Mylohyoid muscle

The Mylohyoid Muscle is a bilateral, flat, and triangular suprahyoid muscle. It forms the primary muscle of the floor of the mouth (diaphragma oris) and extends from the mandible to the hyoid bone. It plays a crucial role in elevating the hyoid bone and the tongue during swallowing, chewing, and speech.

Anatomical Structure

Morphology and Attachment Points

The mylohyoid muscle is a thin, flat structure that constitutes the floor of the mouth. These bilateral muscles converge along the midline to form a diaphragmatic structure, creating an anatomical barrier between the oral cavity and the neck. The muscle originates from the mylohyoid line on the internal surface of the mandible. This line begins at the mandibular symphysis and extends to the last molar tooth. The site of insertion varies according to the orientation of the muscle fibers. The anterior and middle fibers meet along a fibrous band known as the median raphe, while the posterior fibers attach to the anterior surface of the hyoid bone, ensuring functional continuity of the muscle. The superior surface of the mylohyoid muscle contains the sublingual space, which houses important structures such as the sublingual salivary gland, the lingual nerve, the hypoglossal nerve, and the sublingual artery. Inferior to the muscle lies the submandibular space, where the superficial lobe of the submandibular gland is located. The posterior border of the muscle is free, allowing transition between the sublingual and submandibular spaces. This anatomical opening serves as a clinically significant pathway for the spread of infection between these two regions.

Embryological Origin and Classification

The mylohyoid muscle is embryologically derived from the first branchial arch (mandibular arch), a feature that classifies it among the suprahyoid muscles associated with mastication. The first branchial arch gives rise to certain muscles and skeletal structures in the embryonic head and neck region; muscles originating from this arch are innervated by the mandibular division of the trigeminal nerve (n. mylohyoideus, V3). The mylohyoid muscle is motorically innervated by the mylohyoid nerve, a branch of the mandibular nerve, consistent with this innervation pattern. The mylohyoid muscle develops as precursor cells migrate from the paraxial mesoderm to the mandibular arch region, forming a key structure that contributes to the formation of the floor of the mouth and the positional control of the hyoid bone. Considering its embryological origin, neural innervation, and functional properties, the mylohyoid muscle is classified as a specialized suprahyoid muscle involved in mastication as well as in swallowing and speech.

Innervation and Vascular Supply

The mylohyoid muscle is innervated by the mylohyoid nerve, a branch of the inferior alveolar nerve, which itself is a division of the mandibular nerve (V3). The mylohyoid nerve arises from the inferior alveolar nerve just before its entry into the mandibular canal and runs along the mylohyoid groove, providing motor innervation to both the mylohyoid muscle and the anterior belly of the digastric muscle. This innervation enables voluntary contraction of the muscle, contributing to the coordination of mandibular movements during mastication, swallowing, and speech. The fact that the mylohyoid nerve also innervates the anterior belly of the digastric muscle allows for synchronized action of these two structures during jaw-opening movements. The vascular supply of the muscle is primarily provided by the mylohyoid branch of the inferior alveolar artery. Additional contributions may come from neighboring arterial branches such as the submental artery and the lingual artery. These arterial structures support the metabolic demands of the muscle and ensure its functional continuity. The organization of its neural and vascular components is essential not only for the physiological function of the mylohyoid muscle but also for defining critical anatomical parameters during surgical interventions, particularly in oral floor and mandibular surgery.

Function and Clinical Relevance

The primary function of the mylohyoid muscle is to elevate the hyoid bone, thereby stabilizing the floor of the mouth during swallowing and speech. It also assists in depressing the mandible to open the oral cavity. Support of the floor of the mouth enhances the efficiency of tongue movements during chewing and facilitates control of food in the oral phase. The mylohyoid muscle can exhibit significant anatomical variations among individuals. The median raphe may be absent in some individuals, resulting in a fused appearance of the two muscle bellies. It may also be continuous with or even replace the anterior belly of the digastric muscle. Rarely, accessory mylohyoid muscles may be observed. Another clinically significant variation is the herniation of sublingual gland tissue or adipose tissue between the muscle fibers, a condition observed in 10–50% of individuals. Clinically, the mylohyoid muscle is important in understanding the pathways of odontogenic infection spread. Infections originating from anterior teeth typically spread to the sublingual space, while those from posterior teeth commonly extend to the submandibular space. Computed tomography (CT) and magnetic resonance imaging (MRI) are frequently used to evaluate these regions.