[Scientific Writing Sample Dilemma - Any Advice?]

I am applying to for my PhD in cognitive neuroscience.  One of my applications requires a scientific writing sample, but I don't really have one already written.  The person I spoke with said that it could be something you have published or just a paper from one of your college courses.  I graduated with my undergrad degree in psychology some time ago and for the past 6 years I have been a science teacher.  My computer was stolen after I graduated so I don't even have an old paper from college I can revise.  

 

I guess I will have to prepare a fresh sample, but what exactly are they looking for?  Also, how important is this piece of the application?  I only need to submit it to one program, but it is my top choice.  

 

Any advice or links or examples that could help me would be sincerely appreciated.  Thanks!!