Can't figure out the AC to use for my "Previous" and "Next" buttons

Hi guys,

I have a slidshow that stretches over 280 frames which can be seen here:

http://www.paragon-nj.com/newslideshow.html

or the fla:
http://www.paragon-nj.com/newslideshow.fla

There are fade ins and outs and I made buttons numbered 1-5 where you can skip to each new slide. At present I am trying to create the "previous" and "next" buttons but I'm hitting a snag of how you make something like this work. The entire AC I have is this:

a.onRelease=function(){
gotoAndPlay(1);
}

b.onRelease=function(){
gotoAndPlay(55);
}

c.onRelease=function(){
gotoAndPlay(110);
}

d.onRelease=function(){
gotoAndPlay(165);
}

e.onRelease=function(){
gotoAndPlay(220);
}

next_btn.onRelease = function () {
(not sure what to put here) ;
}

previous_btn.onRelease = function () {
(not sure what to put here) ;
}

Thanks in advance for all the input

Wil