Hook for Edit/Update entry in admin

We are trying to build a multi-level approval flow into financial aid requests we will be receiving via a form. We are planning on adding some visible-to-admin-only fields in the form that would be updated from the Entry Detail page. We would ideally like to hook into that edit/update action and send out notifications as the entry is modified. I did not see a predefined hook in the docs but may be overlooking something. Would you have any suggestions about how to accomplish this--especially without mucking with the plugin core? Thanks for any help!