Second messengers are molecules that are generated inside a cell in response to an extracellular signal and that then trigger a cascade of intracellular events. They are called "second messengers" because they are not the original signal, but rather a signal that is generated in response to the original signal.

Second messengers are typically small molecules that can diffuse across the cell membrane. Some of the most common second messengers include calcium ions (Ca2+), diacylglycerol (DAG), and cyclic adenosine monophosphate (cAMP).

When an extracellular signal binds to a receptor on the cell surface, it can activate a G protein. G proteins are membrane-bound proteins that can bind to GTP (guanosine triphosphate). When a G protein binds to GTP, it becomes activated and can then bind to and activate an effector protein.

Effector proteins are enzymes or other proteins that can trigger a cascade of intracellular events. For example, activated Ca2+ channels can lead to an increase in intracellular Ca2+ concentration. Increased intracellular Ca2+ concentration can then activate a variety of cellular processes, such as muscle contraction, neurotransmitter release, and gene expression.

Second messengers are an important part of cell signaling. They allow cells to respond to extracellular signals in a rapid and coordinated manner.