The masseteric nerve is a motor branch of the mandibular division (V3) of the trigeminal nerve (cranial nerve V). It primarily innervates the masseter muscle, one of the muscles of mastication responsible for elevating the mandible, aiding in chewing and biting.
Location
The masseteric nerve originates from the mandibular nerve in the infratemporal fossa. It courses laterally, passing through the mandibular notch, a concave space between the coronoid process and the condyle of the mandible. The nerve then travels deep to the masseter muscle and provides motor innervation to the deep surface of the muscle. In addition to its motor fibers, the masseteric nerve sends a small sensory branch to the temporomandibular joint (TMJ), contributing to the sensation around this joint.
Structure and Anatomy
Origin
The masseteric nerve arises from the anterior division of the mandibular nerve (V3), which is the third branch of the trigeminal nerve (cranial nerve V). It is one of the primary motor branches of the anterior division, contributing to the innervation of the muscles of mastication.
Course in the Infratemporal Fossa
The masseteric nerve originates in the infratemporal fossa, a space located beneath the base of the skull and medial to the mandible. After branching from the mandibular nerve, it moves laterally through the infratemporal fossa, traveling toward the mandibular notch, which lies between the condyle and coronoid process of the mandible.
Path through the Mandibular Notch
The masseteric nerve courses superiorly and passes through the mandibular notch, a concave depression located between the condylar process and coronoid process of the mandible. This region allows the nerve to pass from the infratemporal fossa to the lateral aspect of the mandible. During its passage, the masseteric nerve is closely related to the masseteric artery, a branch of the maxillary artery, which supplies blood to the masseter muscle.
Relationship with the Masseter Muscle
Upon exiting the mandibular notch, the masseteric nerve enters the deep surface of the masseter muscle, one of the key muscles involved in chewing. The masseter muscle has a superficial and deep portion, and the masseteric nerve supplies the deep portion of the muscle. The nerve travels within the muscle fibers, innervating both deep and superficial layers, ensuring that the entire muscle is capable of contracting effectively during mastication.
Sensory Branch to the Temporomandibular Joint (TMJ)
In addition to its motor role, the masseteric nerve provides a small sensory branch to the temporomandibular joint (TMJ). This branch supplies sensation to the joint capsule, helping convey sensory information from the TMJ. Although the masseteric nerve is primarily motor, this sensory contribution plays a role in the proprioception and movement feedback of the jaw.
Relations with Surrounding Structures
The masseteric nerve is in close relation with several key structures, including the masseteric artery and the tendon of the temporalis muscle, which attaches near the coronoid process. It also passes near the zygomatic arch, under which the masseter muscle attaches to the mandible. Its course through the mandibular notch places it near the lateral pterygoid muscle and the capsule of the TMJ, where it provides sensory innervation.
Terminal Branches
The terminal branches of the masseteric nerve fan out within the masseter muscle, innervating the entire muscle from its deep surface. These branches provide motor input to all portions of the masseter, allowing for coordinated muscle contractions during chewing and biting. The masseteric nerve does not extend beyond the muscle, as its primary function is confined to the motor control of the masseter muscle and the sensory input to the TMJ.
Function
The masseteric nerve is a motor nerve responsible for innervating the masseter muscle, a major muscle involved in chewing. In addition to its motor function, the nerve also has a small sensory component related to the temporomandibular joint (TMJ). Below is a detailed breakdown of its functions.
Motor Innervation to the Masseter Muscle
The primary function of the masseteric nerve is to provide motor innervation to the masseter muscle, which plays a crucial role in jaw movement and mastication. The masseter muscle is responsible for:
- Elevation of the Mandible: The masseteric nerve innervates the masseter muscle, enabling it to contract and elevate the mandible (lower jaw). This action is essential for closing the mouth and chewing food. The masseter is one of the strongest muscles involved in mastication, and its contraction generates significant force to break down food during biting and chewing.
- Clenching of the Jaw: When the masseteric nerve stimulates the masseter muscle, it allows the jaw to clench tightly, which is important for tasks like holding objects between the teeth or applying strong force during biting.
- Stabilization of the Jaw: The masseter muscle, innervated by the masseteric nerve, also contributes to stabilizing the mandible during movements such as speaking, chewing, and swallowing. This stabilization is critical for coordinating the complex movements of mastication and maintaining proper alignment of the teeth.
Coordinated Action with Other Muscles of Mastication
The masseteric nerve works in coordination with other motor nerves that innervate the muscles of mastication, such as the deep temporal nerves (which innervate the temporalis muscle) and the nerves to the lateral and medial pterygoid muscles. Together, these muscles, under the control of their respective nerves, allow for:
- Complex Movements of the Jaw: The masseteric nerve enables the masseter muscle to work in synergy with the other muscles of mastication to produce complex jaw movements, such as chewing in a grinding motion or side-to-side movements. The coordinated contraction and relaxation of these muscles allow for efficient and controlled mastication.
- Forceful Biting: The masseteric nerve, along with the nerves that supply the temporalis and pterygoid muscles, enables forceful biting by ensuring that the masseter muscle contracts powerfully during jaw closure. This function is essential for breaking down tougher food substances, such as meat or hard vegetables.
Sensory Input to the Temporomandibular Joint (TMJ)
Though primarily a motor nerve, the masseteric nerve also provides a small sensory branch to the temporomandibular joint (TMJ). The sensory input from this branch serves several important roles:
- Proprioception of Jaw Movement: The masseteric nerve’s sensory component helps provide feedback about the position and movement of the mandible through the TMJ. This proprioceptive function ensures that the brain receives continuous information about the jaw’s position, allowing for adjustments in movement, bite force, and alignment during chewing or speaking.
- Pain Sensation from the TMJ: The sensory fibers also help detect pain or discomfort in the TMJ, such as that caused by inflammation or mechanical stress. This function is crucial for protecting the joint from damage during excessive force or abnormal movement.
Facilitating Chewing and Biting Reflexes
The masseteric nerve plays a role in facilitating the reflexes involved in chewing and biting. By innervating the masseter muscle, it allows for:
The Jaw-Jerk Reflex: This reflex, which is part of the mandibular stretch reflex, involves the masseteric nerve and is responsible for protecting the teeth and jaw from damage. The reflex is triggered by a sudden stretch of the masseter muscle, leading to an involuntary contraction that helps stabilize the jaw and prevent excessive movement.
Clinical Significance
The masseteric nerve is clinically significant due to its role in innervating the masseter muscle and its contribution to proper jaw function. Injury or damage to the masseteric nerve can result in weakness or paralysis of the masseter muscle, leading to difficulty in chewing, speaking, and closing the jaw. This can occur due to trauma, surgical interventions (such as procedures involving the mandible or temporomandibular joint), or nerve compression.
Damage to the masseteric nerve may also affect the temporomandibular joint (TMJ) due to its sensory contribution to the joint capsule, potentially leading to TMJ disorders such as pain, inflammation, or joint dysfunction. Furthermore, the masseteric nerve is often used in nerve grafting or nerve transfer surgeries due to its motor function, particularly in facial reanimation procedures for patients with facial nerve paralysis. Understanding the anatomy and course of the masseteric nerve is essential in preventing nerve injury during head and neck surgeries.
