Ion channels that respond to neurotransmitter molecules are described as gated.

Ion channels that respond to neurotransmitter molecules are described as "ligand-gated."

Here's a step-by-step explanation:

1. Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells. They are key in conducting impulses in nerve and muscle cells.

2. These channels can open or close in response to different types of stimuli. Some ion channels open or close in response to changes in electrical charge across the cell membrane, and these are called "voltage-gated" ion channels.

3. Other ion channels open or close in response to specific molecules that bind to the channel. These molecules are often signaling molecules, like neurotransmitters.

4. When a neurotransmitter molecule binds to a specific site on the ion channel, it causes the channel to open or close. This type of ion channel is called a "ligand-gated" ion channel. The neurotransmitter is the "ligand" that triggers the gate to open or close.

5. So, ion channels that respond to neurotransmitter molecules are described as "ligand-gated."