Depressor labii inferioris muscle
Depressor labii inferioris is a paired facial muscle located in the chin region of the face. Due to its quadrangular shape, it is also called quadratus labii inferioris. It belongs to a broad group of muscles of facial expression, called the buccolabial group. Besides depressor labii inferioris, other members of this group are levator labii superioris alaeque nasi, zygomaticus major, levator labii superioris, zygomaticus minor, levator anguli oris, risorius, depressor anguli oris, mentalis, orbicularis oris, incisivus superior and inferior, and buccinator muscles.
The function of this muscle group is to control the position, shape and movements of the lips. The contribution of depressor labii superioris is limited to the lower lip, where it depresses the lower lip inferolaterally.
|Origin||Oblique line of mandible (continuous with platysma muscle)|
|Insertion||Skin and submucosa of lower lip|
|Action||Depresses lower lip inferolaterally|
|Innervation||Mandibular branch of facial nerve (CN VII)|
|Blood supply||Inferior labial branch of facial artery, mental branch of maxillary artery|
This article will discuss the anatomy and function of depressor labii inferioris muscle.
Origin and insertion
Depressor labii inferioris originates from the oblique line of mandible, between the symphysis menti and mental foramen. From here, the muscle courses superomedially, inserting to the skin and submucosa of lower lip. The mandibular end of depressor labii inferioris is continuous with platysma, while the labial attachment fuses with its counterpart and inferior fibers of the orbicularis oris muscle.
The inferolateral part of depressor labii inferioris lies deep to depressor anguli oris, while its superolateral portion partially covers the mentalis muscle. Marginal mandibular branches of the facial nerve run deep to the muscle, which is important to be aware of when conducting surgical procedures involving the face. Knowing this fact, surgeons make incisions around 1.5 centimeters inferior to the mandibular border in order to avoid cutting the nerve.
Depressor labii inferioris is innervated by the mandibular branch of facial nerve (CN VII).
Vascular supply to depressor labii inferioris comes from the inferior labial branch of facial artery and mental branch of maxillary artery.
Together with the labial part of platysma, depressor labii inferioris is a direct tractor of the lower lip, meaning that it inserts to, and acts directly upon, the lip without an intermediary. When contracting, this muscle pulls the lower lip inferomedially. This action helps in producing the facial expressions associated with sadness, doubt and melancholy.