The length of time a cell takes to respond to an extracellular signal can vary greatly, depending on what needs to happen once the message has been received.

Some extracellular signals act swiftly: acetylcholine can stimulate a skeletal muscle cell to contract within milliseconds and a salivary gland cell to secrete within a minute or so. Such rapid responses are possible because, in each case, the signal affects the activity of proteins that are already present inside the target cell, awaiting their marching orders.

Other responses take more time. Cell growth and cell division, when triggered by the appropriate signal molecules, can take many hours to execute.

This is because the response to these extracellular signals requires changes in gene expression and the production of new proteins.