Second Messenger Systems
All cells of the body respond to their environment. For most cell types, the responses can be both general and be related to the cell's particular function. An example of a general response is the regulation of sugar utilization: When sugar is plentiful, glucose is polymerized to glycogen, a storage form. When sugar is scarce, the biochemistry of the cells will be adjusted so that glycogen is broken down to make sugar available. Stimulation of gland cells to release their secretion is a specialized response. In neurons the regulated specialized functions include properties of ion channels; availability of synaptic vesicles for exocytosis (a process called mobilization); and responsiveness of postsynaptic receptors. These functions can be altered transiently (in a time frame of minutes to hours) or for much longer periods, if not permanently. Short-term changes are produced by modifying existing proteins in the cell, modifications that are rapidly reversed when the second-messenger is removed. Long-term changes result when the second-messenger pathway changes transcription, ultimately producing a change in gene expression and the production of new proteins.
Many of these responses, both general and specialized, are produced by signal transduction, a process in which an extracellular stimulus activates a specific receptor on the surface of the responding cell.
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