HELP! Valves aren't controlled by heart, yet open and close with precise timing??? How is this possible?

Question

Okay, I'm in a bio class and I have no clue about this question. Here it is:
The valves of the heart are not directly controlled by nerves. Yet, in most individuals they open and close at precisely the right points in the cardiac cycle for efficient heart operations. How is this precise timing possible? What does determine just when the valves will open and shut???
Help is appreciated, have to turn it in, in about an hour=/ Yikes! Many thanks=)

Answer 1

The heart has four chambers - two at the top (the atria) and two at the bottom (the ventricles). The normal trigger for the heart to contract arises from the heart's natural pacemaker, the SA node, which is in the top chamber . The SA node sends out regular electrical impulses causing the atrium to contract and to pump blood into the bottom chamber (the ventricle). The electrical impulse then passes to the ventricles through a form of 'junction box' called the AV node (atrio-ventricular node). This electrical impulse spreads into the ventricles, causing the muscle to contract and to pump blood to the lungs and the body. Chemicals which circulate in the blood, and which are released by the nerves that regulate the heart, alter the speed of the pacemaker and the force of the pumping action of the ventricles. For example, adrenaline increases the heart rate and the volume of blood pumped by the heart

Answer 2

The heart valves are controlled by the the flow of blood and the blood pressure.
The heart contracts in sequence,the atrium and the ventrical
These contraction switch the pressure around to different sides of the valves causing them to open or close.

Answer 3

They are controlled by pressure - when the contracting muscles of the heart sections reach a certain pressure, the valves open. When the pressure falls off, they are closed by their shape and the back pressure of the blood.