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Intracellular signaling cascades or pathways relay extracellular signals to the cell interior.

Multiple intracellular signaling cascades, such as the cyclic AMP, MAPK/ERK, and IP3/DAG pathways, transmit and amplify the signals, leading to cellular responses including changes in gene expression or cell metabolism. 

For example, the cyclic AMP signaling cascade can be triggered by various external ligands, such as adrenaline, that bind to transmembrane G protein-coupled receptors.

The binding creates a conformational change in the receptor that enables GDP/ GTP exchange and dissociation of the G protein subunits.

This stimulates the enzyme adenylyl cyclase to convert ATP  into cyclic AMP. Once stimulated, adenylyl cyclase can generate multiple cyclic AMP molecules, resulting in the amplification of the signal.

Cyclic AMP molecules can activate protein kinase A or PKA molecules. This enzyme phosphorylates many other proteins,  including transcription factors that regulate gene expression.