Scroll single row in table on pressing down arrow key

Scroll single row in table on pressing down arrow key

I have table with fixed height and overflow = scroll. when i press down arrow , i am highlighting the next row correctly.
            My doubt is when row reaches bottom of the visible screen, how to move the only one down and also on pressing the up arrow, when the highlighted row reaches the top of visible row, how to move only one  row up in the table.

Thanks in advance.