Suprahyoid muscles

Digastric muscle

The digastric muscle consists of two parts which are separated by an intermediate tendon. The anterior belly originates from the digastric fossa of mandible, while the posterior belly originates from the mastoid notch of temporal bone. Together they insert onto the body of hyoid bone via the intermediate tendon and its fibrous sling.

One particular feature of the digastric is that both bellies have different embryological origins. The anterior belly derives from the first pharyngeal arch and is therefore innervated by the nerve to mylohyoid from the mandibular nerve (CN V3). In contrast the posterior belly arises from the 2nd pharyngeal arch, which is why its nerve supply comes from digastric branch of the facial nerve (CN VII).

The digastric has two important functions that assist with the actions of swallowing, speech and mastication. If the hyoid bone is fixed in place, it causes depression of the mandible. Conversely, if the mandible is fixed, it causes elevation of the hyoid bone. 

Mylohyoid muscle

The mylohyoid muscle runs from the mylohyoid line of mandible to a median tendon, known as the mylohyoid raphe, where the medial fibers of each mylohyoid muscle unite. The mylohyoid raphe continues its course and inserts on the body of hyoid bone. The mylohyoid is supplied by the nerve to mylohyoid from the mandibular nerve (CN V3).

The mylohyoid elevates both the hyoid bone and floor of mouth when the mandibular attachment is fixed. When the hyoid attachment is fixed, it depresses the mandible. 

Geniohyoid muscle

The geniohyoid muscle has its origin on the inferior mental spine, also known as the inferior genial tubercle, at the internal surface of the mandible from where its fibers radiate posteroinferiorly to the body of hyoid. It is innervated by the anterior ramus of spinal nerve C1 via the hypoglossal nerve (CN XII).

The primary function of the geniohyoid is to elevate the hyoid bone and draw it anteriorly. Additionally, if the hyoid is fixed in place, the geniohyoid assists with depressing the mandible and pulling it inwards.

Stylohyoid muscle

The stylohyoid muscle extends from the posterior surface of the styloid process of temporal bone to the body of hyoid bone. In its distal part, the muscle divides into two tendons. The stylohyoid is supplied by the stylohyoid branch of the facial nerve (CN VII).

The primary function of the stylohyoid muscle is to elevate the hyoid bone and draw it posteriorly (retract). This further results in the retraction of the retraction and lengthening the floor of the mouth. Additionally, the stylohyoid helps to keep the pharynx open during inspiration by tensing the floor of the mouth.