The stylohyoid muscle is a small skeletal muscle in the jaw region. It is part of the suprahyoid musculature and contributes to swallowing and opening the jaw. Dysphagia can also affect the stylohyoid muscle and lead to functional impairment.
What is the stylohyoid muscle?
The stylohyoid muscle is a striated muscle involved in opening the jaw and swallowing. It belongs to the suprahyoid group of muscles, also known as the floor of the mouth muscles or upper hyoid muscles, which includes four other muscles in addition to the stylohyoideus muscle: the digastricus muscle, the geniohyoideus muscle, and the mylohyoideus muscle. These muscles work together in a coordinated manner both when swallowing and when opening the jaw. They are controlled by the seventh cranial nerve, the facial nerve, which uses numerous branches (rami) to reach numerous tissue structures in the head. Its fibers not only conduct motor and parasympathetic signals from the central nervous system to the innervated muscles, but they also transport sensory and sensitive nerve signals in the opposite direction.
Anatomy and structure
The origin of the stylohyoid muscle is at the temporal bone (Os temporale), which is part of the skull. Within it lie the inner ear and the middle ear. At the temporal bone, the stylohyoid muscle originates from the styloid process, which is a process of this skull bone. The attachment of the stylohyoid muscle is located at the hyoid bone (Os hyoideum), where a tendon fixes the striated muscle to the bone and where the tendon of the digastric muscle also attaches. The digastric muscle is another suprahyoid muscle, also known as the biceps muscle because of its shape. The ligamentum stylohyoideum – a paired ligament – spans from the stylar process to the hyoid bone, connecting the two bones. Like all striated skeletal muscles, the stylohyoid muscle is composed of muscle fibers that correspond to muscle cells. They have multiple nuclei because the conventional cell structure does not exist in them. Instead, inside a muscle fiber are several myofibrils that run longitudinally through the fiber and are surrounded by the sarcoplasmic reticulum. When the transverse sections of the myofibrils (sarcomeres) shorten because actin/tropomyosin and myosin filaments contained within them push into each other, the muscle contracts as a whole, causing a corresponding movement of the hyoid bone.
Function and tasks
The stylohyoid muscle performs both static and dynamic functions. Together with other muscles and ligaments, it holds the hyoid bone (Os hyoideum), which otherwise has no direct connection to other bones. The hyoid bone is composed of the middle body and the lateral horns; the attachment of the stylohyoid muscle is distributed between the body and the large horn of the bone. The dynamic function of the stylohyoid muscle is to assist swallowing and jaw opening, working in conjunction with the other suprahyoid muscles. The stylohyoid muscle receives the command to contract from the facial nerve. The electrical signal terminates in the terminal knob of the innervating nerve fibers, where it is accompanied by an influx of calcium ions. As a result, some vesicles located in the terminal button unite with the outer membrane and release the neurotransmitters they contain. As a messenger, acetylcholine binds transiently to receptors in the membrane of a muscle cell, causing the influx of ions that generate a new electrical potential: the endplate potential, which passes into the sarcoplasmic reticulum via sarcolemma and tubular T-tubules. Calcium ions from the sarcoplasmic reticulum enter the interior of the myofibrils and bind to the filaments there, which then push into each other. In this way, the muscle fibers of the stylohyoid muscle shorten and pull the hyoid bone backward and upward, for example, during swallowing. In addition to the suprahyoid muscles, the infrahyoid muscles (lower hyoid muscles) also participate in this process.
Diseases
Because the facial nerve connects the stylohyoid muscle to the nervous system, damage to the facial nerve can also affect the stylohyoid muscle.Swallowing disorders are summarized by medicine under the term dysphagia. One of the possible causes is Alzheimer’s dementia, which is characterized by progressive damage to the brain, resulting in functional limitations or failures in the affected areas. Parkinson’s disease, which is based on nerve atrophy in the substantia nigra, or a stroke, the hereditary disease Huntington’s disease or other neurological diseases are also possible causes of swallowing disorders. Injuries to the tongue and fractures to the midface or hyoid bone may damage both the muscles and the innervating nerve fibers. Malformations and neoplasms of the head, diseases of the esophagus, and infectious diseases may also contribute to dysphagia, which is reflected in dysfunction of the stylohyoid muscle and other muscles involved. Psychologically induced swallowing disorders occur, for example, in the context of phagophobia, which is a morbidly severe fear of choking or swallowing and is colloquially known as fear of swallowing. Eagle’s syndrome also manifests in the stylohyoid muscle environment. Watt Weems Eagle was the first to describe the clinical picture; it does not affect the stylohyoid muscle directly, but rather the stylohyoid ligament. In Eagle’s syndrome, calcium salts become deposited in the ligament and cause ossification. The syndrome may also result from the stylohyoid process being too long. Is both cases typically present with swallowing difficulties such as pain in the throat and difficulty swallowing when the head is turned.