The autopilot of an aircraft is much more complex than just heading, altitude and speed. There are some 'modes' of climb and descent controlled by the autopilot and autothrottle. You can sort of 'link' the autothrottle with a vertical mode in which you can climb or descend at a specific airspeed, or at a specific N1, or in such a way you meet the vertical speed selected, etc. Another way is you could simply bring the throttle to idle, and adjust the vertical speed of the aircraft to maintain a specific airspeed: nose down faster, not so down slower. Hope it helps some.