The complexity of G protein signalling

Although it is usually pointless to point out the complexity of biological systems, here it may be appropriate. 

G protein-coupled receptors function as adapters between the virtually boundless multitude and variety of extracellular hormone and neurotransmitter signals and the lower (yet still considerable) number of intracellular G-proteins. Therefore, it is very common to have receptors for multiple transmitters or hormones converge onto the same type of G protein and thus trigger the same response. E g., glucagon and epinephrine both activate adenylate cyclase in the liver, through separate receptors but the very same G protein. 

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