The depressor labii inferioris depresses and everts the lower lip, exposing the lower teeth and inner lip mucosa. It works with the mentalis in lower lip movements and is active during expressions of contempt, irony, and during certain speech sounds. Like other lower facial muscles it is supplied by the marginal mandibular branch of CN VII, making it vulnerable during lower face surgery.
| Origin | Anterior surface of the mandible between the mental symphysis and the mental foramen |
|---|---|
| Insertion | Skin and mucous membrane of the lower lip |
| Nerve Supply | Marginal mandibular branch of the facial nerve (CN VII) |
| Blood Supply | Mental and inferior labial arteries |
| Actions |
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|---|
Everting the lower lip during speech facilitates the production of labiodental sounds and bilabial sounds with lower lip protrusion. Its resting tone contributes to lip seal and competence.
Selective depressor labii inferioris neurectomy is performed as an adjunct to treat the upper facial asymmetry of contralateral facial palsy by weakening the normally-functioning depressor on the healthy side to match the paralysed side. This counter-intuitive approach improves overall facial symmetry at the cost of some lower lip depression function.
Palpable as a flat muscle in the anterior chin lateral to the mentalis during deliberate lower lip depression and eversion.
Lower lip depression weakness from nerve damage producing the drooped lower lip corner and inability to evert the lower lip, causing smile asymmetry and potential drooling.
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