When you smile and laugh or purse your lips, you're using a little-known muscle called the risorius. While most times it works effortlessly with other facial muscles to move your mouth into a smile, in rare instances, complications with the muscle can affect your appearance and ability to be animated.
Location and Function
A study in the Aesthetic Surgery Journal describes this muscle as a band of fibers originating from the fascia (connective tissue) that covers the masseter muscle — the large facial muscle in your cheek that's responsible for the motion of chewing. It attaches to the skin at the corners of your mouth, allowing you make expressions by pulling your lips back toward your ears.
As this muscle contracts, it works closely with other facial muscles, such as the orbicularis oris muscle, which allows you to pucker your mouth and kiss, according to the textbook Anatomy and Physiology. The zygomaticus major and minor muscles are also involved in drawing the lips and face back, enabling you to grin and create other facial expressions. When the risorius is drawn back without the movement of other muscles, your mouth will take the shape of a flat line or grimace rather than a smile.
Connection to the Seventh Cranial Nerve
The seventh cranial nerve, also known as the facial nerve, carries impulses to the muscles on both sides of the face to activate the risorius muscle and all the other muscles responsible for facial expressions, according to StatPearls. In addition, the impulses from this facial nerve control the salivary glands in the oral cavity and the muscles in the ear and transmit sensations from your tongue to your brain. That is why damage to the facial nerve or one of its branches can affect the muscles of facial expression.
Damage to the facial nerve can lead to Bell's palsy, which is a condition causing temporary facial paralysis, notes the National Institute of Neurological Disorders and Stroke. Bell's palsy is the most common cause of facial paralysis and affects up to 40,000 Americans each year. The condition may cause a patient to experience weakness or total paralysis of the facial muscles, as well as twitching and drooping of the eyelid and the corner of the mouth — which is the area controlled by this muscle.
Mild cases of Bell's palsy may go away without treatment within a couple of weeks, but doctors can treat more severe cases with medications after pinpointing the cause. Physical therapy, acupuncture and facial massage might also be effective therapies.
Alternatively, facial paralysis may occur after a stroke, according to Cleveland Clinic. Stroke patients may regain the use of their facial muscles through physical therapy, heat massage and exercises that stimulate the muscles and nerves. If you ever have trouble moving your lips or mouth, or if notice your facial muscles drooping, see a doctor right away to determine the cause and the best course of treatment.
Face-Lift Surgery and the Facial Muscles
Accidents involving the face and neck can tear and damage facial muscles, requiring surgical repairs. But according to the study published in the Aesthetic Surgery Journal, facial muscles are also at risk for damage during surgical face-lift procedures that involve the surgeon tightening and suspending the facial muscles. This is because it can often be difficult to distinguish the exact location of the facial nerve, the masseteric ligament and the fibers of the risorius muscle.
Some of the confusion might be due to a lack of sufficient studies into the location of the risorius muscle relative to the other fascial layers, which are the bands of tissue located underneath the skin on your face, explains the Aesthetic Surgery Journal study. Additionally, the muscle might be located in slightly different places in different individuals. So, for satisfactory treatment results, it's important that the surgeon is experienced and well-versed in facial anatomy, especially regarding the location of the muscles and tissue underneath the skin.
The ability to make animated expressions is important to most everyone. So whether you give just a little smirk when you're amused or erupt in an all-out belly laugh, know that you couldn't do either without the help of a little muscle in the corner of your mouth: the risorius.