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What Is the Depressor Labii Inferioris?

Reviewed by Dan Brennan, MD on April 09, 2021

The depressor labii inferioris, also called the quadratus labii inferioris because of its quadrilateral shape, is a facial muscle. It is a paired facial muscle, which means that one muscle contracts to move the body part, and the other muscle contracts to return the body part back to its original position.  

Where Is the Depressor Labii Inferioris Located?

The depressor labii inferioris is located in the chin area. Muscles attach to bone or tissue at two or more places. If the muscle attaches to a bone that doesn't move during the action of that muscle, the attachment is called an origin. In this case, the depressor labii inferioris originates from the oblique line at the base of the mandible, which is the jaw bone. 

If the muscles attaches at a place that moves during the action of the muscle, the attachment is called an insertion. The depressor labii inferioris inserts into the skin of the lower lip. 

What Is the Function of the Depressor Labii Inferioris?

The depressor labii inferioris is part of a group of muscles called the buccolabial group. The buccolabial group consists of muscles that contribute to our facial expressions.  The function of the muscles in the buccolabial group is to control the lips, including their shape, position, and movement. The depressor labii inferioris works within that group to pull the lower lip down and forward when it contracts. In doing so, it helps to form the facial expressions associated with sadness, melancholy, and doubt. When you frown, you are using your depressor labii inferioris to do so. 

What Conditions Can Affect the Depressor Labii Inferioris?

The depressor labii inferioris is supplied with nerves from the cranial nerve number 7, also known as the facial nerve. Like the other facial muscles, it can be affected by many causes of facial nerve paralysis, including: 

Bell's Palsy. This is the most common cause of facial paralysis in the US. It is a diagnosis of exclusion, which means that your doctor will diagnose this when there are no other apparent causes of facial paralysis. Symptoms of Bell's Palsy include:

  • Sudden onset of facial paralysis
  • Paralysis on one side of the face
  • Ear pain

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Most people fully recover from Bell's Palsy. Bell's Palsy often occurs in connection with other conditions, such as diabetes or pregnancy.

Moebius Syndrome. This is a rare form of facial paralysis that also involves the nerve that controls the sideways motion of the eyes. Moebius Syndrome can also involve other nerves as well. There may be abnormalities of the extremities, chest muscles, eyes, ears, and lips. The cause of Moebius Syndrome is not known:

Surgery. During surgery or medical procedures, facial paralysis may be caused accidentally. Facial paralysis may be part of the procedure if the facial nerves have to be removed. It can be caused by: 

Sometimes facial paralysis after surgery is temporary and will go away over the course of several months. If a nerve has been cut, it will be necessary to reconstruct the nerve.

Trauma within the skull. The facial nerve runs through the temporal bone, which is located at the sides and base of the skull. Any trauma that causes a fracture of this bone can cause facial paralysis. This is often seen in motor vehicle accidents and blast injuries. When the paralysis happens immediately after the accident, surgery may be needed. If the paralysis progresses gradually, monitoring it may be the best option.

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Facial trauma. This type of trauma is usually the result of a penetrating injury, such as from a knife. The treatment for it will depend on the location of the injury. If it is in the middle of the face, it may heal without surgery. If the injury is on the side of the face, it may require surgery to repair the nerve damage. 

Tumors. Facial paralysis can be caused by different types of tumors, including: 

Depending on the location of the tumor, removing it may cause temporary or permanent facial paralysis.

Viruses. Several types of viruses can cause facial paralysis, including:  

Lyme Disease. Lyme Disease is caused by ticks that are commonly found on deer. It is more common on the east coast of the US. Facial paralysis happens in about 11% of Lyme Disease cases. In about 30% of those cases, the paralysis is on both sides of the face. 

WebMD Medical Reference

Sources

SOURCES:

Anatomy.net: "Depressor labii inferioris."

BBC Bitesize: "Muscular system: Antagonistic muscle pairs."

KENHUB: "Depressor labii inferioris muscle."

UT Southwestern Medical Center: "Facial Paralysis Causes."

Visible Body: "Describing Skeletal Muscles: A Review of Muscle Attachments And Actions."

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