Endocrine System Notes

Ch. 16 Notes: Endocrine System

General Principle of Endocrine System Operation

Specific Endocrine Organs and Hormones

Ch. 16 Notes: Endocrine System

General Principles

Endocrine system is a control system: compare/contrast to nervous system

Chemical Messengers: Hormones compared to other chemical messengers

Hormones: Classification by chemical structure. Most are either:

Amino-acid based: Hydrophilic, water-soluble, don’t cross the cell membrane

Cholesterol-based: Hydrophobic, poorly soluble in water, do cross cell membrane into cytoplasm

Hormone mechanisms of action

Amino acid based (except thyroid hormone): Work via plasma membrane receptors and second messengers. The membrane receptors are usually coupled to G proteins which activate when the hormone binds to the rector. (“G-protein coupled receptors”). Know general principles of G protein coupling & 2nd messengers and a couple of examples.

Steroid hormones and thyroid hormone: work via cytoplasmic receptors – know general principles of how these can regulate gene expression , protein synthesis

Other: Insulin receptor is a tyrosine kinase. Cyclic GMP is a second messenger in some cases.

Target Cell Specificity

Refers to ability of a given hormone to have different effects in different tissues, even though the hormones “go everywhere” since they are carried by blood to all tissues.

Possible due to fact that different cells have different receptors and/or different intracellular pathways “connected to” those receptors

Half-life, onset, duration

Regulated by rate of production and rate of removal from the system

Removed by metabolic breakdown in various tissues, esp. liver, and by excretion, esp by kidneys

Interactions at target cells: permissive, synergistic, antagonistic

Control of Hormone Release

Almost always involves negative feedback to keep some physiological quantity in the “good” range

Humoral, i.e. controlled by concentration of a substance in the blood. Examples: glucose, calcium, etc

Neural. Example: action potentials on sympathetic preganglionic fibers to adrenal medulla cause epi & norepi release from adrenal medullary cells

Hormonal: Many hormones are regulated by other hormones, or regulate another hormone, or both

Specific Endocrine Organs

Hypothalamus & Pituitary (functionally & anatomically linked)

Pituitary gland – “master gland”

Other name = hypophysis. Stalk = infundibulum. Location = hypophyseal fossa of sphenoid.

Two parts: posterior pituitary = neurohypophysis; anterior pituitary = adenohypophysis

Post. Pit.:

Secretes oxytocin & ADH (also known as vasopressin) – know what they regulate and what would cause more or less secretion of both. Both are peptides.

Secretion is from axons of hypothalamic neurons (i.e. neurons whose cell bodies are in hypothalamus)

Ant. Pit.

Secretes GH, TSH, ACTH, PRL, FSH, LH. Know what each does and what directly or indirectly would cause more or less secretion of each. LH, FSH covered more in reproduction chapter.

Hormones of ant. pit. are made in the ant. pit., unlike post. pit. hormones that are made in hypothalamus.

Ant pit hormones are controlled by hormones made in other hypothalamic nuclei, which secrete into capillary network at top of infundibulum. Portal veins carry the “control” hormones down the infundibulum to capillaries of the ant pit. (An example of a portal circulation – what does that mean?)

Growth hormone (GH): General metabolic stimulation & more protein synthesis. Releases fat stores into blood for use as fuel for growth. Reulates by 2 hypothalamic hormones which come via the hypophyseal portal circulation: GHRH & GHIH. (Know what the abbrevs in this chapter stand for.) Gigantism, dwarfism if too much or too little during childhood.

TSH (thyrotropin): Stimulates thyroid gland, making it make more thyroid hormone (TH). Regulated by TRH which comes from hypothalamus via portal circulation. Both TRH release from hypothalamus and TSH release from ant pit are under negative feedback control by TH: more TH suppresses TRH and TSH release.

ACTH (=corticotropin): Stimulates adrenal cortex to make & release more cortisol & cortisone, which are themselves hormones (“glucocorticoids”). Stressors of various types can cause more ACTH release. CRH, from hypothalamus, stimulates ACTH synthesis & release from ant pit. Glucocorticoids (cortisol, cortisone) exert negative feedback control over release of CRH and ACTH.

Prolactin, PRL: Stimulates mammary glands to produce milk. PRL production & release is inhibited by PIH, from the hypothalamus. PRL is the slow, long-term stimulator of breast milk synthesis in lactating women. (Oxytocin is the short-term stimulus for milk “letdown” during nursing. Its production & release are stimulated via neural pathways driven by suckling.)

FSH & LH = gonadotropins. LH stimulates ovaries & testes to make more estrogen, progesterone, testosterone. FSH stimulates sperm maturation in men. FSH & LH work together in women to cause one egg per month (usually) to mature and become ready for fertilization. Controlled by GnRH from hypothalamus: more GnRH causes more production & release of LH & FSH. There is negative feedback control of GnRH, LH, and FSH by the gonadal hormones (testosterone, estrogen testosterone).

Thyroid Gland

Thyroid hormones: thyroxine (T4) & tri-iodothyronine (T3) have 4 & 3 iodine atoms

TH stimulates metabolism throughout the body and increases body temperature

Parathyroid Glands

Make PTH

Regulates plasma calcium concentration

Low [Ca] causes more PTH release, which causes

Vitamin D activation, which causes more Ca uptake from gut

Less Ca loss in urine

Stimulation of osteoclasts, which dissolve bone matrix and put Ca into the circulation

Adrenal Glands

Adrenal Cortex

Mineralocorticoids: mainly aldosterone:

Aldo causes [Na] up by reducing the loss of Na in urine, [K] down

Aldo release is controlled by angiotensin (more ang.II causes more aldo release), which is controlled by renin (more renin causes more ang.II to be created), whichis controlled by sympathetic nerves and blood pressure (low BP and high symp nerve activity cause more renin release).

Whole system is called renin-angiotensin-aldosterone system. R-A-A system is a key slow and long-term regulator of blood pressure

Glucocorticoids

Cortisol, cortisone, corticosterone – mostly the first. All are steroid-backbone hormones.

Regulated by ACTH, which is regulated by CRH – see above. More ACTH causes more cortisol release. Cortisol feeds back to inhibit release of CRH and ACTH, so the system controls itself by negative feedback.

Stress stimulates more glucocorticoid release. Glucocorticoids helps keep blood glucose levels and blood pressure up. High levels of glucocorticoids inhibit immune system function and inhibit inflammatory responses. This can be good or bad. Prednisone is a potent synthetic analog of cortisol. May be prescribed to help during a flare-up of rheumatoid arthritis and other autoimmune diseases, or even for a bad case of poison ivy, but side effects of prednisone make its long-term use dangerous.

Adrenal Medulla

Epi & norepi – mediate fight-or-flight responses

Pineal Gland

Makes & secretes melatonin, which promotes sleep

Bright light inhibits melatonin release

Other Endocrine Organs

These organs have endocrine and non-endocrine functions. This list is not complete.

Pancreas

Insulin

Glucagon

Diabetes mellitus (as opposed to diabetes insipidus, a disease caused by absence of ADH)

Pancreas is also an exocrine organ – more in GI chapter

Gonads & Placenta

Make gonadal hormones: estrogen, progesterone, testosterone

Also make eggs & sperm

Heart: ANP

Kidneys: erythropoietin (EPO) & renin