Cell signaling systems link the extracellular environment to changes in cellular behavior. Several classes of receptor proteins sense different types of extracellular inputs and, in response, stimulate a variety of intracellular biochemical processes. The common feature of these signaling systems is that they amplify signals through enzyme-mediated reactions. This signaling occurs through ligand binding to receptor proteins, which causes conformational changes in the receptor proteins that affect the biochemical activities of intracellular proteins. The G protein–coupled receptor (GPCR) illustrates the specific GPCR signaling in human sensory pathways required for sight, smell, and taste. The insulin receptor, which is a type of receptor tyrosine kinase, illustrates the downstream signaling pathways initiated by insulin binding to its cognate receptor. Lastly, glucocorticoid receptor (GR) signaling (GR is a type of nuclear receptor) reduces the physiologic effects of stress and inflammation.