Endocrine System (BIO-169)

What is the main difference between the nervous and endocrine system?

The nervous system uses electrical signals through neurotransmitters for communication that are fast but have short-term effects. The endocrine system uses hormones through chemical signaling hormones that travel through the blood for communication that are slower but have long-term effects.

What are the functions of the endocrine system?

Helps maintain homeostasis by regulating:

• Regulation of metabolism

• Maintenance of homeostasis

  • Secretion of waste

  • Blood pressure and blood osmolarity

  • Sleep

• Growth and Development

• Reproduction

What are the 3 types of chemical signaling in the endocrine system?

1. Paracrine signaling: hormone affects neighboring cells

2. Autocrine signaling: hormone affects the same cell that released it

3. Endocrine signalin: hormone travels through the blood to affect cells throughout the body

Endocrine vs. Exocrine (2 ways of exocytosis)

Endocrine gland secretion releases product into the bloodstream or extracellular fluid. Exocrine gland secretion releases product into a duct that carries product to a body surface.

What is a target cell?

A cell with a receptor for a particular hormone; binding of a hormone to a receptor initiates intracellular signaling.

9 Major Endocrine Organs

1. Hypothalamus

2. Pituitary gland

3. Pineal gland

4. Thyroid gland

5. Parathyroid gland

6. Adrenal gland

7. Thymus

8. Pancreas

9. Gonad (testes and ovaries)

What glands have purely endocrine functions?

Anterior pituitary, adrenals, parathyroid, thyroid

Which glands are mixed (endocrine and exocrine)?

Gonads & pancreas

What are the 3 types of hormones based on chemical structure?

1. Steroid hormones that are lipid-based and can cross the cell membrane easily (e.g., testosterone & estrogens)

2. Amine-based hormones that are modified amino acids and are water-soluble but cannot cross the cell membrane (e.g., melatonin, epinephrine, norepinephrine).

3. Peptide and Protein hormones that are made from amino acid chains and are water-soluble but cannot cross the cell membrane (e.g., antidiuretic hormone & insulin).

Which hormones can and cannot pass through the cell membrane?

Steroid hormones can pass through the cell membrane because they are lipid-soluble but only once target cells bind to a receptor. Amine hormones and Peptide & Protein hormones cannot pass through the cell membrane.

Which hormone(s) require transport proteins to travel in the blood?

Only steroid hormones (not amine and protein & peptide hormones)

How are steroid hormones produced and stored?

They cannot be stored and are made on demand in the endocrine cell by modifying cholesterol molecules in the smooth ER. They are secreted in the blood and bind to transport proteins to reach target cells where they are released from transport protein and bind to intracellular receptors within the target cell

How are protein hormones produced and stored?

They are made by rough ER or ribosomes in the endocrine cell, stored in vesicles until release; they are secreted in the blood and travel in a free state without transport proteins to reach target cells and bind to receptors on a surface cell since they cannot cross the cell membrane (intiate second messenger systems)

Where are receptors for steroid hormones located?

They are intracellular (maybe in the cytosol or nucleus) because they are lipid-soluble and can pass through the cell membrane.

Where are receptors for water-soluble hormones located?

They are on the surface of the cell membrane because they cannot cross the cell membrane

Explain the second messenger system.

1. Hormone binds to receptor on cell membrane

2. Receptors that activates G protein

3. G protein activates adenylyl cyclase

4. Adenylyl cyclase converts ATP to cyclic adenosine monophosphate (cAMP)

5. cAMP activates protein kinase

6. Protein kinases phosphorylate proteins to activate these proteins

7. These proteins cause alterations to cell activity

8. Phosphodiesterase (PDE) breaks down cAMP to quickly stop internal intracellular changes

What is downregulation?

A decrease in receptor number when hormone levels are chronically high; cells become less sensitive to hormone

What is upregulation?

A increase in receptor number when hormone levels are chronically low; cells become more sensitive.

What are the 3 categories of hormone secretion regulation?

Regulated by negative feedback loops (as hormone level rises, secretion will stop or slow down).

1. Hormonal (most common) - tropic hormones that control release of other hormones

2. Humoral - chemical levels of nutrients or ions within blood stimulate release

3. Neural - nerve fibers in sympathetic NS stimulate release

What is the hypothalamus often called, and why?

