[jQuery] stuck on a simple traversal problem - help!

yo guys,
screenshot here: http://www.ldexterldesign.co.uk/screenshot.jpg
at present this jquery function adds the .collapsed class to
ALL .articleBody's. i only want the button to function for
the .articleItem in question only. i'm sensing some parentage is in
order?
a nudge in the right direction would be appreciated,
lewis