Dem/Rep Response

This post is not meant to be an attack on Bobby Jindal's speech from the other night (there has been enough of that). Rather, it is a question that I hope to have answered. When the President gives a State of the Union (or this case, Nation) address, the opposing party to that of the President's gives a response. My question is: who picks them? Is it the party Chair? Is there a vote? Or does someone just come forward and volunteer. Someone please enlighten me.