xlswrite to next available cell

I am trying to create a database of sorts in which the GUI user can input data regularly. Is there a way to have xlswrite automatically write the information into the next available cell?

For example: If column N is for number of bacteria in a test, and N1 is filled with last week's data, can I make matlab recgonize N2 has being the point of entry? And continue to do so for N3, N4, etc. for subsequent weeks? I don't want to have to update my script everyweek with N3, N4, etc.

So far I have this in the callback of the edit text box:

number = get(handles.numbenter,'String')
 xlswrite('filename',number,'Sheet1','[what do I put here?]')

Let me know if I need to clarify.

Thanks!