Final answer:
The intracellular signal molecule is called a second messenger.
Step-by-step explanation:
The intracellular signal molecule that translates a signal from a neurotransmitter or hormone into an intracellular response is called a second messenger. In this process, the neurotransmitter or hormone, referred to as the first messenger, binds to a receptor protein on the extracellular surface of the cell, which then initiates the activity of a G protein. The G protein physically moves from the receptor protein to an effector protein, activating it. The effector protein catalyzes the generation of a new molecule, known as the second messenger, that serves as the intracellular mediator of the signal.