What nerve controls the left lip?

Innervation of the Left Lip

The left lip is primarily controlled by the facial nerve (cranial nerve VII), with the marginal mandibular branch innervating the lower lip and the buccal and zygomatic branches innervating the upper lip. 1

Facial Nerve Anatomy and Distribution

The facial nerve (CN VII) is responsible for innervating the muscles of facial expression, including those that control lip movement. After exiting the skull through the stylomastoid foramen, the facial nerve branches into several divisions:

• Upper lip: Innervated by the buccal and zygomatic branches of the facial nerve
• Lower lip: Primarily innervated by the marginal mandibular branch of the facial nerve 2, 3

Lower Lip Innervation Details

The marginal mandibular branch of the facial nerve controls several muscles affecting the lower lip:

• Depressor anguli oris
• Lower orbicularis oris
• Mentalis muscles 3

Recent research has revealed that the depressor labii inferioris (DLI), which is responsible for lower lip depression during smiling, is actually innervated by cervical branches of the facial nerve that originate approximately 2 cm below the angle of the mandible 3.

Sensory Innervation

While the facial nerve provides motor control, sensory innervation to the lip comes from:

• Upper lip: Maxillary branch (V2) of the trigeminal nerve
• Lower lip: Mandibular branch (V3) of the trigeminal nerve, specifically via the mental nerve 4

The mental nerve branches run upward from the mental foramen in the submucosal tissue. Near the free margin of the lip, some branches pass through the orbicularis oris muscle toward the lip skin, while others follow a J-shaped course along the free margin 4.

Clinical Implications

Facial Nerve Palsy

Damage to the facial nerve or its branches can result in:

• Asymmetrical smile
• Inability to depress or elevate the lip on the affected side
• Difficulty with oral competence and articulation 2, 5

Diagnostic Approach

When facial nerve dysfunction affecting the lip is suspected:

1. MRI is the preferred imaging modality for evaluating both intracranial and extracranial portions of the facial nerve 1
2. CT provides useful information regarding temporal bone fractures, trauma, and bone-related pathologies 1
3. Clinical observation is recommended for isolated minor symptoms without other neurological deficits 6

Treatment Options

For marginal mandibular branch palsy affecting the lower lip:

1. Botulinum toxin injection can provide temporary correction of asymmetry
2. Anterior belly of digastric muscle transfer offers superior cosmetic results with less donor site morbidity
3. Free extensor digitorum brevis transfer is preferred when the digastric muscle is absent or damaged 2, 5

For more extensive facial nerve damage, functional reconstruction using forearm flaps combined with gracilis muscle transfers can restore sphincter function 7.

Key Points to Remember

• The facial nerve provides motor innervation to the muscles controlling lip movement
• The trigeminal nerve provides sensory innervation to the lip surface
• Recent research shows that lower lip depression during smiling is controlled by cervical branches of the facial nerve, not just the marginal mandibular branch as traditionally taught 3
• Proper understanding of this anatomy is crucial for surgical approaches to the face and neck to prevent iatrogenic nerve damage