Which muscle elevates the upper lip in a smile expression?

The muscle responsible for elevating the upper lip during a smile is the Quadratus labi superioris. This muscle is located in the facial region, and its primary function is to pull the upper lip upwards, contributing to facial expressions that denote happiness or joy. When contracted, it facilitates the elevation of the lip, which is a key component of a smile.

In contrast, the other muscles listed serve different purposes related to facial expression. For instance, the Quadratus labi inferioris is involved in lowering the lower lip, the Depressor labii inferioris also serves to lower the lower lip, and the Risorius is primarily involved in pulling the corners of the mouth sideways, which contributes to smiling but does not specifically elevate the upper lip. Understanding the specific functions of these muscles helps clarify how different facial expressions are formed and the roles that various muscles play in those expressions.