Question

when the acetylcholine receptor sites on sodium ion channels are not occupied ______. multiple choice a ligand is formed sodium ions diffuse through and enter the cell the sodium channels open sodium ions diffuse through and leave the cell the sodium channels remain closed

Explanation:

Acetylcholine (a ligand) binds to its receptor sites on sodium ion channels to open them, allowing sodium ions to enter the cell. If the receptor sites are not occupied, the sodium channels remain closed. Let's analyze each option:

• "a ligand is formed": Ligand (acetylcholine) binding is about occupancy, not formation here. Eliminate.
• "sodium ions diffuse through and enter the cell": This happens when channels are open (receptor occupied). If not occupied, channels are closed, so sodium can't enter. Eliminate.
• "the sodium channels open": Channels open only when receptor is occupied by acetylcholine. Eliminate.
• "sodium ions diffuse through and leave the cell": Sodium's concentration gradient is into the cell (higher outside), so it doesn't leave when channels are closed or open (enters). Eliminate.
• "the sodium channels remain closed": Correct, as the receptor occupancy is needed to open the channels; without it, channels stay closed.

Answer:

the sodium channels remain closed (the option with this text)