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.
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