Electrotonus causes the nerve irritability to

• A. Decrease at cathode
• B. Increase at cathode
• C. Increase at anode
• D. No change at anode

Answer: (B)

The main idea is how an external electric field shifts the nerve’s membrane potential and thus its excitability. Under the cathode, the nerve segment experiences depolarization—the inside becomes less negative because the local current makes it easier for positive ions to enter. This moves the membrane closer to the threshold, so irritability increases. Conversely, under the anode the membrane hyperpolarizes, making it harder to reach threshold and reducing excitability. So the correct statement is that nerve irritability increases at the cathode. The other options describe effects that don’t occur under electrotonus: depolarization at the cathode increases, not decreases, irritability; hyperpolarization at the anode would decrease, not increase, irritability; and there is a change in excitability, not no change, at the anode.