How to Set focus to next cell inside a gridview after post back

I have a gridview having ajaxtoolkit combobox,textbox and button.Combobox and text box have some functon which do their work during post back.
I want to change the focus to next cell when postback occurs from combo box and when postback occours from textbox it the a new row will be added and focus set to the next row combo box.
Please help me