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Intracellular Signaling or Second Messenger System



  • First Messenger: Takes signal from the cell that secreted it, to the receptor of the target cell.

  • Second Messengers: Take the signal from the receptor to the protein that initiates the response.

Transfer of Signal from Receptor to the Target Protein

  • By series of second messengers downstream of the receptor.

  • Upstream messenger passes the signal to the downstream messenger.

  • Transfer of signal occurs in form of activation or inactivation of the downstream messenger.

  • Finally, the signal reaches the target protein that initiates the ultimate response.

  • Type of Target Proteins and Responses are as follows:

Transport Protein

  • Alters ion transport across the cell membrane.

Metabolic Enzyme

  • Alters metabolic reactions occurring in the cell.

Gene Regulatory Protein

  • Alters gene expression.

Cytoskeletal Protein

  • Alters the shape or movement of the cell.

Cell Cycle Protein

  • Alters growth or division of the cell.


Amplification of Signal

One upstream messenger can activate or generate multiple downstream messengers

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Signal is amplified as it travels towards the target protein.


Integration of Signals

Converging Signals
  • Allows different first messengers to affect the same response of the cell.

Different ligand/receptors

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Activate the same intracellular signaling pathway

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Affect the same function


For example

Adrenaline activates adrenergic receptors and glucagon activates glucagon receptors

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Their second messenger pathway converges at cAMP

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Same signaling downstream this

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Affect same function e.g. increase in triacylglycerol breakdown


Diversify Signals
  • Allows same first messenger to produce different responses in different cells.

  • By activating different intracellular signaling pathways in different cells.

  • For example, Acetylcholine:

    • In heart muscle cells:

Follows cAMP pathway

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Decreases contractility

  • In pancreatic acinar cells:

Follow IP3-DAG pathway

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Induces secretion of digestive juice

 

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