Week 1 - The Endocrine System, Endocrine Organs and Their Hormones

Question 1

8 glands

How is the endocrine system organised

Answer 1

• 8 PRIMARY (main) endocrine glands
  - glands are ductless (exocrine glands release hormones via ducts)
  - glands control homeostatsis (i.e. gorwth, metabolism)
• 4 secondary glands (= secreting hormone is NOT their main function)
  - e.g. skin, heart, GI tract, kidneys

8 Primary glands
- Pituitary gland
- hypothalamus (in brain) links CNS to endocrine system, it releases hormones to control this gland
- Thyroid gland
- Parathyroid gland
- Pancreas (largest)
- Adrenal gland
- Ovaries / Testes
- Pineal gland (smallest)
- Thymus

Question 2

What is the function of the endocrine system

Answer 2

• to control + maintain physiological functions via the release of hormones
  
  • prevent endocrine disorders (due to imbalance ~ hypo / hyper secretion or hyporesponsiveness of target)
    - disturbance in hormone release patten will affect homeostasis

Question 3

What are hormones + what do they do

Answer 3

WHAT THEY ARE:
- biologically active substance produced + secreted by glands
- trigger response in organs + tissues by binding to receptors
endocrine hormones = have biological effect far away

WHAT THEY DO:
- alter rate of enzyme mediated reactions
- stimulate mitosis
- control movement across plasma membrane (by changing permeability or potential)
- induce secretory activity
- regulate rate of gene expression

Question 4

Describe the regulation of hormone synthesis and secretion

Answer 4

Synthesis:
- can be produced synthetically
- need to ensure don’t have similar shape to other molecules or will bind to other receptors = cross reactivity = side effects

Secretion:
- secreted from glands into extracellular fluid then into blood stream then to target organ / tissues

Question 5

List the 3 classifications of hormones

Answer 5

Classified according to structure, chemical nature, MoA, effects, stimulation of endocrine glands

1. Amine Hormones
   - derived from tyrosine (e.g. dopamine, adrenaline)
   - derived tryptophan (e.g. melatonin)
   - have a NH2 group
2. Steroid Hormones
   - lipid-derived (lipohphillic)
   - e.g. testosterone, steroids, cortisol
3. Peptide Hormones
   - are lipophobic
   - 2 to 50 a.acids
   - e.g. endorphin, glucagon

Question 6

Describe the MoA for steroid hormones

Answer 6

• lipid-derived (lipohphillic)
  - require protein transporters to be carried around in blood (separates from carrier before entering cell + binding to receptor in cell cytoplasm)
• NOT stored, synthesised on demand

1. Diffuse into cytosol or nucleus
2. act as transcription factor (in nucleus) to change gene activity by
   - repressing or activating transcription rate
   - lag period (before effect is seen due to complex taking time to translocate to nucleus)

How is Steroid synthesis regulated
- negative feedback loop (↑ transcription = ↓ synthesis / release)
- phosphorylation (stop transcription)
- ligand binding to transcription factors

Question 7

Describe the MoA for peptide / protein hormones

Answer 7

• cause effect by binding to receptors on extracellular membrane
  - have cellular action mechanism
• are lipophobic
  = have to bind to extracellular surface
• most work via G-protein coupled receptors, other via tyrosine kinase receptors (e.g. insulin)

Question 8

Describe how hormones are synthesised

Answer 8

1. Hormones begin as PROHORMONES
   - pro hormones that encode more than one hormone is called PRE-PROHORMONES
2. PROHORMONE is processed into HORMONE and other peptides
   - e.g. proinsulin (precursor) is cleaved into insulin (A and B chain) and c-peptide chain

PROHORMONES:
- allow transport + storage of hormone in inactive form
- transformation of pro hormone may occur before or after storage

Question 9

Describe the mechanism of action for the types of hormones

Humoral, Hormonal and Neural Release

Answer 9

1. Humoral Trigger
   hormone release in response to changes in extracellular fluid e.g. substance level (ion conc.) in blood
   Example:
   - decrease in Ca2+ levels in blood
   - causes parathyroid glands to release PTH (parathyroid hormone) into blood
   - increases Ca2+ releases by stimulating / binding to receptors on Ca2+ stores
2. Neural Release
   hormone release stimulated by neurotransmitter signals in the nervous system
   Example:
   - sympathetic system releases noradrenaline which stimulates hormone release by adrenal gland
3. Hormonal Release
   when one hormone causes increase or decrease in response to another
   Example:
   - hypothalamus releases hormone that stimulates anterior pituitary gland
   - anterior pituitary gland secrets hormones that can cause thyroid, adrenal or testes gland to secrete hormone
   -

Question 10

How is target cell activated

Answer 10

• blood levels of hormone (how much hormone released)
• affinity between receptor + hormone
• no. of receptors on / in target cell