The Masseter Muscle is the most superficial and powerful of the chewing muscles. It plays a key role in grinding food by elevating the mandible (lower jaw). It is particularly well-developed in herbivorous mammals.

Anatomy

Morphology and Connection Points

The Masseter Muscle, one of the strongest masticatory muscles, is located on the outer surface of the mandible. Structurally, it consists of two main layers: a superficial and a deep layer. The superficial layer originates from the zygomatic arch and the inferior border of the zygomatic arch, inserting into the angulus and ramus regions of the mandible. The deep layer terminates with vertical fibers that extend from the inner surface of the zygomatic arch to the coronoid process of the mandible. This multilayered and fibrous structure allows the masseter muscle to generate high levels of force during jaw movements. The posterior portion of the muscle is covered by the parotid gland, while the inferior portion is connected to the platysma muscle. It also works in conjunction with the temporalis and pterygoid muscles to coordinate chewing movements.

Embryological Origin and Classification

The Masseter Muscle originates embryologically from the first pharyngeal arch. Like all muscles developing from this arch, the masseter muscle is of mesodermal origin and is classified among the muscles responsible for mastication. Anatomically, it is divided into two layers: superficial and deep. Functionally, it is among the primary muscles responsible for elevation of the mandible and works synergistically with the temporalis and medial pterygoid muscles. Its inclusion in the group of masticatory muscles is significant both in terms of its neural innervation and developmental origin.

Nerve Innervation and Vascular Supply

The Masseter Muscle receives its innervation from cranial nerve V₃, the mandibular branch of the trigeminal nerve. This nerve controls muscle contraction via motor fibers. Signal transmission begins in the motor cortex and travels through the internal capsule and trigeminal motor nucleus to the masseter muscle. Its vascular supply is provided by the masseteric artery, a branch of the maxillary artery. This artery supplies both the superficial and deep layers of the muscle, ensuring adequate oxygen and nutrient supply. Furthermore, the course of the vascular structures along the muscle is an anatomical detail that requires careful attention during surgical interventions.

Function and Clinical Relationships

The Masseter Muscle is the primary chewing muscle responsible for the upward movement of the mandible. When contracted bilaterally, it pulls the mandible upward, enabling mouth closure. Capable of generating a force of approximately 400–600 newtons, it is considered one of the strongest muscles in the human body. Working in conjunction with the medial pterygoid muscle, it helps grind food during chewing. This muscle, particularly effective in breaking down hard foods, is the primary source of chewing force. It also contributes to the stability of the mandible and plays a role in maintaining the lower facial structure. The masseter muscle can be associated with a variety of clinical conditions. One of the most common is masseteric hypertrophy. This condition can develop as a result of bruxism (tooth clenching and grinding), intense gum chewing habits, or genetic factors. Hypertrophy can cause significant swelling at the corners of the jaw; this swelling can be bilateral or unilateral and can lead to facial asymmetry. Treatment options include reducing muscle volume with botulinum toxin (Botox) injections or, rarely, surgical resection. Bruxism can cause trigger points within the muscle, resulting in headaches or temporomandibular joint (TMJ) disorders. In these cases, conservative treatments such as night guards, physical therapy, and manual therapy are preferred. Clinical evaluation can include palpation of the muscle volume and stiffness when the patient is asked to clench their teeth. When necessary, ultrasonography or magnetic resonance imaging (MRI) can be used to differentially diagnose hypertrophy or potential masses.