Endocrine System

Range-Based Signals

1. Autocrines: chemicals that exert effects on same cells that secrete them

2. Paracrines: locally acting chemicals that affect neighboring cells

3. Hormones: long-distance chemical signals that travel through blood

Endocrine Organs

Hypothalamus: neuroendocrine organ

• Endocrine Tissue: pancreas, gonads, placenta

• Produce Hormones: stomach, intestine, heart, kidneys, skin, thymus, bone, adipose

Hormone Release Stimuli

1. Humoral: altered levels of critical ions/nutrients

2. Neural: neural input

3. Hormonal: another hormone (tropic)

• Controlled by - feedback

Posterior Pituitary

Action potentials travel down axons of hypothalamic neurons —> hormone release from axon terminals in posterior pituitary

• Made by hypothalamic neurons + stored in posterior pituitary

Oxytocin

Peptide (neurons in paraventricular nucleus of hypothalamus)

• Targets: uterus (contractions + labor) + breast (milk ejection)

• Stimulated: stretching of uterine cervix + breast suckling

• Inhibited: stress + lack of stimuli

Antidiuretic Hormone (ADH)

Peptide (neurons in supraoptic nucleus of hypothalamus)

• Targets: kidneys (reabsorb water from urine back into blood)

• Stimulated: increased blood solute concentration, low blood volume, pain

• Inhibited: hydration + alcohol

• Low: diabetes insipidus, High: syndrome of inappropriate ADH secretion

Anterior Pituitary

Hypothalamic hormones released into special blood vessels (hypophyseal portal system) control release of anterior pituitary hormones

Growth Hormone (GH)

Protein (somatotropic cells)

• Targets: liver, muscle, bone, cartilage, tissues (increases blood glucose + fat breakdown, growth effects mediated by IGFs)

• Stimulated: GHRH (low blood levels of GH, deep sleep, low blood glucose, exercise)

• Inhibited: increased GHIH (somatostatin), decreased GHRH, feedback inhibition by GH + IGFs

• Low: pituitary dwarfism, High: gigantism

Thyroid-Stimulating Hormone (TSH)

Glycoprotein (thyrotropic cells)

• Targets: thyroid gland (releases thyroid hormones)

• Stimulated: TRH + cold temp

• Inhibited: GHIH + feedback inhibition by thyroid hormones

• Low: hypothyroidism, High: hyperthyroidism

Adrenocorticotropic Hormone (ACTH)

Peptide (corticotropic cells)

• Targets: adrenal cortex (releases glucocorticoids + gonadocorticoids)

• Stimulated: CRH (peaks in morning)

• Inhibited: feedback inhibition by glucocorticoids

• High: Cushing’s disease

Follicle-Stimulating Hormone (FSH)

Glycoprotein (gonadotropic cells)

• Targets: ovaries (ovarian follicle maturation + estrogen production) + testes (sperm production)

• Stimulated: GnRH

• Inhibited: feedback inhibition by inhibin + estrogens in females, testosterone in males

• Low: failure of sexual maturation

Luteinizing Hormone (LH)

Glycoprotein (gonadotropic cells)

• Targets: ovaries (ovulation + ovarian production of estrogens + progesterone) + testes (testosterone production)

• Stimulated: GnRH

• Inhibited: feedback inhibition by estrogens + progesterone (females), testosterone (males)

• Low: failure of sexual maturation

Prolactin (PRL)

Protein (prolactin cells)

• Targets: breast secretory tissues (lactation)

• Stimulated: decreased PIH, estrogens, breast suckling

• Inhibited: PIH

• Low: poor milk production, High: inappropriate milk production + cessation of menstruation in females, impotence in males

Parathyroid Glands

• Up to 8

• Oxyphil cells + parathyroid cells that secrete parathyroid hormone (PTH)

Parathyroid Hormone (PTH)

1. Stimulates osteoclasts to digest bone matrix + release Ca2+ into blood

2. Enhances reabsorption of Ca2+ + secretion of PO43- by kidneys

3. Promotes activation of vitamin D by kidneys —> increased Ca2+ absorption by intestinal mucosa

• Increases blood Ca2+ levels

Thyroid Gland

Antagonist to PTH: inhibits osteoclast activity + Ca2+ release from bone matrix, stimulates Ca2+ uptake into bone matrix

