The thyrohyoid muscle is part of the lower hyoid (infrahyoid) musculature and is innervated by the ansa cervicalis. It is active during swallowing, closing the larynx to prevent food or fluid from entering the airway. Disorders of the thyrohyoid muscle can therefore lead to increased swallowing.
What is the thyrohyoid muscle?
The thyrohyoid muscle is a lower hyoid (infrahyoid) muscle that occurs once in each half of the body. The infrahyoid muscles also include the omohyoideus muscle, the sternohyoideus muscle, and the stylohyoideus muscle. Their counterpart above the hyoid bone is represented by the suprahyoid muscles. Both groups must contract in coordination with each other during the swallowing process. The common nerve supply through the ansa cervicalis helps to ensure that coordination is successful. The thyrohyoid muscle owes its name to the fact that it forms a connection between the hyoid bone (Os hyoideum) and the thyroid cartilage. An outdated name for the thyroid cartilage of the larynx is “thyroid”. Despite the name, which often misleads laypersons, the thyrohyoid muscle is accordingly not associated with the thyroid gland (thyroid glandula).
Anatomy and structure
The origin of the thyrohyoid muscle is at the larynx, where it arises from the thyroid cartilage (cartilago thyroidea). In addition to the thyroid cartilage, the larynx has four other cartilaginous structures. Cartilago thyroidea is among the larger of them and has an oblique notch, which anatomy calls linea obliqua. It provides support for the muscle. The thyrohyoid muscle attaches to the hyoid bone (Os hyoideum), where it is attached to the greater horn (Cornu majus). The base of the thyrohyoid muscle is quadrangular. Its tissue consists of individual fibers corresponding to muscle cells. Thinner myofibrils extend through the elongated fibers, which are divided into sarcomeres. Each sarcomere is capable of shortening on its own. Over the length of the entire fiber, this process results in contraction of the muscle. Neuronal innervation in this process originates from the ansa cervicalis, which begins at the cervical plexus and is a loop of nerves in the neck. Its fibers originate from the spinal nerves of the first through third cervical spinal cord segments (C1-C3).
Function and Tasks
During the swallowing process, the function of the thyrohyoid muscle is to protect the trachea from the entry of fluid and food. The swallowing act consists of four roughly definable phases. In the oral preparation phase, the teeth grind food, mixing it with saliva produced by the glands of the oral and pharyngeal mucosa. The oral transport phase uses the tongue muscles to press the tongue against the palate and move the food to the pharynx. The hyoglossus and styloglossus muscles are particularly active in this process. Subsequently, the palatine elevator and tensor move the soft palate upward to close the entrance to the nose together with Passavant’s annular bulge. This step heralds the beginning of the pharyngeal transport phase in the swallowing process. The vocal folds (labia vocalia) and epiglottis close, while the hyoid bone (os hyoideum) and larynx lift. This process is also known as laryngeal elevation. During this process, the thyrohyoid muscle lifts the larynx and moves it proportionally closer to the hyoid bone. Together, the digastric muscle, mylohyoid muscle, and stylohyoid muscle also pull the hyoid bone upward to assist in the upward movement. In this way, the thyrohyoid muscle and the other muscles involved prevent fluid or food from entering the trachea. To complete the pharyngeal transport phase in the swallowing act, the middle and lower pharyngeal constrictor muscles (musculus constrictor pharyngis medius and musculus constrictor pharyngis inferior) continue to push food backward in the pharynx until it enters the esophagus, where it finally arrives in the stomach after 8 to 20 seconds as part of esophageal transport.
Diseases
If the thyrohyoid muscle is unable to elevate the larynx during swallowing and thus contribute to the closure of the trachea, increased swallowing may occur. This causes liquid or solid substances to enter the airway and trigger a cough reflex.If this is not strong enough (or is also disturbed), the substance can get into the lungs. Medicine calls this process foreign body aspiration. Due to the anatomical structure of the respiratory tract, foreign bodies enter the right bronchial tree more frequently, since the access slopes more steeply than the path to the left bronchial tree. Ingested food and fluid can damage the delicate tissues of the respiratory system. In addition, they carry the risk of causing infections. Doctors therefore usually use an endoscope to remove the foreign body. In doing so, they insert the instrument through the mouth and the respiratory tract. Disorders of the thyrohyoid muscle are often accompanied by other complaints, since the small muscles in the mouth and neck area are not only anatomically close together, but are also closely interconnected neuronally. The ansa cervicalis innervates the thyrohyoid muscle and the three other infrahyoid muscles – lesions on this nerve loop therefore affect the entire muscle group. If the nerve pathway is damaged before it exits the spinal cord, depending on the extent of the damage, there may be extensive paralysis affecting all areas of the body below the injury. Spinal cord lesions are caused by tumors, herniated discs and vertebral injuries, for example. Laryngeal carcinoma can also affect the thyrohyoid muscle. In this case, the mass either affects the muscle directly or obstructs nerve fibers that innervate the infrahyoid muscle.
