Topic 7: Endocrine System Flashcards
Endocrine System: Overview
- regulates growth, reproduction, metabolism (long-term events)
- glands and tissues secrete hormones which travel in blood to target cells (= cells with specific receptors for that hormone)
- bind to receptors and change cell activity
- receptors (proteins) found:
- on the cell membrane
- intracellular (nuclear)
Hormone Types
- Water soluble
- Lipid soluble
Water Soluble Hormones
- peptides, proteins, catecholamines (= 1st messenger)
- steps:
- hormone binds to cell membrane receptors (do not enter the cell for their actions)
- hormone-receptor complex activates membrane proteins e.g. G-proteins
- G-proteins then activate 2nd messenger systems
e. g. cAMP, Ca2+
example using cAMP (cyclic adenosine monophosphate) as second messenger
- hormone binds to cell-surface receptor and activates a G-protein
- G-protein activates adenylate cyclase (membrane protein)
- adenylate cyclase converts ATP to cAMP (=second messenger) ∴ ⇑ [cAMP]
- cAMP activates protein kinases (in cytosol)
- protein kinase acts on other proteins (phosphorylates) to alter their activity ∴ changes cell activity
e. g. epinephrine on liver cells (activates cAMP) ⇒ causes breakdown of glycogen to glucose ⇒ released to blood
Why use 2nd messenger systems?
- hormone can’t enter cell (water soluble)
b) rapid acting (enzymes already present - just activate) - 1 hormone molecule ⇒ many enzyme molecules activated ⇒ multiplies signal
- limited - messenger broken down or removed
e. g. cAMP broken down by phosphodiesterase in the cell
Lipid Soluble Hormones
- steroids (e.g. cortisol) and thyroid hormones
- trigger protein synthesis
- takes time ∴ slow, but long lasting response
Lipid Soluble Hormones steps for action
- enter target cell and bind to intracellular (nuclear) receptors in cytosol or nucleus
- hormone-receptor complex binds to a specific region on DNA (activates genes) ⇒ starts gene transcription – produces messenger RNA (mRNA)
- mRNA attaches to ribosomes to produce proteins (translation)
Regulation of Hormone Secretion into Blood
stimuli acting on an endocrine gland may be:
- Humoral Stimulus
- Neural Stimulus
- Hormonal Stimulus
Humoral Stimulus
stimulus = ions/nutrients
e. g.1: ↑blood glucose (after eating carbs)
- pancreatic β-cells (of Islets of Langerhans) detect glucose and release insulin ⇒ ↓blood glucose
e. g.2: ↓Blood Ca2+
- parathyroid gland detects ↓Ca2+ and releases parathyroid hormone (PTH) ⇒ ↑bone resorption (breakdown) by - ↓osteoblast activity and ↑osteoclast activity ⇒ ⇑ blood Ca2+
Neural Stimulus and Hormonal
-see notes for diagram
Stress
- any extreme external or internal stimulus e.g. surgery, infections, strong emotions, exams
- triggers a set of body changes called General Adaptation Syndrome
- all co-ordinated directly or indirectly by the hypothalamus
Stress Phases
- Phase 1: Alarm Reaction (Fight or Flight Response)
- Phase 2: Resistance Reaction
- Phase 3: Exhaustion
Phase 1: Alarm Reaction (Fight or Flight Response)
immediate = NS
effects of SNS + endocrine:
-⇑ blood glucose (glucose)
-SNS inhibits insulin release
-Epi, NE trigger conversion of glycogen to glucose in the liver
-⇑ HR, force of contraction
-⇑ respiration rate
-⇓ blood flow to skin + abdominal viscera
-∴ more available to skel. and card. m., and brain (O2 and glucose to working organs)
-⇓ digestion, urine production
Phase 2: Resistance Reaction
- long term ⇒ endocrine
- permits recovery from the effects of 1) (tissue repair, etc) or response to longer term stress (e.g. starvation)
- hypothalamic hormones initiate phase 2)
Phase 2: Resistance Reaction-hypothalamic hormones initiate phase 2)
- GH
- Cortisol
-GH
stimulates growth (protein production), cell reproduction
-Cortisol
- released within 30 sec of the stress but the response not for hours – steroid hormone - acts at nuclear receptors
- inhibits insulin release
Phase 2: Resistance Reaction-Release of hormones cause
- ⇑ blood glucose:
- inhibition of: immune system, bone formation, formation of CT (delayed healing)
- release of aldosterone and antidiuretic hormone (ADH)
⇑ blood glucose
- liver stimulated to produce new glucose from fats and later from proteins
- little insulin (because of inhibition) - glucose not taken up well, especially by skeletal muscle (at rest) and adipose tissue. ∴:
