Mylohyoid nerve

The mylohyoid nerve is a small branch of the inferior alveolar nerve, which itself is a branch of the mandibular division of the trigeminal nerve (cranial nerve V3). This nerve primarily provides motor innervation to the mylohyoid muscle and the anterior belly of the digastric muscle, both of which play roles in the movement of the floor of the mouth and jaw.

Location

The mylohyoid nerve branches off the inferior alveolar nerve just before the inferior alveolar nerve enters the mandibular foramen. After branching, the mylohyoid nerve runs along the inner surface of the mandible in the mylohyoid groove. From there, it descends to innervate the mylohyoid muscle and the anterior belly of the digastric muscle, located in the floor of the mouth. It is located in the mandibular and submandibular region, near the base of the jaw.

Structure and Anatomy

The mylohyoid nerve is a small but important motor branch of the inferior alveolar nerve, itself a branch of the mandibular division of the trigeminal nerve (cranial nerve V3). Its primary role is to innervate the mylohyoid muscle and the anterior belly of the digastric muscle. Below is a detailed description of the mylohyoid nerve’s origin, course, and anatomical relationships.

Origin

The mylohyoid nerve branches off from the inferior alveolar nerve just before the inferior alveolar nerve enters the mandibular foramen on the medial side of the ramus of the mandible. The inferior alveolar nerve continues its path through the mandibular canal to provide sensory innervation to the lower teeth, but the mylohyoid nerve diverges and travels along a separate course to supply motor innervation to specific muscles.

Course

The course of the mylohyoid nerve can be divided into two segments: the intraosseous and extraosseous pathways.

• Intraosseous Course:The mylohyoid nerve branches off from the inferior alveolar nerve at the mandibular foramen, located on the medial aspect of the mandible. It does not follow the inferior alveolar nerve into the mandibular canal. Instead, it takes a distinct course along the inner surface of the mandible.
• Extraosseous Course:After branching off from the inferior alveolar nerve, the mylohyoid nerve travels downward and anteriorly along the mylohyoid groove of the mandible. This groove is located on the medial surface of the mandible, just below the level of the mandibular foramen.
• The nerve continues along this groove until it reaches the floor of the mouth. Here, it splits into branches that innervate the mylohyoid muscle and the anterior belly of the digastric muscle.

Branches

The mylohyoid nerve divides into small branches to innervate its target muscles:

• Branches to the Mylohyoid Muscle:The primary branches of the mylohyoid nerve innervate the mylohyoid muscle, a flat muscle that forms the floor of the mouth. These branches provide motor innervation, allowing the muscle to function in elevating the floor of the mouth during swallowing and speaking.
• Branches to the Anterior Belly of the Digastric Muscle:The mylohyoid nerve also gives off branches to the anterior belly of the digastric muscle. This muscle runs from the digastric fossa of the mandible to the hyoid bone and plays an important role in jaw movement and stabilization.

Anatomical Relationships

The mylohyoid nerve is located in the submandibular region, and it has important anatomical relationships with various structures in the neck and mandible:

• Inferior Alveolar Nerve:The mylohyoid nerve is a branch of the inferior alveolar nerve, which is the main sensory nerve to the lower teeth and part of the mandibular division of the trigeminal nerve. While the inferior alveolar nerve enters the mandibular canal to provide sensory innervation to the teeth, the mylohyoid nerve branches off outside the canal to supply motor innervation.
• Mandibular Foramen and Mylohyoid Groove:The mylohyoid nerve arises near the mandibular foramen, where the inferior alveolar nerve enters the mandible. From here, the mylohyoid nerve travels in the mylohyoid groove, a small groove along the medial surface of the mandible. This groove provides a protective pathway for the nerve as it travels toward the mylohyoid muscle.
• Submandibular Gland:The mylohyoid nerve is located in proximity to the submandibular gland, a major salivary gland located beneath the mandible. This anatomical relationship is important during surgeries in the submandibular region, as the nerve may be encountered during glandular dissection.
• Submental Artery and Vein:The mylohyoid nerve runs close to the submental artery and submental vein, which are branches of the facial artery and facial vein, respectively. These vessels supply blood to the chin and floor of the mouth, and their close proximity to the mylohyoid nerve is important in surgical interventions involving the lower jaw and neck.
• Facial Nerve (Marginal Mandibular Branch):The marginal mandibular branch of the facial nerve runs in the vicinity of the mylohyoid nerve, particularly near the lower border of the mandible. The two nerves are distinct but can be encountered together during surgeries, particularly in procedures such as submandibular gland removal or neck dissections.

Variations

Although the general anatomy of the mylohyoid nerve is consistent, there can be variations in its course or branching patterns:

• Accessory Branches:In some cases, the mylohyoid nerve may give off accessory branches that innervate additional structures in the floor of the mouth or provide extra sensory innervation to nearby tissues.
• Double Mylohyoid Nerves:Rarely, individuals may have two distinct mylohyoid nerves, one supplying the mylohyoid muscle and another supplying the anterior belly of the digastric muscle. This variation is uncommon but can occur.

Termination

The mylohyoid nerve terminates by innervating the mylohyoid muscle and the anterior belly of the digastric muscle. After branching from the inferior alveolar nerve and traveling along the mylohyoid groove, the nerve splits into terminal branches that enter these muscles, providing motor innervation essential for their function.

