Using arrows-keys to navigate

I am wondering if there was a possibility to navigate with arrow keys through a table I created with JS(using jQuery)? I mean jumping from cell to cell...The script is for Greasemonkey.

The alert, however, works. I just got no idea how to make it well-functioning.

$(document).keydown(function(e){
    if (e.keyCode == 37) { 
       alert( "left pressed " );
       return false;
    }
    if (e.keyCode == 38) { 
       alert( "up pressed " );
       return false;
    }
    if (e.keyCode == 39) { 
       alert( "right pressed " );
       return false;
    }
    if (e.keyCode == 40) { 
       alert( "down pressed " );
       return false;
    }
});
;

Any hint or whatever is much appreciated. Thanks in advance, Faili

Update

It seems like I explained myself wrong. Give me another try: Demo

This one may give you an idea of what I wanted. After selecting one input-field a navigation with arrow keys is possible. My problem is that I just can't realise it via GM and jQuery. Any idea?

Thanks again for your time and effort.

Finally it was like:

function myTest_analysis1(e,leftkey,up,right,down){
    myTest(e,'','','field_analysis2','field_communication1')

function myTest(e,leftkey,up,right,down)
{
  if (!e) e=window.event;
  var selectArrowKey;
  switch(e.keyCode)
  {
  case 37:
    // Key left.
    selectArrowKey = leftkey;
    break;
  case 38:
    // Key up.
    selectArrowKey = up;
    break;
  case 39:
    // Key right.
    selectArrowKey = right;
    break;
  case 40:
    // Key down.
    selectArrowKey = down;
    break;
  }
  if (!selectArrowKey) return;  
  var controls = window.document.getElementById(selectArrowKey);
  if (!controls) return;
  controls.focus();
}
}
 $('#field_analysis1').keydown (myTest_analysis1);

That's how it worked out for me. I bet there is a smarter solution, but I couldn't figure it out right now.

Thank you sooo much for your time and effort.