Scrollbar component

I have an empty textbox with the basic scroll bar component. Nothing is in the text box so the arrows are greyed out on the scrollbar. I have a button that loads text into the textbox, but the component does not update itself to accomodate text that is larger than the text box.

how can I make it so that the scrollbar changes when text is added…also If I put text in the text box in flash, when I publish it gets the scrollbar fine, but it will not adjust itself when I click on the button that loads different text into the text box. what happens is the scroll bar stays the same regardless on the size of the loaded text.