The lingual nerve, or tongue nerve, innervates the anterior two-thirds of the tongue and includes both sensory and sensitive fibers. It is part of the mandibular nerve, which is subordinate to the trigeminal nerve. Lesions can cause taste disturbances, discomfort during swallowing, and physiologic speech disorders.
What is the lingual nerve?
The lingual nerve runs through the area of the lower jaw. It represents a branch from the mandibular nerve, which in turn is a branch of the trigeminal nerve. The trigeminal nerve is the fifth cranial nerve; neural information from the entire facial region converges in it. In addition to the mandibular nerve, the trigeminal nerve has two other main branches: the ophthalmic nerve or eye branch and the maxillary nerve or maxillary branch. The lingual nerve innervates the anterior two-thirds of the tongue and receives both sense-specific (sensory) information from the taste buds and non-specific (sensory) signals regarding pressure, temperature, touch, and pain sensations. The latter are more than just strong touch stimuli; the human body has its own pain receptors (nociceptors), which are often free nerve endings. Because the lingual nerve primarily connects the tongue to the nervous system, it is also known as the lingual nerve.
Anatomy and structure
The end of the lingual nerve is located in the tongue under the mucosa. From there, the nerve fibers pass under part of the submandibular gland (glandula submandibularis) before continuing between it and one of the tongue muscles (musculus hyoglossus). At this point, the lingual nerve is located to the side of the tongue. As it continues, it crosses first one of the external tongue muscles (musculus styloglossus) and then the superior pharyngeal lacing muscle (musculus constrictor pharyngis superior), which is one of the muscles of the pharynx. Next, the lingual nerve passes up the face with the posterior mandibular ramus (ramus mandibulae) on one side and the medial pterygoid muscle on the other, also passing the internal and external wing muscles (medial pterygoid muscle and lateral pterygoid muscle), both of which belong to the masseter muscles. As the mandibular nerve, it continues to the skull. Already in the cranial cavity, the trigeminal nerve divides into this and two other branches.
Function and tasks
The function of the lingual nerve is to transmit nerve signals. Different fibers within the pathway can be grouped to do this. Sensory fibers carry electrical impulses that neurons produce in response to sensory stimulation. In this case, these are gustatory or taste stimuli on the tongue. The sensitive fibers of the lingual nerve must be distinguished from these. They carry information concerning touch, pain and temperature. The sensitive fibers form the majority within the nerve. A human has about 100,000 chemical receptors on the tongue and throat that are responsible for taste perception. Several of them are combined in each taste bud. Saliva helps to dissolve water-soluble molecules from the food so that the taste receptors can react to the individual substances. The molecules either act directly on the ion channels or bind to the receptors, which then open ion channels in the cell membrane. In both cases, depolarization of the sensory cell is the result: an electrical signal is generated. The individual nerve fibers that make up the lingual nerve are grouped into bundles. A layer of connective tissue demarcates the 1-3 bundles within the nerve from each other. This enveloping layer, rich in collagen, constitutes the perineurium. Physiology refers to the interior of a fascicle as the endoneurium – it contains the actual nerve fibers through which information travels from the tongue to the brain in the form of electrical impulses.
Diseases
Damage to the lingual nerve can result in various sensory disorders of the tongue. Such lesions are possible, for example, as a result of surgical intervention on the jaw, which may be part of dental or orthodontic treatment or may be for the removal of cysts, tumors, and other tissues. A typical example is tonsillectomy.Needle insertions, such as those required for local anesthetics, can also accidentally hit the lingual nerve: Although muscles, nerves and other structures in the human body basically follow the same course and structure – minor deviations are possible in individual cases. The exact location of the lingual nerve can therefore not be assessed with absolute certainty in every case. In the context of treatments and examinations, medicine also refers to such damage as iatrogenic. In addition, injuries in the facial region carry the risk of a lesion on the lingual nerve. Regardless of the exact cause, signal transmission in the nerve may fail completely or be only partially impaired. Medical science summarizes disorders of gustatory perception as dysgeusia. Destroyed nerve fibers that no longer transport stimuli may result in a complete loss of the sense of taste in the affected area of the tongue (ageusia). In hypogeusia, on the other hand, sensitivity to gustatory stimuli is only reduced. Numbness and perceptual disturbances related to temperature, pressure, pain, and touch are also possible. Because the lingual nerve does not innervate the entire surface of the tongue, but only the two anterior thirds, a lesion on this nerve does not usually result in absolute loss of tasting. Most of the chemical receptors that a person uses to perceive gustatory stimuli are located in the posterior third of the tongue. In addition to taste disorders, several other complaints may manifest as a result of a lesion on the lingual nerve: Dysphagia and motor difficulties with speech are also possible.
