When a motor nerve has been sectioned, the chronaxie

• A. Decreases
• B. Disappears
• C. Remains unaltered for a period of a week to 10 days
• D. Undergoes a longitudinal reaction

Answer: (C)

Chronaxie is the minimum pulse duration required to excite a tissue with twice the rheobic current, reflecting the tissue’s excitability. When a motor nerve is cut, the initial excitability of the surviving proximal nerve portion remains essentially unchanged for about a week to 10 days because the basic membrane properties haven’t yet altered by denervation. The degenerative changes that alter excitability—Wallerian degeneration and denervation effects—take time to develop, so the threshold timing stays the same during that early period. After this window, the denervation processes progress and chronaxie begins to increase as the muscle and nerve fibers lose conductive and excitability properties. Thus the best description is that chronaxie remains unaltered for a week to 10 days.