3-4/35 The human body is composed of billions of cells, each with a distinct function. However, the function of cells is tightly coordinated and integrated by external chemical signals, including hormones, neurotransmitters, growth factors, odorants, and products of cellular metabolism that serve as chemical messengers and provide cell-to-cell communication. Light and mechanical and thermal stimuli are physical external signals that also coordinate cellular function. Chemical and physical messengers interact with receptors located in the plasma membrane, cytoplasm, and nucleus. Interaction of these messengers with receptors initiates a cascade of signaling events that mediate the response to each stimulus. These signaling pathways ensure that the cellular response to external messengers is specific, amplified, tightly regulated, and coordinated. This chapter provides an overview of how cells communicate via external messengers and discusses the receptors and intracellular signaling pathways that process external information into a highly coordinated cellular response. In subsequent chapters, details on signaling pathways in the nervous system, muscle, cardiovascular system, respiratory system, gastrointestinal system, kidneys, and endocrine system wiff be discussed in greater detail.