The “godfather” because it is part of the dicencephalon of the brain, regulates secretion of hormones from the pituitary gland, synthesizes oxytocin and antidiuretic hormones. It is connected to the posterior pituitary gland by the infundibulum and connected to the anterior pituitary by hypothalamic-hypophyseal portal system.

Anterior pituitary

It is composted of glandular tissue; blood from the hypothalamus travels through the hypophyseal vein to the anterior pituitary

Posterior pituitary

Axons from hypothalamus project through infundibulum. Hormones stored and released into the blood by this. Stores and releases oxytocin and antidiuretic hormone (ADH) that produced in the hypothalamus.

What does oxytocin do?

Release regulated by positive feedback loop, responsible for milk ejection reflex, promotes uterine contractions, and contributes to social bonding behavior

What does antidiuretic hormone (ADH)?

Released in response to high blood osmolarity; conserves body fluids by increasing water reabsorption by kidney, causes constriction of blood vessels, increases blood pressure, and inhibited by alcohol

Diabetes Insipidus (DI)

Results from chronic underproduction of ADH, so kidneys do not reabsorb adequate amounts of water. Leads to excessive thirst, increased water consumption, osmotic imbalance (ionic imbalances can occur in severe cases of DI)

What are the six (6) hormones in the anterior pituitary?

• They are proteins or peptides that act through second-messenger systems; secretion is regulated by trophic hormones from the hypothalamus and they travel from it to the hypothalamic-hypophyseal portal system

• Growth hormone (GH), Prolactin, Thyroid-stimulating hormone (TSH), Adrenocorticotropic hormone (ACTH), Follicle-stimulating hormone (FSH), Luteinizing hormone (LH)

Growth hormone (affects nonendocrine targets)

Anabolic hormone that promotes protein synthesis and tissue building, lipolysis, and blood glucose levels for energy; affects final body size. Release regulated by GHRH and GHIH from the hypothalamus and causes the production of insulin-like (IGFs) in target tissues.

Prolactin (affects nonendocrine targets)

Promotes milk production

Thryoid-stimulating hormone (TSH; tropic hormones)

Causes release of thyroid hormones from thyroid gland.

Adrenocorticotropic hormones (ACTH; tropic hormones)

Stimulates release of cortisol from adrenal cortex.

Follice-stimulating hormone (FSH; tropic hormones)

Promotes gamete production.

Luteinizing hormone (LH; tropic hormones)

Promotes release of sex hormones and initiates ovulation.

Pituitary dwarfism

Decreased stature due to decreased secretion of growth hormone during childhood.

Gigantism

Substantially increased height due to excessive GH secretion during childhood.

Acromegaly

Causes increased growth of cartilage, leading to larger hands, feet, and ears from excessive growth hormone during adulthood. May cause cardiovascular complications due to a diabetogenic effect.

Thyroid gland

Located anterior to trachea and inferior to larynx; two lateral lobes connected by isthmus.

• Functions:

  • increase basal metabolic rate (increase ATP)

  • promote protein synthesis

  • increase effectiveness of epinephrine and norepinephrine

  • increase body temperature

  • required for adequate growth and development of skeletal and nervous tissue in childhood.

How is thyroid hormone (TH) synthesized

Made from follicular cells that bind to iodine to thyroglobulin proteins in colloid; intermediaries are combined to form T3 (triiodothryonine) and T4 (tetraiodothyronine or thyroxine). Released when TSH stimulates follicular cells if necessary.

Describe the negative feedback loop regulating TH

1. Low levels of TH stimulate TRH release from hypothalamus.

2. TRH stimulates release of TSH from anterior pituitary.

3. TSH stimulates of TH from thyroid gland.

Goiter

An enlarged thyroid caused by the accumulation of colloid.

Hypothyroidism

Insufficient production of thyroid hormones which may lead to weight gain and cold intolerance.

Hyperthyroidism

Excessive production of thyroid hormones which may lead to weight loss and increased body temperature.

What does calcitonin do and where is it secreted?

Secreted by parafollicular cells of the thyroid in response to high blood calcium, so it lowers calcium by inhibiting osteoclasts and stimulating osteoblast activity, decreases calcium absorption by intestine, and increases calcium loss in urine.

What does parathyroid hormone (PTH) do?

