The digastric muscle is a pair of muscles with both an anterior and posterior belly. Innervated by the mylohyoid nerve, these muscles coordinate with other muscle groups to perform essential actions like swallowing.
Normal digastric muscles consist of anterior and posterior bellies connected by an intermediate tendon. Their embryological source lies within the first pharyngeal arch.
Anatomy
The Digastric Muscle is a pair of neck muscles located behind the ears that curve back toward each other from their respective origins on either side. They attach to both mastoid processes (part of temporal bone located behind ear), to symphysis menti (suture connecting two halves of lower jaw), belonging to suprahyoid muscle group and aiding with opening and closing mouths. Both derived embryologically from first Pharyngeal Arch; however there are differences in their embryonic structures; as mylohyoid muscle.
The two muscles exhibit clear distinction in their embryological origin and are supplied by distinct nerves: digastric muscle is fed by both palatine and facial nerves while mylohyoid receives its nourishment through lingual nerve. An intermediate tendon connects their anterior and posterior bellies via which to the stylohyoid muscle in the neck.
On physical examination, it’s common to find various abnormalities in the anatomy of the digastric muscle. For instance, its anterior belly may be duplicated, or accessory muscle bundles fusing to it; while its posterior belly can expand or contract abnormally and even lengthen downward; Mylohyoid may develop double-lobed muscular structures.
Understanding anatomical variations is vital, since they may interfere with normal physiology. For example, improper functioning between the anterior and posterior bellies of the digastric can result in symptoms like speech impediments or lockjaw.
An extended posterior belly of the digastric muscle may affect the stability and position of the hyoid bone in your neck. Furthermore, damage to its intermediate tendon could cause symptoms like difficulty swallowing and neck or jaw pain.
To conduct an external examination, a patient should be placed in a supine position and their thumbs placed on either side of their neck and placed against the hyoid bone’s inferior surface, found in the submental triangle region. From here, they should gently glide along its lateral borders until locating their digastric muscle.
Function
The digastric muscle is a suprahyoid muscle made up of two muscular bellies connected by tendon. The anterior belly originates in the digastric fossa of the mandible while its posterior counterpart originates from mastoid process of temporal bone; they both connect through an intermediate tendon that attaches directly to hyoid bone. Together these bellies depress mandible while elevating hyoid bone for swallowing and speech functions; in addition it also assists in creating submental and submandibular triangles.
The two bellies of the digastric muscle have different embryological origins and nerve innervations; its anterior belly arises from the 1st pharyngeal arch and is supplied by mylohyoid nerve (CN V), while its posterior belly arises from 2nd arch and supplied by facial nerve (CN VII).
Kim and Loukas reported numerous morphological variations of the digastric muscle. According to them, these changes likely result from complex morphogenesis of 1st pharyngeal arches. Most commonly encountered variations involve its anterior belly region or where an intermediate tendon sits.
Kim and Loukas’ study, performed both a clinical and cadaveric investigation. They studied the anatomy of ten fresh cadaver dissections using standard incision methods; and 10 parotid surgeries using this same approach; additionally mapping all digastric muscle landmarks against distance from facial nerve trunk.
Researchers discovered that the anterior and posterior bellies of the digastric muscle were arranged sling-like, connected by an intermediate tendon which passed through the stylohyoid bone before attaching itself to both body and greater cornu of hyoid bone.
Digastric muscles feature a sling-like arrangement between their anterior and posterior bellies to provide support to submental and submandibular spaces, acting like anchors that protect these spaces from being compromised by submental and submandibular cavities. Recognizing these structures is crucial as they may cause symptoms like jaw pain, tinnitus, headaches or any combination thereof – thus helping physicians provide more precise evaluations and treatment plans for their patients.
Diagnosis
The digastric muscle is a paired muscle that connects with both the mastoid process of the temporal bone behind the ear and suture that connects two halves of the lower jaw, as well as mylohyoid, tensor veli palatini, tensor tympani muscles, mylohyoid, mylohyoid muscle and medial and lateral pterygoids of temporalis, masseter, medial/lateral pterygoids temporalis/masseter/ medial/lateral pterygoids temporalis. Facial nerve runs from parotid gland and travels posteriorly and downward to this muscle while its counterpart from trigeminal nerve usually travels deep through this muscle; finally this muscle supplies its counterpart of neck supply which supplies its counterpart sternocleidomastoid muscle of neck supply sternocleidomastoid muscle of neck supply as well.
The digastric muscle exhibits many morphological variations due to the complex development of its first pharyngeal arch.
Anterior Bellies may duplicate, causing extra slips that extend toward mylohyoid or hyoid bone, fuse with each other via intermediate tendon fusion, or fuse entirely as one muscle, with both anterior and posterior bellies fused into one entity. Furthermore, there can be an anomaly whereby the anterior belly of a muscle extends toward splenius capitus or sternocleidomastoid without touching hyoid bone.
Damage to the intermediate tendon of the digastric muscle can result in symptoms including speech impediments and jaw lock. Additionally, it may cause headaches, face pain and neck ache; therefore it is vitally important that one understands both its anatomy and morphological variations.
The submental triangle is a triangular area on either side created by the posterior belly of the digastric muscle superiorly, the superior belly of omohyoid muscle laterally and the sternocleidomastoid muscles laterally; and contains submental lymph nodes and the anterior jugular vein. The carotid triangle on both sides forms from this, the inferior belly of digastric muscles inferiorly, the sternohyoid muscle laterally as well as their respective muscular fasciae all of which connect to common carotid artery, internal/external carotid arteries as well as hypoglossal nerve.
Treatment
The digastric muscle is part of the suprahyoid muscle group and plays an essential role in chewing and swallowing, speech production and articulation, depressing mandible elevation and elevation of the hyoid bone depressing mandible depression and elevation respectively. A dysfunction of this muscle can result in pain in both upper cervical spine and jaw areas requiring treatment via massage, chiropractic manipulation or physical therapy modalities.
The anterior belly of the digastric muscle lies beneath mylohyoid muscle and is supplied by an extension of mandibular nerve known as mylohyoid nerve. Meanwhile, its posterior belly begins at mesoderm of second pharyngeal arch and receives its own nerve supply through facial nerve (cranial nerve VII). Furthermore, its surface lies underneath sternocleidomastoid and stylohyoid muscles and submandibular gland and retromandibular vein which provide additional innervation of digastric muscle.
Numerous variations of the digastric muscle have been documented in literature. These variations could result from embryological issues or differences in muscle insertion to body or greater cornu of the hyoid bone. According to one study by Kim et al., submental arteries that supply mylohyoid and digastric muscles run deep to their anterior belly in 70% of cases.
Trigger points in the posterior digastric muscle can refer pain into the upper cervical spine (SCM), as well as to occiput and earlobes. Furthermore, this muscle may become tender with overuse during chewing or speaking.
Treatment options for this condition include manual manipulation of the thoracic facet joints – joints that run from the base of the skull to scapula – as well as acupuncture which may relieve pain by stimulating specific meridians in the neck. Massage and physical therapy may help increase blood flow to muscles while relieving inflammation; patients can also perform self-stretches by protruding their mandible to tighten muscles further.
