Introduction

The nerve innervation in the face divides between the facial nerve (cranial nerve VII) and the trigeminal nerve (cranial nerve V). The facial nerve provides the motor innervation to the muscles that participate in facial expression. The trigeminal nerve is the source of sensory innervation to the face. Along with sensory innervation, the trigeminal nerve also provides motor innervation to the muscles used for mastication.

The sensory map of the face can further divide between the three main branches of the trigeminal nerve. The first branch that arises from the trigeminal nerve is the ophthalmic nerve (CN V1). The ophthalmic nerve provides sensory innervation to the eye region and parasympathetic innervation. The second branch of the trigeminal nerve is the maxillary nerve (CN V2). The sensory territory of the maxillary nerve is mainly below the eye extending to the upper lip. The last branch from the trigeminal nerve is the mandibular nerve (CN V3). The mandibular provides motor innervation to the muscles of mastication. The sensory innervation territory of the mandibular nerve correlates with the mandibular bone. These regions include the jawline, lower lip, and chin mainly.

The branches of the trigeminal nerves will further branch into different nerves to provide sensory innervation to their sensory territories; for example, the mandibular nerve branches into the alveolar nerve. The alveolar nerve will travel within the mandible bone and then branches into the mental nerve.[1] The mental nerve will be responsible for providing sensory innervation to the lower lip and the chin region. 

Structure and Function

As the trigeminal nerve exits the pons, it will divide into the ophthalmic nerve, the maxillary nerve, and the mandibular nerve. The mandibular nerve will exit the cranial valve via the foramen of ovale. The mandibular nerve eventually branches into the inferior alveolar nerve. The inferior alveolar nerve travels in the alveolar foramen. Once the inferior alveolar nerve gets near mental foramen. The inferior alveolar nerve branches into the mental nerve. The mental nerve will provide sensory innervation to the anterior territory of the chin, lower lip, and the buccal gingivae. The mental nerve divides into three branches. The division of the mental nerve occurs beneath the depressor anguli oris muscle. The first branch of the mental nerve descends toward the chin to give sensory innervation to the skin of the anterior chin. The other two branches will ascend to provide sensory innervation to the lower lip and buccal gingivae around the premolar teeth.[1][2]

Embryology

During embryology, the nerves derive from the ectodermal germ layer. The ectodermal germ layer that participates in nervous tissue formation further subdivides into neuroectoderm and neural crest cells. The nerves that are in the central nervous system will develop from neuroectoderm while the peripheral nerves will originate from the neural crest cells. The trigeminal nerve and its branches will develop from the neural crest cells, making the mental nerve a derivative of neural crest cells. 

The development of the trigeminal nerve in the face derives from the brachial apparatus. The brachial arches will develop into the muscles, vessels, bone, and nerves in the face and neck. The brachial arch is made up mainly of mesoderm and neural crest cells. The first brachial arch develops into the mandibular nerve. The mandibular nerve will give rise to the inferior alveolar nerve, which branches into the mental nerve.

Blood Supply and Lymphatics

The blood supply to the mental nerve will come from four main arteries:

• Inferior labial artery
• Mental artery
• Submental artery
• Inferior alveolar artery 

These arteries are the main artery that perfuses the mental nerve and its sensory territory. The vascular network around the lips forms from many anastomoses. The network of vessels can provide collateral blood flow to the lips and chin regions.

The lymphatic drainage for the mental nerve and its sensory territory is towards the submental lymph nodes or the submandibular lymph nodes. These lymph nodes will eventually drain back into the central circulation. The right side of the chin will drain back into the central circulation via the right lymphatic duct. The left side of the chin will drain back into the central circulation via the thoracic duct.

Nerves

The mental nerve is one of the branches of the inferior alveolar nerve (branch of the mandibular nerve). The sensory innervation of the mental nerve may have some small overlap with the contralateral mental nerve. The sensory innervation of the mental nerve divides between three smaller branches. One of the branches from the mental nerve will innervate the skin on the chin. The other two nerves provide sensory innervation to the gingivae, the mucosa, and the lower lip. The mental nerve also has some communication with the facial nerve.

Muscles

The only muscle that the mental nerve has an anatomical association with is the depressor anguli oris muscle. As the mental nerve exits the mental foramen, it divides into its three branches underneath or posterior to the depressor anguli oris muscle.

Physiologic Variants

The branching of the mental nerve may vary slightly. In some individuals, there may be more or fewer branches that come from the mental nerve. In some individuals, there are multiple mental foramina. The various foramina lead to variations in the branches of the mental nerve exiting. Even with the differences in the branching of the mental nerve, the sensory territory is consistent.[3][4]

Surgical Considerations

The knowledge of the mental nerve location and its sensory territory is essential in surgery. Damage to the mental nerve is avoidable during surgery.

In plastic surgery, blocking of the mental nerve is common during chin reconstructive surgeries. The surgeon may choose to inject a local anesthetic into the mental nerve to reduce the sensation of pain. In mentoplasties and other chin surgeries, the patient may be awake — the nerve block aids in safe and successful surgery with reduced potential complications. The surgeon may also block the mental nerve block in jaw and lower lip surgeries.[5][6]

Clinical Significance

The mental nerve can be injected with an anesthetic when it comes to repairing lacerations of the lower lip and soft tissues of the lower face. Proper anesthetic is necessary to prevent tissue distortion and pain.[6][7] The nerve block may prevent the need to inject a local anesthetic into the tissue itself. The injection of local anesthetic into the tissue can distort the tissue leading to potentially poor aesthetic appearance during suturing. However, it is important to know that a mental nerve block does not provide adequate anesthesia for dental work or soft tissue of the upper face. When properly performed, a mental nerve block can significantly lower the pain in the mental nerve sensory territory.[8][9][10]

Other Issues

Since the mental nerve is a branch of the inferior alveolar nerve. Any damage or aesthetic affecting the inferior alveolar nerve will also affect the mental nerve's sensory territory. While damage or compromise of the mandibular nerve or the trigeminal nerve will also affect the sensory area of the mental nerve.[8]