Chief cells secrete PTH in response to low calcium levels, increases blood calcium levels (stimulates osteoclasts), inhibits osteoblasts, and stimulating calcitriol production to increase absoprtion of dietary calcium.

Hyperparathyroidism

Excessive secretion of PTH leads to excessive resorption; aka leads to increased blood levels of calcium- decreased bone density, reduced responsiveness of nervous system, and increased calcium deposits in tissues and organs.

Hypoparathyroidism

Insufficient production or secretion of PTH, which leads to low blood levels of calcium, causing muscle twitching, cramping, convulsions, or paralysis.

Adrenal glands

Triangular glands on top of each kidney that is covered by capsule; divided into adrenal (superficially) and medulla (deeper). Adrenal cortex has three zones.

Zona glomerulosa (adrenal cortex)

Most superficial region of adrenal cortex that secretes mineralocorticoids. The main one is aldosterone, which increases sodium and water reabsorption by kidney, increases blood pressure, and involved in renin-anglotensin aldosterone system (RAAS).

Zona fasciculata (adrenal cortex)

Intermediate region of adrenal cortex that secretes glucocorticoids (cortisone and cortisol) in response to stress, stimulated by ACTH, surpresses immune system, and stimulates breakdown of stored nutrient for energy (glycogenolysis, lipolysis, gluconeogenesis).

Zona reticularis (adrenal cortex)

Deepest region of adrenal cortex that secretes androgens which are male sex hormones and the main one is dehydroepiandrosterone (DHEA), supplements testosterone in males, and promotes libido in females.

Adrenal medulla

Releases epinephrine and norepinephrines called catecholamines produced by chromaffin cells; released during fight-or-flight responses of the sympathetic nervous system, raising heart rate, blood pressure, and blood glucose, and dilates small passageways of lungs.

Cushing’s disease

A hypersecretion of cortisol that results in hyperglycemia and lipid deposits around the face and neck. Some symptoms are moon-shaped face, buffalo hump on back of neck, rapid weight gains, and hair loss. Some complications include an increase in type 2 diabetes and decreased immunity.

Addison’s disease

A hyposecretion of cortisol that results in hypoglycemia and low blood levels of sodium (hyponatremia). Some symptoms may result in general weakness, weight loss, nausea, vomitting, sweating, and craving salty foods.

Pancreas

Located within abdomen posterior to stomach; both endocrine and exocrine glands.

• exocrine function is to secrete digestive enzymes

• endocrine cells in pancreatic islets

  • alpha cells: secrete glucagon

  • beta cells: secrete insulin

  • delta cells: secrete somatostatin

  • PP cells: secrete pancreatic polypeptide

Regulation of blood glucose

Insulin lower blood glucose by:

• stimulating uptake by cells, glycogensis, lipogenesis

Glucagon increases blood glucose by:

• glycogenolysis, gluconeogenesism lipolysis

Thymus

Located within the mediastinum, superior to heart. Secretes thymosins which aids in the development and differentiation of T-lymphocytes. Most active in childhood and decreases with age.

Pineal gland

Hangs from the roof of the third ventricle of the brain; secretes melatonin that is believed to trigger body's sleep/wake cycle and reproductive system

Heart

The heart secretes atrial natriuretic peptide (ANP) which decreases blood pressure by increasing sodium and water loss through the kidneys when blood volume or pressure is too high

Kidneys

Produces renin that is involved in renin-angiotensin-aldosterone system (RAAS) that regulates blood pressure. Also secretes calcitriol, which aids in the regulation of calcium homeostasis, and produces erythropoietin (EPO), which stimulates the production of red blood cells. 

Liver

Secretes insulin-like growth factors (IGF) in response to growth hormone, angiotensinogen that increases blood pressure, thrombopoietin that stimulates platelet production, and secretes hepcidins that regulate iron levels

Gonads

Produce sex cells (exocrine function) and sex hormones.

Ovaries (gonads)

Produces ova (eggs) and two hormones:

• estrogen: stimulates development of secondary female characteristics and matures female reproductive organs

• proesterone: acts with estrogen to enact menstrual cycle, helps implant embryo, and aids in lactation

Testes (gonads)

Produces sperm and several androgens. Testosterone is most important androgen that is responsible for secondary sex characteristics, promotes maturation of male reproductive system, and sperm cell production.