What is a small intracellular ligand generated in response to an extracellular signal called?

The term for a small intracellular ligand generated in response to an extracellular signal is known as a second messenger. These molecules play a crucial role in signal transduction pathways. When an extracellular signal, such as a hormone or neurotransmitter, binds to a receptor on the cell surface, it often triggers the generation of secondary messengers inside the cell. These second messengers then amplify the original signal and initiate a cascade of cellular responses.

Common examples of second messengers include cyclic AMP (cAMP), calcium ions (Ca²⁺), and inositol triphosphate (IP3). They are essential for relaying signals from cell surface receptors to target molecules within the cell, thus facilitating processes such as metabolic regulation, gene expression, and cell growth.

The other terms referenced in the question represent different concepts: hormones are signaling molecules that can act over longer distances; neurotransmitters are chemical messengers used specifically in neural communication; and enzymes are biological catalysts that facilitate biochemical reactions but do not function as signaling molecules in this context. Thus, the definition and function of second messengers align perfectly with the question's requirements.