Control Movie Clip using Buttons

Hello,

I am new to Action Script or any kind of programming language, but determined to try and understand it.
My question is probably very simple, all I am trying to do is control “myMovie_mc”.
I have created four buttons “up_btn”, “down_btn” etc.
I can get the movie clip to move each time I rollover a button.
How do I get continuous movement when I keep my cursor over a button?
Thanks in advance for your help.

up_btn.onRollOver = function() {
_root.myMovie_mc._y -= 10;
updateAfterEvent;
}