From ligand binding to cellular response
A G-protein-coupled receptor spans the cell membrane seven times. When a ligand binds the extracellular face, the receptor changes shape and acts on an associated G-protein inside the cell, prompting it to swap GDP for GTP and split into active subunits. Those subunits then switch effector enzymes on or off, which raise or lower the level of a second messenger such as cyclic AMP or calcium, and the change in second messenger drives the downstream cellular response.
Because one activated receptor can switch on many G-proteins and each effector enzyme makes many second-messenger molecules, the cascade amplifies a small chemical signal into a large intracellular effect.
Why this pathway is such a common drug target
GPCRs sense an enormous range of signals, including hormones, neurotransmitters, light, and odorants, which is why a large share of marketed medicines act on them. Beta-blockers, antihistamines, opioids, and many others work by either mimicking or blocking ligands at these receptors.
The cascade also self-regulates: receptors are desensitised and internalised after prolonged stimulation, which contributes to tolerance for some drugs. This is general educational information about receptor biology, not medical advice.