B747-400 flight crew reported severe turbulence in cruise flight.

Narrative

We experienced severe turbulence during cruise at FL400. We had some severe downdrafts followed by updrafts; which the auto throttle and autopilot could not keep up with. We would lose 100 feet or gain 100 feet; and it kept cycling greater and greater to 200. Then 300. As I overrode the autothrottles; I saw that as the speed was trending towards an overspeed; reducing power was causing the autopilot to overcorrect with pitch. So I disconnected the autopilot. This happened three times in this area of turbulence. I did the best I could to maintain altitude and airspeed; but it go temporarily into the 'red ants' 2 or 3 times; no more than 5 to 10 seconds each time. But I did call the duty pilot and he put me in a touch with Maintenance Control and I was advised to write the overspeed in the logbook. Of course in cruise; flaps up; and no freight; I didn't suspect damage; but wanted to be sure.Cause: We just had a lot of turbulence along our route.Suggestion: It was worse than I expected it to be. But in the future; I'll see if dispatch can give me a better routing.