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