Player looking/aiming at mouse

I am currently working on a project that uses a 2D player "sprite" (using planes for the body, arms, head, etc.) and is in a 3D play space using an orthographic camera and wasd to move. I want the arms to aim at the mouse, as well as the head to look at the mouse by rotating. I have tried several times and still cannot get it to work properly. Most attempts result in the head rotating out of place and not following the mouse cursor at all.

Here is an example of how I want it to rotate, but in a 3D play space rather than 2D:

Unity (4.3) - 2D Platformer w/ Mouse Aiming

I am still learning how to code and have been trying to get it to work for several days now but to no avail. Any help is greatly appreciated.