The only means by which an extracellular chemical messenger can bring about a desired intracellular response is to activate a second messenger system.

Question

Quizplus · Accepted Answer

While the use of second messenger systems is a common mechanism by which extracellular chemical messengers can induce intracellular responses, it is not the only means. Some messengers directly impact ion channels or receptors on the cell surface, while others can enter the cell and interact with intracellular proteins.

The Only Means by Which an Extracellular Chemical Messenger Can