ligand Gated channel

Meaning: A ligand-gated ion channel (also called an ionotropic receptor) is a transmembrane protein with a central pore. A specific ligand binds at a site on the protein, causing a conformational change that opens the pore. Ions then move by passive diffusion down their electrochemical gradient, with no ATP used. Most MCAT examples are neurotransmitter receptors. The nicotinic acetylcholine receptor lets Na⁺ in and depolarizes the muscle cell. The GABA-A receptor lets Cl⁻ in and usually hyperpolarizes the neuron. A few channels are opened by intracellular ligands, such as cyclic nucleotide-gated channels.

On the MCAT: Passages often compare ligand-gated, voltage-gated and G protein-coupled receptors. You may be asked to predict whether a given ion flow is excitatory or inhibitory, or what happens when a competitive antagonist occupies the binding site (fewer channels open, so the response shrinks). Ligand-gated responses are fast, on the millisecond scale, because no second messenger cascade is needed. They also show saturation: once all channels are bound, a higher ligand concentration does nothing more.

Common confusion: The ligand does not travel through the channel. It binds and signals, and the ions are what cross. Also, a GPCR binds an extracellular ligand too, but it acts through G proteins and second messengers, so it is slower and not itself a pore.