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Depressor labii inferioris muscle

The depressor labii inferioris muscle is a small, flat, facial muscle located in the lower jaw region, responsible for depressing the lower lip. This muscle plays a critical role in facial expressions, particularly in exposing the lower teeth and creating expressions of sadness or doubt. It forms part of the muscles of facial expression and is clinically important in both anatomical and radiological evaluation due to its superficial position and contribution to lower lip movement.

Synonyms

  • Quadratus labii inferioris muscle

Function

  • Depresses the lower lip, drawing it downward and laterally

  • Aids in expressions such as sadness, doubt, and irony

  • Assists in exposing the mandibular (lower) teeth

Nerve Supply

  • Facial nerve (cranial nerve VII)

    • Specifically, the marginal mandibular branch of the facial nerve

Artery Supply

  • Inferior labial branch of the facial artery

  • Mental branch of the inferior alveolar artery

Vein Supply

  • Corresponding veins: inferior labial vein and tributaries of the facial vein

MRI Appearance

  • T1-weighted images:
    The depressor labii inferioris muscle appears as a low to intermediate signal intensity structure, similar to other skeletal muscles of the face.

  • T2-weighted images:
    The muscle displays low signal intensity,  generally isointense to adjacent facial muscles.

  • STIR (Short Tau Inversion Recovery):
    The muscle shows low signal intensity under normal conditions; however, any edema or pathology will increase signal intensity, making pathology more conspicuous.

CT Appearance

  • The depressor labii inferioris muscle appears as a thin, soft tissue density band, distinguishable from adjacent fat and bony structures, best appreciated in axial or coronal facial views. It is iso-attenuating to other facial muscles and can be visualized adjacent to the body of the mandible, inserting into the skin of the lower lip.

MRI images

Depressor labii inferioris muscle MRI AXIAL IMAGE