The hypoglossal nerve is the twelfth cranial nerve. The motor nerve innervates the tongue muscles. Paralysis of the nerve results in speech and swallowing disorders.
What is the hypoglossal nerve?
The tongue is a mucosa-covered muscular organ. As such, it is involved in everyday human life with countless movements. Humans need the tongue and its mobility for communication, for example. Tongue movements are vital in the context of food intake. The mobility of the tongue is ensured by a motor nerve, which connects the muscle to the central nervous system and sends it voluntary motor commands in the form of excitation. This motor nerve is the hypoglossal nerve. In Greek, “hypoglossus” means “under the tongue”. The hypoglossal nerve corresponds to the twelfth and thus last cranial nerve. Like all cranial nerves, the hypoglossal nerve arises directly from specialized nerve cell assemblies or cranial nerve nuclei within the brain. In addition to the tongue muscle, the nerve also innervates the floor of the pharynx with its fibers. Its nucleus is called the nucleus nervi hypoglossi and lies paramedian and elongated on both sides in the caudal portion of the medulla oblongata and floor of the rhomboid fossa in the trigonum nervi hypoglossi. This point lies at approximately the same level as the nuclei of cranial nerves ten and eleven.
Anatomy and structure
The hypoglossal nerve emerges from the medulla oblongata laterally of the pyramid with its up to twelve root fibers. From there, it travels on the brain surface in about three trunks to the hypoglossal canal at the os occipitale, where it exits the cranial cavity near the foramen magnum. For the time being, the motor nerve runs outside the skull between the internal jugular vein and the two arteries, internal and external carotid. In the trigonum caroticum, ventral branches from the superior cervical nerves attach to the nerve. Some of the fiber tracts further accompany the hypoglossal nerve to proportionately innervate the hyoid muscles. All other fibers of the nerve bend at the cranial trigonum caroticum, from where they continue under the stylohyoid muscle and venter posterior of the digastric muscle to the trigonum submandibulare. At this point, they enter the musculature of the tongue from the side of the floor of the mouth between the mylohyoideus and hyoglossus muscles. Like all motor nerves, the hypoglossal nerve is in contact with the motor endplate of the target muscle. In the case of the twelfth cranial nerve, the target muscle corresponds to the external and internal tongue muscles. In addition to efferent fibers, the nerve contains afferent fibers of the muscle spindle and Golgi tendon organ within the tongue. The twelfth cranial nerve supplies the superior and inferior longitudinalis muscles, the verticalis linguae and transversus linguae muscles, and the external tongue muscles chondroglossus, genioglossus, hyoglossus, and styloglossus muscles
Function and tasks
Motor nerves such as the hypoglossal nerve transmit signals from the central nervous system to the muscle fiber via the motor endplate of their target muscles. With this signal transmission, the muscles are induced to contract. Because of their direction of excitation conduction, motor nerves are called efferent fibers because they transmit away from the central nervous system. Via the afferent sensory fibers from the Golgi tendon organ and the muscle spindle of the tongue, the hypoglossal nerve conducts stimulus perceptions from the tongue muscle toward the central nervous system despite its predominantly efferent motor fibers. These stimulus perceptions primarily communicate the current muscle tone to the nervous system. Only through this information can purposeful voluntary movements of the tongue take place, which allow a precise change in muscle tone. The nerve performs the functions described for the Musculus longitudinalis superior and inferior, the Musculus verticalis linguae and transversus linguae and the external tongue muscles Musculus chondroglossus, genioglossus, hyoglossus and styloglossus. Thus, the nerve is involved in virtually all tongue movements. In addition to the comminution of food, the act of swallowing and the formation of sounds depend on the position of the tongue. Consequently, the hypoglossal nerve is irreplaceable for everyday human life and communication. Since linguistic communication is sometimes referred to as a species-specific human characteristic, the nerve with its functions in communication contributes significantly to the typically human characteristic.
Diseases
When the hypoglossal nerve is damaged on one side, the result is hemiplegia of the tongue. The tongue deviates to the damaged side. This results in speech disorders and difficulties in taking in food and liquids. Since a unilateral paralysis can be compensated relatively well by the other side, this damage is usually not perceived as a severe disability. The paralyzed side of the muscles degrades over time. Thus, paretic atrophy occurs. Serious problems in everyday life occur when the hypoglossal nerve is damaged on both sides. In this phenomenon, there is a complete paralysis of the tongue, in which the entire tongue musculature atrophies over time. The tongue remains motionless on the floor of the pharynx in bilateral paralysis, resulting in severe speech disorders and severely impaired food and fluid intake. People with bilateral paralysis of the twelfth cranial nerve sometimes also run the risk of inhaling their own saliva, so that severe pneumonia is a sometimes frequent consequence. Unilateral paralysis can be caused by diseases such as multiple sclerosis or by a stroke. Bilateral paralysis occurs at some stage of ALS, for example. In apoplexy (stroke), in terms of cerebral circulatory disturbance, usually only partial dysfunction of the hypoglossal nerve occurs.
