth and development § Growth Hormone (GH) Targets most body tissues, but especially liver, bone, muscle & cartilage, affecting growth, repair & mobilization of energy (mostly glucose) reserves. Works indirectly through release of “somatomedins” (hormones) from liver. § Follicle stimulating hormone (FSH) & Luteinizing hormone (LH) Targets gonads (ovaries/testes) for gamete (egg/sperm) production & development, secretion of hormones including estrogens and progesterone by ovaries, and testosterone by testes

Overview of Anterior Pituitary Hormones ACTH Adrenal cortex GH TSH Liver FSH LH Somatomedins Thyroid gland Glucocorticoids (cortisol, corticosterone) ) Testes Ovaries Bone, muscle, cartilage, other Thyroid hormones (T 3, T 4) Sperm Testosterone Estrogen Progesterone ovulation Egg

Posterior Pituitary Gland § Comprised of axons, telodendria and terminals of neurons that originate in the hypothalamus § 2 hormones (antidiuretic hormone (ADH) & oxytocin (OT) are produced by the hypothalamic neurons & transported through the axons running within infundibulum to terminals § Hormone release controlled via AP stimulation

Effects of Posterior Pituitary Hormones § Antidiuretic hormone (aka vasopressin) – targets kidneys to decrease amount of urine produced (increased water reabsorption), & blood vessels causing vasoconstriction to increase BP § Oxytocin – targets uterus during childbirth (positive feedback), mammary glands for milk release (“letdown”); In non-pregnant females & in males, may contribute to orgasm; “bonding” hormone

Thyroid Gland § Located anterior to trachea, just below thyroid cartilage of larynx § Two lobes connected by isthmus § Lobes contain thyroid follicles § Follicular cells secrete 2 hormones under the influence of TSH – Thyroxine (T 4) & Triiodothyronine (T 3)

Thyroid Gland § Thyroid hormones target almost every body cell § Can enter cells & bind to intracellular receptors on mitochondria & in nucleus § Effects include: § increased ATP production § increased cellular metabolism, energy utilization & oxygen consumption § increased body temperature § growth & development of skeletal, muscular & nervous system in fetus & children

Thyroid Disorders Hyperthyroidism – different causes but resulting increased production of thyroid hormones may lead to: increased metabolic rate, increased heart rate & blood pressure, increased body temperature, excitability & nervousness, weight loss, possible goiter, possible exopthalmos Hypothyroidism – decreased secretion of thyroid hormones may cause: lower metabolic rate, sluggishness, fatigue, decreased heart rate & BP, decreased body temperature, weight gain, possible goiter, subcutaneous swelling (myxedema), hair loss, dry skin Hypothyroidism during pregnancy or in early childhood may lead to “cretinism” – abnormal skeletal & nervous development

Parathyroid Glands § Four small glands attached to the posterior surface of thyroid gland § When blood Ca 2+ levels fall below homeostatic normal levels, Chief cells secrete Parathyroid hormone (PTH) § Targets osteoclasts (increases activity), osteoblasts (decreases activity), kidneys (decreases urinary Ca 2+ excretion) & digestive tract (increases Ca 2+ absorption)

Adrenal (Suprarenal) Glands § Lie superior to each kidney § Comprised of two parts: outer adrenal cortex & inner adrenal medulla

Adrenal Cortex § Divided into 3 functional zones with each zone producing different types of “corticosteroids” § Middle zone secretes steroid hormones known as “Glucocorticoids” (cortisol (hydrocortisone), cortisone, corticosterone), under the control of ACTH § Glucocorticoids target most body cells, especially liver, muscle, adipose & white blood cells Effects include: § increased glucose synthesis, “gluconeogenesis” & glycogen formation mainly in liver § release of fatty acids from adipose tissue & protein breakdown in muscle cells for energy use § helps provide resistance to stress due to increase in available energy § inhibits activity of WBCs & immune responses – anti-inflammatory effects but slow wound healing & resistance to disease

Adrenal Medulla § Comprised of cells that are postganglionic sympathetic neurons § Sympathetic stimulation results in release of epinephrine (adrenaline) and norepinephrine (noradrenaline) into bloodstream § Effects are sympathomimetic

Pancreas § Endocrine & exocrine (digestive) gland § Endocrine function by cells within pancreatic islets (islets of Langerhans) § Alpha (α) cells secrete glucagon § Beta (β) cells secrete insulin § Both regulate blood glucose levels

Regulation of Blood Glucose

Diabetes Mellitus Most common endocrine disorder of the pancreas 2 major types: § Type 1 (insulin dependent/ ”juvenile”) – actual deficiency of beta cells; sudden onset; may be autoimmune § Type 2 (non-insulin dependent/ “adult”) – insulin resistance due to decreased sensitivity (or number) of insulin receptors Initial symptoms include: hyperglycemia, glycosuria, polyurea, polydipsia, polyphagia, weight loss, fatigue Long term symptoms: neuropathy, retinopathy, gangrene, kidney failure…

Hormones & Stress – General Adaptation Syndrome (GAS)