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The digastric muscle (also digastricus) (named digastric as it has two 'bellies') is a bilaterally paired suprahyoid muscle located under the jaw.Its posterior belly is attached to the mastoid notch of temporal bone, and its anterior belly is attached to the digastric fossa of mandible; the two bellies are united by an intermediate tendon which is held in a loop that attaches to the hyoid bone.
The four classical muscles of mastication elevate the mandible (closing the jaw) and move it forward/backward and laterally, facilitating biting and chewing. Other muscles are responsible for opening the jaw, namely the geniohyoid , mylohyoid , and digastric muscles (the lateral pterygoid may play a role).
These four muscles have different actions, but in general assist in elevating the hyoid bone and widening the esophagus during swallowing.When the two bellies of the digastric contract, they pull upward on the hyoid bone; but if the hyoid is fixed from below, the digastric assists in extreme opening of the mouth such as yawning or taking a large bite of an apple. [1]
Mylohyoid: by its own nerve, a branch of the inferior alveolar (from the mandibular division of trigeminal nerve), which arises just before the parent nerve enters the mandibular foramen, pierces the sphenomandibular ligament, and runs forward on the inferior surface of the mylohyoid, supplying it and the anterior belly of the digastric.
The trigeminal motor nucleus contains motor neurons that innervate muscles of the first branchial arch, namely the muscles of mastication, the tensor tympani, tensor veli palatini, mylohyoid, and anterior belly of the digastric. [1] It is situated in the upper pons, inferior to the lateral part of the floor of the fourth ventricle. [2]
The stylohyoid branch of facial nerve provides motor innervation to the stylohyoid muscle. [1] [2] It frequently arises from the facial nerve (CN VII) in common with the digastric branch of facial nerve. [1] It is long and slender. [3] It enters the stylohyoid muscle at the middle portion of the muscle. [1]
The digastric branch of facial nerve provides motor innervation to the posterior belly of the digastric muscle. [1] [2] It branches from the facial nerve (CN VII) near to the stylomastoid foramen [1] as the CN VII exits the facial canal (it thus branches proximal to the parotid plexus of facial nerve). [2]
The mylohyoid muscle is flat and triangular, and is situated immediately superior to the anterior belly of the digastric muscle. It is a pharyngeal muscle (derived from the first pharyngeal arch) and classified as one of the suprahyoid muscles. Together, the paired mylohyoid muscles form a muscular floor for the oral cavity of the mouth. [2]