Basic terminology – In classic cell signaling, a hormone or extracellular ligand is best described as which “messenger” in the communication chain?

Introduction:
Signal transduction uses a hierarchical terminology: the extracellular signaling molecule, the receptor and associated enzymes, and intracellular second messengers. This question focuses on the correct label for the ligand itself in this sequence.

Given Data / Assumptions:

• Ligands (hormones, neurotransmitters, growth factors) originate outside the cell or at its surface.
• Second messengers are generated inside the cell after receptor activation.
• Downstream effectors (e.g., kinases) interpret second messengers.

Concept / Approach:
The extracellular ligand that binds a receptor is termed the first messenger. Receptor engagement triggers intracellular enzymes to produce second messengers such as cAMP, cGMP, IP3, DAG, and Ca2+. These propagate the message to effectors that alter cell behavior.

Step-by-Step Solution:

Verification / Alternative check:
Canonical GPCR signaling: epinephrine (first messenger) → β-adrenergic receptor → adenylyl cyclase → cAMP (second messenger) → PKA activation, confirms the terminology.

Why Other Options Are Wrong:

• Second messenger: reserved for intracellular mediators.
• Both/none/third messenger: do not match standard nomenclature.

Common Pitfalls:
Calling Ca2+ or cAMP “hormones”; they are intracellular messengers, not extracellular ligands.

Final Answer:
First messenger