These 10 animal facts will amaze you. The platysma is the muscle that allows us to create and control our facial expressions. It spreads out in a wide sheet that begins at the chest, runs up over the collarbone, and then stretches up along the sides of the neck to the chin. The primary function of the platysma is to help move the lower lip and jaw downward or to the side. The contraction of this muscle is the cause behind a variety of facial expressions, including smiling, frowning, and grimacing. The platysma muscle is also a large contributor to the condition known as the double chin.