What is considered the 'first messenger'?

The term 'first messenger' typically refers to extracellular signaling molecules, such as hormones or neurotransmitters, that transmit signals from one cell to another. These molecules bind to specific receptors on target cells, initiating a cascade of intracellular events. This process is crucial for regulating various physiological functions and responses in the body. Examples of first messengers include insulin, adrenaline, and neurotransmitters like dopamine.