When a reaction occurs in a cell, it receives signals transmitted from outside the cell, and then the cell responds according to the information it received. Signal transduction refers to a series of enzymatic reaction pathways that exert extracellular molecular signals into the cell through the cell membrane. These extracellular molecular signals (ligands) include hormones, growth factors, cytokines, neurotransmitters and other small molecule compounds. The whole procedure from receiving external signals from cell receptors to making a comprehensive response is not only a signal transduction process, but also a process of gradually amplifying external signals.
Receptor proteins convert extracellular signals into intracellular signals, which are amplified, scattered and regulated by signal cascades and finally produce a series of comprehensive cellular responses including regulation of downstream gene expression, changes of intracellular enzyme activities, cytoskeleton type and DNA synthesis. Not all of these changes are caused by one single signal, some responses can also be generated by different combinations of several signals.