Platsyma is a muscle of facial expression and therefore is innervated by the facial nerve (cervical branch) and derived from the 2nd pharyngeal arch. It arises from the fascia surrounding deltoid and pectoralis major and inserts into the mandible. Its action is to wrinkle the skin of the neck pulling down the jaw and depressing the sides of the mouth.
Platysma is a superficial muscle and therefore covers sternocleidomastoid and trapezius. As a result of its superficial position it must be incised during a thyroidectomy.