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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


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


Activate the same intracellular signaling pathway


Affect the same function

For example

Adrenaline activates adrenergic receptors and glucagon activates glucagon receptors


Their second messenger pathway converges at cAMP


Same signaling downstream this


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


Decreases contractility

  • In pancreatic acinar cells:

Follow IP3-DAG pathway


Induces secretion of digestive juice


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