• Parafollicular Cells: secrete calcitonin in response to high blood Ca2+ levels

• Follicular Cells: secrete thyroid hormone

Thyroid Hormone Regulation

• T3 + T4 transported by Thyroxin-Binding Globulins (TBGs) —> bind to target receptors

• Removal of I- converts T4 —> T3

• TH release regulated by - feedback w/ TSH

Thyroid Hormone Effects

1. Basal metabolic rate (BMR): O2 use, calorigenesis, sympathetic NS

2. Carb/lipid/protein metabolism: glucose catabolism, mobilizes fats, protein synthesis, liver's synthesis of cholesterol

3. GI system: GI motility, increases secretion of digestive juices

4. Cardiovascular system: normal heart functioning

5. Reproductive system: normal female reproductive ability + lactation

Adrenal Gland

Medullary chromaffin cells:

• Synthesize catecholamines epinephrine (80%) + norepinephrine (20%) from tyrosine

• Increases BP, diverts blood to heart + skeletal muscle, increases blood glucose levels

Mineralcorticoids (Aldosterone)

Produced by zona glomerulosa

• Targets: kidneys (increase blood Na+ levels + decrease blood K+ levels)

• Stimulated: renin-angiotensin-aldosterone system, elevated blood K+ levels, ACTH

• Inhibited: increased blood volume + pressure, decreased blood K+ levels

• Low: Addison's disease, High: Aldosteronism

Glucocorticoids (Cortisol)

Produced by zona fasciculata

• Targets: body cells (gluconeogenesis, hyperglycemia, energy metabolism, protein catabolism, resist stress, depress inflammation + immune responses)

• Stimulated: ACTH

• Inhibited: feedback inhibition exerted by cortisol

• Low: Addison's disease, High: Cushing's disease

Gonadocorticoids (Androgens: Testosterone/Estrogens)

Produced by zona reticularis

• Targets: female libido, pubic + axillary hair in females, source of estrogens after menopause

• Stimulated: ACTH

• High: masculinization of females (adrenogenital syndrome)

Pineal Gland

Hangs from roof of 3rd ventricle in diencephalon

1. Pinealocytes: secrete melatonin

• Synthesized from serotonin, blood levels rise + fall in diurnal cycle (peaks at night)

2. Melatonin: prevention of oxidative damage in cells, antigonadotropic effect, targets hypothalamus to influence rhythm (body temp, sleep, appetite)

Pancreas

Insulin + glucagon regulate blood glucose levels

• High blood glucose: pancreas releases insulin —> liver produces glycogen —> blood glucose falls

• Low blood glucose: pancreas releases glucagon —> liver produces glucose —> blood glucose rises

Other Hormone Secreting Organs

1. Stomach, intestine, heart, kidneys, skin, thymus, bone, adipose tissue

2. Gonads

• Ovaries: estrogens, progesterone, inhibin —> maturation of reproductive organs, 2nd sex characteristic, breast development, menstrual cycle

• Testes: androgens (testosterone) + inhibin —> maturation of reproductive organs, 2nd sex characteristic, sex drive, sperm production

• Placenta: temporary endocrine organ (secretes estrogens, progesterone, human chorionic gonadotropin (hCG)

Hormones of Other Organs

1. Adipose Tissue: leptin, resistin, adiponectin —> suppresses appetite + increases energy expenditure in brain, resistin antagonizes insulin + adiponectin enhances in fat/muscle/liver

2. Stomach: gastrin, ghrelin

3. Duodenum: secretin, CCK, GIP

4. Heart: ANP —> inhibits Na+ reabsorption + renin release in kidney, inhibits secretion of aldosterone + decreases BP in adrenal cortex

5. Kidney: EPO —> stimulates production of RBCs

6. Skeleton: osteocalcin —> increases insulin production + sensitivity

7. Skin: cholecalciferol —> active transport of Ca2+ across cell membranes of small intestine

8. Thymus: thymulin, thymopoietins, thymosins —> T-lymphocyte development + immune responses