Which receptor is a ligand-gated ion channel?

• A. Nicotinic receptor
• B. Beta-adrenergic receptor
• C. Adenosine receptor
• D. Tyrosine kinase receptor

Answer: (A)

Ligand-gated ion channels open an ion pore in the membrane right after a neurotransmitter binds, producing a rapid change in membrane potential. The nicotinic acetylcholine receptor is a classic example of this kind of receptor: it’s a pentameric channel that opens when acetylcholine binds, allowing sodium (and some calcium) to flow in and quickly depolarize the cell.

The other options are different signaling types. The beta-adrenergic receptor is a G protein–coupled receptor that, when activated, triggers a signaling cascade via second messengers rather than opening an ion channel. The adenosine receptor is also a GPCR, leading to various second-m messenger effects. The tyrosine kinase receptor is a receptor tyrosine kinase that signals through phosphorylation and kinase cascades, not by forming an ion channel.