You've probably heard that you use far more muscles to frown than you do to smile. While the jury is still out on the exact number, there is one specific muscle that lets you pull down your lips to make a frown: the depressor anguli oris (DAO) muscle. Learn more about what the DAO does and where it's located.
Anatomy and Function of the Depressor Anguli Oris Muscle
Fun fact: You actually have two DAO muscles. They are located opposite one another in your lower cheeks. The muscles start at the mandible, also known as the lower jawbone, and connect to the lower corners of your mouth, as Anatomy Next explains.
StatPearls notes that the muscles in the face serve one of two purposes. They help you chew, and they help you make various facial expressions — in fact, most of the facial muscles play a role in creating facial expressions. The DAO muscles help you frown by pulling down the corners of your mouth on each side, as the Loyola University Medical Education Network explains.
Like many of the other facial muscles, the DAO is controlled by the seventh cranial nerve, also known as the facial nerve. This nerve controls the sensations you feel in your face, some of your taste buds, your salivary glands and your facial muscle movements, notes Anatomy Next. The branch of the seventh cranial nerve that's specifically responsible for controlling the DAO muscles is the marginal mandibular branch, which controls the facial muscles in the chin area.
Problems With the Depressor Anguli Oris Muscle
Missing DAOThough it's very rare, it's possible for a person to be born without one or both DAO muscles. This condition is known as congenital hypoplasia of the depressor anguli oris muscle (CHDAOM). It's usually benign and is very rare, only occurring in three to six of every 1,000 live births, as a BMJ Case Report notes. Doctors are able to diagnose CHDAOM once they notice the asymmetry in the mouth of a crying baby or toddler. Usually, children with CHDAOM outgrow the condition, meaning that the asymmetry in the mouth becomes less visible as they get older.
Overactive DAOIt's also possible for the DAO muscle to be too active. Hyperactivity in the muscle can make it look as if you're constantly frowning and contribute to the development of jowls in the lower part of the face, as an article in the journal Plastic and Reconstructive Surgery notes. While these aren't exactly medical concerns, they might make some individuals feel self-conscious. As a review in the National Journal of Maxillofacial Surgery describes, one possible treatment for this is to have a plastic surgeon or another qualified medical professional inject botulinum toxin A into the area. This can help the DAO muscle relax so that it isn't constantly pulling down the corners of the mouth. However, the procedure requires the knowledge, skill and recommendation of an experienced clinician.
If you feel like you're frowning all of the time, or if people constantly ask you what's wrong, there's a chance you might have an overactive DAO. If that's a concern for you, talk to your doctor about your options for relaxing the muscle and turning your frown upside down.