As the external tongue muscle, the hyoglossus muscle is involved in swallowing, speaking, sucking, and chewing, pulling the tongue backward and downward. Functional limitations are often due to problems with the hypoglossal nerve, which supplies the muscle neuronally.
What is the hyoglossus muscle?
The hyoglossus muscle is one of a total of four external tongue muscles, which also include the genioglossus muscle, the styloglossus muscle, and the chondroglossus muscle. Because of its location in the body, the hyoglossus muscle is also known as the hyoid-tongue muscle. Contraction of the muscle results in the tongue moving backward and downward. Its antagonist is the styloglossus muscle, which is another external tongue muscle and is primarily involved in swallowing. When it contracts, it pulls the tongue backward and upward, partially relaxing the hyoglossus muscle. Experts disagree on whether the chondroglossus muscle is part of the hyoglossus muscle and splits off from it – or whether it is an independent muscle. The chondroglossus muscle is two centimeters long and, like the hyoglossus muscle, pulls the tongue back and down. It originates from the hyoid bone and attaches to the tongue.
Anatomy and structure
The origin of the hyoglossus muscle is in the lower posterior region of the oral cavity at the hyoid bone (Os hyoideum). The hyoid bone is a bone held in place by muscles and ligaments without being directly connected to other bones-but its supporting muscles do not include the hyoglossus muscle. Instead, it in turn relies on the hyoid bone for firm support. The insertion of the hyoglossus muscle is attached to the aponeurosis linguae. The tendon plate is located between the tongue muscles and the oral mucosa and passes into the lingual septum (septum linguae), with which it is fused. In its basic form, the hyoglossus muscle forms an approximately square, thin surface. It belongs to the striated skeletal musculature, the structure of which consists of individual fibers. Such a muscle fiber or muscle cell results from cell division and has many cell nuclei, which, however, are not located in a respective delimited cell as is usually the case. Instead, they form a tissue with a superordinate organization. A muscle fiber combines many myofibrils. Transverse striated muscle owes its name to its microscopic appearance: light and dark stripes appear alternately. They occur because hair-like fibers of actin and myosin are shifted closer or farther into each other.
Function and tasks
The hyoglossus muscle participates in swallowing, speaking, sucking, and chewing. The cranial nerve XII or hypoglossal nerve, which also innervates the other tongue muscles, is responsible for its control. The nerve carries commands to tense the muscles in the form of electrical impulses that travel along the nerve fiber. At the muscle, the fiber ends in a motor end plate: Within it sit vesicles filled with neurotransmitters. The incoming electrical stimulus causes the release of the transmitters into the synaptic cleft between nerve and muscle. Once at the muscle cell membrane, the molecules open ion channels, which slightly alters the charge state of the cell. This transient electrical charge of the muscle cell is also known as the endplate potential. It travels via the sarcolemma and T-tubules to the sarcoplasmic reticulum, which then releases calcium ions. Calcium binds to the fine structures of the myofibrils and causes its actin and myosin filaments to push into each other. This causes the irritated muscle fibers to shorten longitudinally and simultaneously pull the tongue back and down, which is necessary during swallowing, speaking, sucking and chewing. Humans are able to consciously control these movements; however, automatic reflexes also have an effect on the control of the hyoglossus muscle. For example, the sucking reflex in newborns is not the result of a voluntary action but is part of an innate behavioral program.
Diseases
Because the hyoglossus muscle is located far inside the head, direct lesions of the tissue are rare. Functional deficits and discomfort of the hyoid muscle are often due to damage to the hypoglossal nerve, which is responsible for its control.Medicine distinguishes between unilateral and bilateral lesions, both of which lead to various disorders of chewing, swallowing, sucking and speaking. The causative lesion of the hypoglossal nerve, in turn, may be due to injury, neurodegenerative disease, or stroke, for example. A bilateral lesion is reflected in complete tongue paralysis: the tongue is completely unable to function because the hypoglossal nerve not only innervates the hyoglossus muscle but is also responsible for controlling the other tongue muscles. If the nerve damage persists for a long time, the muscle tissue disappears (atrophy) as the body gradually breaks it down. Therefore, if the lesion on the hypoglossal nerve is reversible, training of the affected muscles is often required after tongue paralysis. Targeted exercises stimulate the body to rebuild the tissue. The extent to which a complete return to normal is possible depends on the individual case. In contrast to complete tongue paralysis, hemiplegia results from a unilateral lesion on the hypoglossal nerve. As a result, the tongue hangs down on the affected side. Conversely, however, a slight deviation in tongue position does not necessarily indicate nerve damage, as it may be due to other factors and is not always pathologic.
