Cells are constantly engulfed in an environment of electrical and chemical stimuli. Specific receptors enable cells to differentiate among the stimuli and relay a specific message that leads to a defined cellular response. The process that translates the stimulus-receptor interaction into an intracellular chemical message is termed signal transduction. Signal transduction pathways are involved in the regulation of cell function in four broad physiological functions: 1) Reproduction, 2) Growth and Development,. 3) Maintenance of Internal Environment (e.g. fluid volume, blood pressure, electrolyte balance, etc.) and 4) Regulation of Energy Availability (i.e. an organism must be able to convert the calories contained in food into energy).