- glucose spared for use by NS
- metabolism of non-nervous tissue directed to use fats for energy (control = GH, cortisol) - if stress continues, cortisol inhibits GH release and proteins are then also used
- Overall: ⇑ blood FA and aa ⇒ energy (except brain)
release of aldosterone and antidiuretic hormone (ADH)
reduces salt and water loss at kidney to maintain blood volume
Phase 2: Resistance Reaction-Long term effects
⇓ weight, ⇑ bp, ⇑ HR, immune suppression (cortisol), ⇓ bone density, ⇑ risk of type 2 diabetes (because of ⇑ blood glucose)
Phase 3: Exhaustion
Results from:
- depletion of body resources i.e. lipid reserves
- loss of K+ (aldosterone effect)
- damage to organs (heart, liver, kidneys)
Functions of Testosterone
- development of organs of reprod. tract + 2° sex characteristics
- stimulates bone growth at epiphyseal plate (converted in bone to estrogen (E) to stop growth = closure of plate)
- promotes protein anabolism
- directly stimulates spermatogenesis
♀ Reproductive Hormones-Hormone Functions
- FSH
- LH
- Estrogen
- Progesterone (P)
FSH
- stimulates 1° to become 2° follicle
- inhibited by ⇑ progesterone (P) (∴ FSH ⇑ when P ⇓)
LH
- stimulates Estrogen (E) production from theca + granulosa cells of follicle
- surge in LH ⇒ ovulation and then formation of corpus luteum from remnants of follicle
- in follicular phase - E from 2° follicle rises for a few days ⇒ stim LH release (via GnRH) ⇒ stim. follicle to ⇑ E secretion etc (+ve feedback) ⇒ leads to LH surge
- luteal phase - P inhibits LH release
Estrogen
- required for ovulation
- development of of 2° sex characteristics
- stim. growth of + maintains endometrium
- ⇑ bone growth, closure of epiphyses
Progesterone (P)
- from corpus luteum
- prepares uterus for pregnancy
Ovarian/Uterine Cycle (~28 days)
- Days 1-14
- Day 14: Ovulation
- Days 15-28
- If fertilization occurs
- If NO fertilization
Days 1-14
- Ovary: Follicular (preovulatory) phase
- Uterus: (at same time as follicular phase)
Ovary: Follicular (preovulatory) phase
- Early on: P low ∴ LH + FSH secreted - some 1° follicles ⇒ 2° follicles (due to FSH)
- follicles secrete E ∴ blood E rises
- Later on: one (usually) 2° follicle becomes vesicular follicle
Uterus: (at same time as follicular phase)
Menstrual phase (days 1-5): -stratum functionalis shed (outer layer of endometrium) and denuded areas bleed -∴ menstrual flow = blood, cells, and secretions Proliferative phase (days 6-14) -E ⇒ repair + proliferation of stratum functionalis (due to mitosis in stratum basalis)
Day 14: Ovulation
- due to LH surge
- LH triggers:
- completion of meiosis I ⇒ 2° oocyte
- rupture of vesicular follicle with release of 2° oocyte
Days 15-28
Ovary: Luteal Phase
-High P from corpus luteum inhibits GnRH (∴ LH + FSH) ∴ no follicles develop
Uterus: Secretory Phase
-Progesterone from corpus luteum:
-prepares endometrium for implantation - becomes vascular, thick, and stores glycogen
-inhibits uterine contractions
If fertilization occurs
- placenta secretes human chorionic gonadotropin (hCG) - hCG maintains corpus luteum (similar structure to LH)
- corpus luteum ⇒ P, E for about 6 weeks, then the placenta takes over (secretes P, E)
- FSH, LH inhibited by high P (no new follicles develop)
If NO fertilization
corpus luteum ⇒ corpus albicans (no hCG, low LH)
∴ ⇓ P and E ∴:
-no longer inhibit LH, FSH ⇒ LH, FSH ⇑
-no longer maintain endometrium ⇒ menstruation
Contraceptives
- Oral Contraceptives
- Implants
- Morning After Pill
Oral Contraceptives
high E + P ⇒ inhibit GnRH secretion ∴ low FSH, LH (mimics luteal phase) - no follicle maturation, no ovulation
Implants
e.g. progestin – similar mechanism to oral contraceptives
Morning After Pill
- high E and progestin or progestin only
- prevents implantation, ovulation or fertilization
Placenta
-formed from chorion (fetus) and endometrium (maternal)
-blood vessels of mother and fetus in close proximity (no blood mixing)
functions:
-exchange site
-secretes hormones
exchange site
- gases, nutrients/wastes, hormones, antibodies (passive immunity)
- drugs e.g. alcohol, morphine, nicotine
- viruses e.g. measles polio
secretes hormones (placenta)
Estrogen + Progesterone
hCG
-maintains corpus luteum for ~6 weeks post-fertilization
-detected by pregnancy tests
-stimulates testosterone secretion by fetal testes
