Final answer:
A ligand-gated channel opens when a signaling molecule, called a ligand, binds to the extracellular region of the channel, causing ions to cross the membrane and change its charge.
Step-by-step explanation:
A ligand-gated channel opens because a signaling molecule, a ligand, binds to the extracellular region of the channel. This type of channel is also known as an ionotropic receptor because when the ligand, known as a neurotransmitter in the nervous system, binds to the protein, ions cross the membrane changing its charge.