The trigeminal nerve owes its name to the tripartite structure of the ocular, maxillary, and mandibular branches. Its main function is trigeminal perception as well as the transmission of neuronal signals from the brain to specific muscles in the three areas. Typical diseases affecting the trigeminal nerve include lesions, trigeminal neuralgia and neurinoma, and meningitis, increased incranial pressure, and sinusitis.
What is the trigeminal nerve?
Unlike other cranial nerves, the trigeminal nerve does not transmit neural signals that originate from specific sensory cells. Instead, it plays a role in trigeminal perception, which is named after it and is based on chemical stimuli. These stimuli can originate from different areas, but have one thing in common: they have an irritating effect and, when strong, usually elicit a defensive response. Examples are strong odors such as ammonia or the irritation of the eye with onion vapors. In accordance with its function, the trigeminal nerve does not form a single nerve cord, but stretches its branches in three directions: the ocular region, the maxilla, and the mandible. All three branches are located near other nerves that process modality-specific stimuli. Furthermore, the trigeminal nerve represents the fifth of twelve cranial nerves and is one of the gill arch nerves. This designation is due to the location of the nerves during embryonic development: they arise from the first gill arch. Part of the trigeminal nerve, the mandibular branch, corresponds essentially to the nerve of the first gill arch in adult humans.
Anatomy and structure
The anatomy of the trigeminal nerve distinguishes three branches of the cranial nerve, each covering different fields in the head from which it both receives signals and transmits commands to muscles. The ophthalmic branch (ophthalmic nerve) extends to the vicinity of the photoreceptors, while the maxillary branch (maxillary nerve) provides a sensitive connection to the skull. The third branch of the trigeminal nerve is the mandibular branch (mandibular nerve); it forms the signal bridge to the mandible with the tongue, soft palate, masseter muscles, and tympanic membrane tensioner. The individual branches of the trigeminal nerve are characterized by further ramifications, so that they can each cover a larger field. In addition to its three branches, the trigeminal nerve also has four cranial nuclei in the brain: nucleus motorius nervi trigemini, nucleus mesencephalicus nervi trigemini, nucleus pontinus nervi trigemini, and nucleus spinalis nervi trigemini.
Function and tasks
At the functional level, the trigeminal nerve is responsible for trigeminal perception, which is named after it. This type of perception involves the processing of stimuli that do not originate from only one specific modality. A trigeminal stimulus usually triggers irritation in the body and initiates protective and defensive mechanisms. For example, olfactory cells in the nose register strong ammonia odor, which is an indicator of harmful conditions in the immediate environment or signals inedible food. The olfactory stimulus triggers electrical potentials in the olfactory cells that travel to the brain via the olfactory nerve. Strong stimuli lead to the generation of many action potentials in succession, which is a strong signal. The finely branched nasal branches (rami nasales) pick up the information transmit it through other parts of the maxillary branch to the central nervous system (CNS). In reverse, the CNS can now command various muscles to contract – for example, to recoil, to contort the face (which should stem the flow of air into the nose), or even to react with disgust and nausea. The nucleus motorius nervi trigemini is a motor nucleus responsible for movement control and is located in the rhomboid brain, while the other three nuclei are located in the brain stem and are sensory nuclei responsible for receiving sensory perceptions.The task of the nucleus mesencephalicus nervi trigemini (literally “brainstem nucleus of the trigeminal nerve”) consists essentially in unconscious depth perception, the nucleus pontinus nervi trigemini (“bridge nucleus of the trigeminal nerve”) assumes conscious depth perception, pressure, tension, position perception, etc., and the nucleus spinalis nervi trigemini (“spinal nucleus of the trigeminal nerve”) is protopathically sensitive. And the spinal nucleus nervi trigemini (“spinal nucleus of the trigeminal nerve”) is protopathically sensitive, i.e., it receives temperature, strong pressure, and itch stimuli.
Diseases
Damage to the trigeminal nerve sometimes results in peripheral or central sensory loss. The peripheral lesion usually affects only one of the three branches and results in perceptual impairment or complete loss of trigeminal perceptual ability in that specific area. In contrast, the central lesion does not affect the branches of the trigeminal nerve, but rather the nuclei. If the nucleus spinalis nervi trigemini is damaged, the affected person experiences circular deficits. In contrast, trigeminal neuralgia manifests itself in severe pain, which, along with cluster headaches, is one of the most intense types of pain, occurring in the form of pain attacks and lasting up to two minutes. Doctors use medications to treat trigeminal neuralgia, but may also perform surgical procedures in persistent cases. Another disease of the trigeminal nerve is trigeminal neurioma, a tumor in the nerve sheath. Meningitis, increased incranial pressure (intracranial pressure) and sinusitis can also affect the trigeminal nerve and typically trigger pressure pain via it. Doctors therefore check the trigeminal pressure points of both halves of the face if a corresponding disease is suspected, in order to detect specific changes. In these cases, therapy is based on the specific cause.
