SN6
SHORT NOTE
5M
★ High-yield
Enumerate the second messenger systems and describe the cyclic AMP pathway.
Definition
Second messengers = intracellular signalling molecules generated in response to a first messenger (hormone or neurotransmitter) binding to a surface receptor, which then amplify and transmit the signal inside the cell.
WHY Needed
- Water-soluble hormones cannot cross the lipid bilayer
- Provide enormous amplification — one hormone molecule → millions of product molecules
Camp System
Hormone + receptor → Gs protein activated → Adenylyl cyclase → ATP → cAMP → Protein kinase A → Phosphorylates enzymes → Response
- Terminated by phosphodiesterase
- Hormones — ACTH, TSH, LH, FSH, glucagon, PTH, ADH (V2), calcitonin, β-adrenergic
- Inhibited via Gi — α2-adrenergic, somatostatin
Ip₃ / Dag System
Hormone + receptor → Gq protein → Phospholipase C → PIP2 → IP3 + DAG → IP3 → releases Ca2+ from ER; DAG → protein kinase C → Response
- Hormones — GnRH, TRH, oxytocin, ADH (V1), angiotensin II, α1-adrenergic
Other Second Messenger Systems
| System | Mechanism | Examples |
|---|---|---|
| CGMP | Guanylyl cyclase → cGMP | ANP, nitric oxide, rod photoreceptors |
| Ca2+–calmodulin | Ca2+ binds calmodulin | Smooth muscle contraction |
| Tyrosine kinase | Receptor autophosphorylation | Insulin, growth factors |
| JAK–STAT | Cytokine receptor associated kinase | Growth hormone, prolactin, interferons |
| Intracellular receptor | Direct gene transcription | Steroids, thyroxine, vitamin D |
Applied Aspects
- Cholera toxin → permanently activates Gs → ↑↑ cAMP → massive Cl− and water secretion
- Pertussis toxin → inactivates Gi → ↑ cAMP
- Sildenafil → inhibits phosphodiesterase-5 → ↑ cGMP → vasodilatation
- Theophylline and caffeine → inhibit phosphodiesterase → ↑ cAMP
All questions in General Physiology & Body Fluids
- Q1Describe the structure of the cell membrane. Classify the mechanisms of transport across it and describe passive transport in detail. ★
- Q2Define active transport. Classify it and describe the sodium–potassium pump with its functions. Add a note on secondary active transport. ★
- Q3Enumerate the body fluid compartments with their volumes and ionic composition. Describe the methods used to measure them. ★
- Q4Define resting membrane potential. Describe its ionic basis and the factors responsible for it. Add a note on the Nernst equation. ★
- Q5Define homeostasis. Describe the components of a control system and explain negative and positive feedback with examples. ★
- SN1Define osmosis and osmotic pressure. Explain tonicity with examples. ★
- SN2Write a short note on the Gibbs–Donnan equilibrium.
- SN3Define oedema. Describe the Starling forces and enumerate the mechanisms by which oedema is produced. ★
- SN4Classify cell junctions and describe their functions.
- SN5Write a short note on facilitated diffusion. ★
- SN6Enumerate the second messenger systems and describe the cyclic AMP pathway. ★
- SN7Define apoptosis. Differentiate it from necrosis and describe its pathways.