Summary of the major signalling pathways used by cells to regulate cellular processes
Cells have a number of signalling systems that are capable of responding either to external stimuli or to internal stimuli. In the case of the former, external
stimuli acting on cell-surface receptors are coupled to transducers to relay information into the cell using a number of different signalling pathways
(Pathways 1–17). Internal stimuli derived from the endoplasmic reticulum (ER) or from metabolism activate signalling pathways independently of
external signals (Pathways 18 and 19). All of these pathways generate an internal messenger that then acts through an internal sensor to stimulate
the effectors that bring about different cellular responses. As described in the text, the names of these signalling pathways usually reflect a major
component(s) of the pathway.