The geniohyoid muscle is one of the suprahyoid muscles that together open the jaw and participate in swallowing. The hypoglossal nerve is responsible for the nervous supply to the geniohyoid muscle. Accordingly, hypoglossal nerve palsy impairs the function of the muscle and causes dysphagia, which can occur in the context of numerous neurologic, muscular, and other diseases.
What is the geniohyoid muscle?
One of the suprahyoid muscles in the jaw region of humans is the geniohyoid muscle, also known as the hyoid muscle. In addition to the geniohyoideus muscle, the suprahyoidal muscle group also includes the digastricus muscle, the mylohyoideus muscle, and the stylohyoideus muscle. When swallowing and opening the jaw, these four muscles work together. The hyoid muscle is one of the skeletal muscles that can be influenced voluntarily. It is also involved in various reflexes, for example in automated swallowing and vomiting. The vomiting center in the brainstem reacts to potentially toxic substances and can trigger the emptying process. To do this, it coordinates the interaction of various nerves, muscles and glands. The position of the geniohyoid muscle is a feature that distinguishes modern humans (Homo sapiens) from Neanderthals: The latter possessed a horizontal hyoid muscle, whereas the geniohyoid muscle in Homo sapiens is slightly oblique. It is possible that this difference affects articulation ability.
Anatomy and structure
The geniohyoideus muscle originates from the spina mentalis, which forms a projection in the mandible bone (Os mandibulare) and is found there on the inner surface (Facies interna). The base of the muscle is located on the hyoid bone (Os hyoideum). In its fine structure, the geniohyoid muscle consists of transversely striated muscle tissue, whose name derives from the easily recognizable fibrous structure. The individual elongated muscle fibers are each surrounded by a layer of connective tissue; inside them are the filamentous myofibrils. Wrapped around them is the sarcoplasmic reticulum, which corresponds to the endoplasmic reticulum of other cells. The myofibrils can be divided into transverse sections known as sarcomeres. A Z-disc delimits each side of the sarcomere and serves as a support for tiny filaments. Following the zipper principle, filaments of actin and tropomyosin on one side and myosin on the other are arranged alternately so that they can slide into each other when the muscle contracts. The geniohyoid muscle receives such neuronal signals via the hyoglossal nerve, which is connected to the spinal cord via spinal segment C1 and also innervates the other suprahyoid muscles.
Function and Tasks
The function of the geniohyoid muscle is to assist in jaw opening and swallowing, pulling the tongue forward. In addition, it is involved in lateral movements of the jaw and, together with the other suprahyoid muscles, forms the musculature of the floor of the mouth. Motor fibers of the hypoglossal nerve transmit signals to the geniohyoid muscle by releasing neurotransmitters at the junction between the nerve fiber and the muscle cell. These messengers attach reversibly to receptors located on the outside of the muscle cell membrane. An activated receptor opens ion channels through which charged particles flow into the cell and cause an electrical endplate potential in the muscle. This spreads across the tissue of the geniohyoid muscle and stimulates the sarcoplasmic reticulum to release calcium ions. The ions bind to the actin/tropomyosin filaments of the fine myofibrils bundled in the muscle fiber, thus changing their spatial structure. As a result, the myosin filaments find attachment with their “heads” to the actin/tropomyosin strand. As a result, the myosin filaments push further between the complementary fibers, actively shortening the sarcomere and ultimately the entire muscle. Contraction of the geniohyoid muscle in turn pulls the tongue forward.
Diseases
A lesion on the hypoglossal nerve can affect the function of the geniohyoid muscle when the innervating fibers no longer transmit nerve signals to the muscle. Typically, hypoglossal palsy affects not only the geniohyoid muscle but also the other tongue muscles.Often, the nerve suffers damage in only one side of the face, resulting in hemiplegia of the tongue. On a functional level, this paralysis often leads to swallowing disorders (dysphagia) and motor problems during speech. The position of the tongue often deviates from its normal position in the mouth. Persistent hypoglossal palsy gradually leads to atrophy of the affected muscles, resulting in the easily recognizable asymmetry, which is particularly visible when the tongue is stuck out. Various causes can be considered for hypoglossal paralysis, one of which is stroke or cerebral infarction. In Germany, 160-240 out of 100,000 people suffer an ischemic stroke every year, which is the most common form of cerebral infarction and is due to the undersupply of blood to the brain. Symptoms can vary depending on the area affected. Hypoglossal palsy can also be permanent damage if the nerve tissue is permanently damaged. Particularly in the advanced course of Alzheimer’s dementia, swallowing disorders may also become apparent. The neurodegenerative disease initially manifests itself in short-term memory disorders and leads to increasing symptoms such as agnosia, apraxia, speech and language disorders, apathy, and ultimately to bedriddenness and numerous motor disorders. Neuromuscular diseases, in addition to malformations and neoplasms, are other possible causes of dysphagia involving the geniohyoid muscle and other muscles. Direct injury to the geniohyoid muscle is possible during implant placement and other facial injuries and fractures.