The omohyoid muscle is divided into a superior and inferior belly. The inferior belly begins at the superior border of scapula, ascends craniomedially and merges into an intermediate tendon at the height of the lateral cervical region. This tendon is connected to the carotid sheath which surrounds the neurovascular bundle (including the common carotid artery, internal jugular vein and vagus nerve). Distally the muscle attaches to the body of hyoid.
The sternothyroid muscle originates from the dorsal surface of the manubrium and inserts on the oblique line of thyroid cartilage. That is why the sternothyroid is the only hyoid muscle that does not directly attach to the hyoid bone. The sternothyroid lies in close relation to the capsule of the thyroid gland.
The thyrohyoid muscle constitutes the continuation of the sternothyroid, beginning at the oblique line of thyroid cartilage and inserting on the body of hyoid and greater cornu.
The sternohyoid muscle has its origin at the dorsal surface of manubrium and the sternoclavicular joint and its insertion on the body of hyoid. The sternohyoid is the most superficial of all the infrahyoid muscles.
All four infrahyoid muscles are supplied by the deep ansa cervicalis (C1-C3), which arises from the cervical plexus. The thyrohyoid receives additional supply through branches of the superior root of ansa cervicalis accompanied by the hypoglossal nerve.
The infrahyoid muscles are responsible for the positioning of the hyoid bone along with the suprahyoid muscles. They play an active role in swallowing and the movement of the larynx. More specifically, all infrahyoid muscles (except the sternothyroid) depress the hyoid. The sternothyroid depresses the larynx whereas the thyrohyoid elevates it (when the hyoid bone is fixed). The omohyoid has an additional function due to its attachment to the carotid sheath: by pulling the sheath, it maintains a low pressure in the internal jugular vein, this way increasing the blood return from the head to the superior vena cava.
Infrahyoid Muscles Paralysis
Trauma in the region of the cervical spine can damage the ansa cervicalis, resulting in paresis or even paralysis of the infrahyoid muscles. Clinically, those may be presented as swallowing difficulties, a hoarse voice and throat tightness. Incorrect positioning during surgery or medical interventions in the cervical region can also lead to nerve injuries.
A common type of surgery in which the ansa cervicalis is often deliberately “sacrificed” is neck dissection. During that type of operation, which is indicated for malignant head and neck tumors, lymph nodes, fat tissue, muscles and even nerves and blood vessels are radically removed from the cervical region. Only the most important structures (e.g. carotid artery, vagus nerve, etc) are preserved.