Function

The mylohyoid nerve primarily serves as a motor nerve, responsible for innervating the mylohyoid muscle and the anterior belly of the digastric muscle. These muscles are essential in the movement of the jaw, the floor of the mouth, and for functions related to swallowing and speaking. Below is a detailed description of the mylohyoid nerve’s functions.

Motor Innervation to the Mylohyoid Muscle

The mylohyoid nerve provides motor innervation to the mylohyoid muscle, a broad, flat muscle that forms the floor of the mouth.

• Elevation of the Floor of the Mouth:The mylohyoid muscle is essential for elevating the floor of the mouth during the initial phase of swallowing (oral phase). As the mylohyoid muscle contracts, it raises the floor of the mouth, which aids in pushing the bolus of food toward the pharynx for further passage through the digestive tract.
• Jaw Depression:The mylohyoid muscle, along with the digastric muscle, assists in jaw depression, meaning it helps open the mouth. This function is particularly important when chewing or speaking. By lowering the mandible, the mylohyoid muscle plays a role in opening the mouth.
• Oral Cavity Support:The mylohyoid muscle provides structural support to the floor of the mouth, helping maintain the shape of the oral cavity and assisting in proper positioning of the tongue during speech and eating.

Motor Innervation to the Anterior Belly of the Digastric Muscle

The mylohyoid nerve also innervates the anterior belly of the digastric muscle, which is a paired muscle that lies below the mandible. It works in conjunction with the posterior belly of the digastric muscle (which is innervated by the facial nerve) to perform key actions.

• Jaw Depression and Opening of the Mouth:
• • The anterior belly of the digastric muscle works with the mylohyoid muscle to depress the mandible, helping to open the mouth. This action is particularly important during chewing, speaking, and yawning.
  • The anterior belly also assists in stabilizing the hyoid bone during jaw movement.
• Elevation of the Hyoid Bone:During swallowing, the anterior belly of the digastric muscle elevates the hyoid bone, which in turn assists in moving the larynx upward and forward. This movement is essential for allowing the bolus of food to pass safely into the esophagus and for preventing food from entering the airway.

Coordination with Other Muscles of Mastication and Swallowing

The mylohyoid nerve plays a role in the broader coordination of muscles involved in mastication (chewing), swallowing, and other movements of the lower jaw and oral cavity. While its role is primarily motor, it works in conjunction with other cranial nerves and muscles to accomplish these actions.

• Mastication (Chewing):The mylohyoid muscle and anterior belly of the digastric muscle, both innervated by the mylohyoid nerve, assist in the complex movements required for chewing. The mylohyoid muscle helps move the tongue and floor of the mouth to position food between the teeth, while the digastric muscle assists in jaw depression, enabling the mouth to open.
• Swallowing (Deglutition):During swallowing, the mylohyoid muscle elevates the floor of the mouth, pushing the bolus of food toward the pharynx. At the same time, the anterior belly of the digastric muscle helps elevate the hyoid bone and larynx, protecting the airway by helping to close the epiglottis and directing food into the esophagus.

Role in Speech

The mylohyoid nerve indirectly contributes to speech through its motor control of the mylohyoid and digastric muscles.

• Articulation Support:Proper elevation of the floor of the mouth, controlled by the mylohyoid muscle, is necessary for articulating certain speech sounds. By stabilizing and supporting the tongue during speech, the mylohyoid muscle contributes to the precision of oral movements needed for clear speech.
• Jaw Movements During Speech:The anterior belly of the digastric muscle assists in jaw opening and stabilization during speech, enabling fluid jaw movements required for the articulation of various sounds.

Integration with Other Cranial Nerves

The function of the mylohyoid nerve is closely integrated with the actions of other cranial nerves involved in the control of jaw and mouth movements.

• Trigeminal Nerve (V3):The mylohyoid nerve originates from the mandibular division of the trigeminal nerve (cranial nerve V3), which also controls the muscles of mastication (e.g., the temporalis, masseter, and lateral/medial pterygoid muscles). The actions of the mylohyoid and digastric muscles complement the work of these larger masticatory muscles, helping coordinate jaw opening and closing during chewing.
• Facial Nerve (Cranial Nerve VII):While the mylohyoid nerve innervates the anterior belly of the digastric muscle, the posterior belly of the digastric muscle is innervated by the facial nerve (cranial nerve VII). This dual innervation allows for the coordinated contraction of both muscle bellies during swallowing, speech, and jaw movement.

Clinical Significance

The mylohyoid nerve, responsible for motor innervation to the mylohyoid muscle and the anterior belly of the digastric muscle, plays an essential role in jaw movement, swallowing, and the elevation of the floor of the mouth. Its clinical significance is particularly evident in dental surgeries, such as inferior alveolar nerve blocks used for anesthesia during lower dental procedures. Since the mylohyoid nerve branches from the inferior alveolar nerve, inadequate anesthesia may occur if this nerve is not properly blocked, leading to pain during dental procedures.

• Surgical Considerations: The mylohyoid nerve is susceptible to injury during procedures like submandibular gland surgery, mandibular fracture repair, or dental implant placement. Damage to the nerve may result in difficulties with jaw movement or swallowing.
• Mylohyoid Pain: Compression or irritation of the mylohyoid nerve can lead to referred pain in the jaw, floor of the mouth, or lower teeth, contributing to conditions like myofascial pain syndrome.