Depressor labii inferioris muscle: anatomy
- Muscle
- Highlighted area
Quick facts
- Origin
- Oblique line of the mandible, between the symphysis and the mental foramen.
- Insertion
- Skin and mucosa of the lower lip, blending with the orbicularis oris and with the muscle of the opposite side.
- Innervation
- Marginal mandibular branch of the facial nerve.
- Function
- Lowers the lower lip and moves it slightly to the side, showing the lower teeth.
- Relevance in facial harmonization
- Spread of toxin injected into the depressor anguli oris or the mentalis can reach it and alter the smile.
The depressor labii inferioris is a small quadrilateral muscle of the chin that lowers the lower lip and moves it slightly to the side. It arises from the mandible, between the midline and the mental foramen, and rises to the skin and mucosa of the lower lip. It lies between two muscles that are often treated with toxin, the depressor anguli oris and the mentalis, which is why it is a central piece in explaining an asymmetric smile after toxin.
Where is the depressor labii inferioris?
| Point | Description |
|---|---|
| Origin | Oblique line of the mandible, between the symphysis and the mental foramen |
| Insertion | Skin and mucosa of the lower lip; fibers blend with the orbicularis oris and with the muscle of the other side |
| Lateral relation | Partly covered by the depressor anguli oris; in its lower part, it receives fibers from the platysma |
| Medial relation | The mentalis lies more medial and deeper, near the midline |
| Innervation | Marginal mandibular branch of the facial nerve |
Near the origin of the muscle are the mental foramen, with the mental nerve and vessels, and higher up runs the inferior labial artery.
How does it act in expression?
When it contracts, it pulls the lower lip down and outward, exposing the lower teeth. It acts in expressions of displeasure, irony and effort, and in articulating some sounds. In a broad smile, it helps lower the lower lip symmetrically, which keeps the curve of the smile balanced.
Why does it matter in facial harmonization?
- Asymmetric smile after toxin: toxin injected into the depressor anguli oris or the mentalis can spread and weaken the depressor labii inferioris. The lower lip stops dropping on one side in smiling and speech. See asymmetric smile after toxin.
- Injury to the marginal branch: paralysis of this branch of the facial nerve produces a similar picture, with the lower lip not dropping on the affected side. The assessment asks about jaw surgery and trauma.
- Lip and mouth corner filler: the muscle is part of the balance of forces in the lower lip, a topic covered in mouth corner filler.
How does it show up in the assessment?
- Broad smile and speech: height of the lower lip and exposure of the bottom teeth on each side.
- Asking the patient to show the bottom teeth: isolates the action of the muscle and reveals pre-existing asymmetries.
- Photos and video before toxin: record the baseline pattern and protect the professional against later complaints.
Also called: depressor of the lower lip, quadratus labii inferioris (old name), musculus depressor labii inferioris.
Frequently asked questions
What is the function of the depressor labii inferioris?
It lowers the lower lip and pulls it slightly to the side, as when showing the bottom teeth or making an expression of irony or displeasure. It also takes part in speech and in lip mobility during chewing.
Why does the smile look crooked after toxin in the chin or mouth corners?
One possible cause is toxin reaching the depressor labii inferioris, which lies between the depressor anguli oris and the mentalis. With it weakened on one side, the lower lip does not drop symmetrically when smiling and speaking. The effect is usually temporary.