Signalling pathways use different messenger systems acting through specific sensors and effectors to control a great variety of cell types (Module 7: Table cell inventory). Each specialized cell type has developed control mechanisms to suit their particular functions. For example, skeletal and cardiac muscle cells have very rapid signalling mechanisms capable of driving contraction within milliseconds. Smooth muscle cells tend to act more slowly, and are far more diverse in both their function and control mechanisms. Some cells contract in a conventional way in response to an external agonist, while others have more complex control mechanisms responsible for maintaining the tonic contraction of the smooth muscle cells surrounding blood vessels.
Another example is cell secretion, where there is similar diversity in function and control. Preformed secretory products packaged into vesicles are released through a process of exocytosis. In exocrine glands and the intestine, there is a process of fluid secretion that depends upon the active transport of ions to provide the driving force for a parallel flow of water. Secretion is evident in many different cell types and cellular processes, such as transmitter release during presynaptic events in neurons, enzyme and fluid secretion by pancreatic acinar cells, release of insulin by insulin-secreting β-cells and inflammatory mediators by mast cells.
Many of these mammalian cell types are controlled by more than one signalling pathway, and the object here is to understand how signalling pathways and their downstream sensors and effectors function to bring about a change in cellular activity in some typical examples of specialized cell types.
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