Automatically update custom object when lead changes

I am sure this question has been asked before but I can't find a definitive answer.

I have created custom objects which store deatils about our courses eg url links, general overview of course etc. This allows us to include this information in emails (using velicity scripts) to prospective students when they enquire about a course. To acheive this I had to create 2 custom objects; a custom object that stores the course information and an intermediary custom object that links this the custom object and the lead together. In fact the example in the custom object information is almost exactly what I set up.

I have linked the existing leads to the custom objects using a once off import. Now I would like these to automatically update whenever the link field changes in the lead. Is this possible? I find it strange that I need to ask this question since this seems like fundamental database behaviour but I find everything to do with custom objects a but under done.

Any help would be appreciated.