The small molecule acetylcholine acts to signal different cellular responses in many different cell types, including skeletal muscle cells, heart muscle cells, cells lining blood vessels, liver cells, and pancreatic cells. In each case, there is a membrane receptor that specifically binds acetylcholine. Upon binding, acetylcholine triggers a _______ to elicit a specific cellular response.

The signal transduction pathway occurs when a molecule triggers an orderly sequence of chemical reactions within a cell. All of these reactions are mastered by enzymes, after a molecule activates the first reaction through signals or stimuli. Reading the text in the question above, we can see that, after binding, acetylcholine triggers a signal transduction pathway, creating a sequence of reactions, to elicit a specific cellular response.