Tetrodotoxin is a selective blocker of which ion channels?

• A. Sodium channels
• B. Potassium channels
• C. Calcium channels
• D. Chloride channels

Answer: (A)

Tetrodotoxin’s action shows why voltage-gated sodium channels are essential for nerve and muscle excitability. It binds to the outer part of the sodium channel pore and blocks Na+ from entering the cell. Because the rapid influx of Na+ through these channels is what drives the rising phase of the action potential, blocking them stops depolarization and prevents the action potential from firing or propagating. Potassium channels, calcium channels, and chloride channels carry out different roles in cells—repolarization, various calcium-dependent processes, and inhibitory or other conductances—but tetrodotoxin is highly specific for the voltage-gated sodium channel. So the toxin’s primary effect is to prevent sodium influx, making sodium channels the correct target.