Body messaging: The endocrine systems

Abstract

Cells of multicellular organisms communicate via chemical messengers secreted into the extracellular fluid. This form of communication can be found in even the simplest animals like sponges and is phylogenetically older than neural transmission (Hartenstein 2006). Some of the diffusible signals travel short distances and influence cells in the vicinity (paracrine signaling), whereas others reach their target cells via the circulatory system (endocrine and neuroendocrine signaling). Transported within body fluids, these highly specific messengers, called hormones (from the Greek word hormaein, to excite), reach most cells throughout the body, where they bind to specific receptor proteins. Their interaction with target cells depends on the presence and sensitivity of these specific receptors. By binding to different receptor subtypes, most hormones are able to trigger more than one and in many cases even opposing responses. In interaction with the nervous system, hormones coordinate the activities of many different cells and regulate growth, metabolic processes, water balance, immune function, digestion, and reproduction. The ability to cooperatively respond to environmental (exteroceptive) or internal (interoceptive) stimuli enables the body to maintain a dynamic equilibrium, the so-called homeostasis.