In acute demyelination, what happens to the length constant?

• A. Decreases
• B. Increases
• C. Remains unchanged
• D. Fluctuates

Answer: (A)

The length constant describes how far along an axon a voltage change can spread before it decays significantly. It is determined by the membrane resistance and the internal resistance, with lambda proportional to the square root of rm/ri. In acute demyelination, the myelin sheath is lost, making the membrane leakier and reducing membrane resistance (rm). If the internal resistance (ri) stays about the same, the ratio rm/ri decreases, so the length constant becomes smaller. That means the depolarization decays more quickly with distance, leading to poorer propagation of the action potential. (Note: time constant may increase due to higher membrane capacitance, but the length constant specifically decreases because rm falls.)