Ligand-Gated Channel

A ligand-gated channel is a type of ion channel that opens or closes in response to the binding of a specific chemical messenger (the <u>ligand</u>). In other words, when a ligand (such as a neurotransmitter) binds to the channel's receptor site, the channel protein changes conformation to allow ions to flow through the membrane.

• Ligand-gated ion channels translate a chemical signal into an electrical one. For example, when <u>acetylcholine (ACh)</u> is released from a neuron and binds to the nicotinic ACh receptors on a muscle cell, those receptor channels open to let Na+ ions in, depolarizing the muscle cell and initiating contraction.
• In neuroscience, ligand-gated channels are also called <u>ionotropic receptors</u> because the ligand binding directly causes ion flow. (By contrast, G protein-coupled receptors are <u>metabotropic</u> - they act through metabolic signaling cascades rather than an immediate ion flux.)
• Common examples of ligand-gated channels include nicotinic acetylcholine receptors (Na+/K+ channels activated by ACh), GABAA receptors (Cl- channels activated by GABA), and glutamate AMPA receptors (Na+ channels activated by glutamate). Recognizing these examples can be helpful for exam questions.

• If a question describes a channel that opens when a neurotransmitter or hormone binds (for instance, Na+ channels opening when ACh binds at the neuromuscular junction), that's referring to a <u>ligand-gated ion channel</u>.
• Be ready to identify classic ligand-gated channel scenarios: e.g., a problem might mention ACh causing an influx of sodium into a muscle cell - the mechanism is that ACh bound to nicotinic receptors (ligand-gated Na+ channels) on the muscle, causing them to open.
• Exam trap: Don't mix up ligand-gated channels with GPCRs. If the effect is immediate ion flow due to binding, it's a ligand-gated channel; if the effect is mediated by a G protein and second messengers, it's a GPCR.