The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017
Trigeminal Nerve
The trigeminal nerve (CN V) is the fifth of 12 pairs of cranial nerves. It carries primarily somatosensory (temperature, pressure, and pain) information of the head and face, but also some motor information to the front third of the head. The nerve is named three (tri-) twins (-geminus) in Latin after its three major branches. Because of these distinct branches, each division is named after the region it innervates. Specifically, the branches are named ophthalmic (V1), maxillary (V2), and mandibular (V3). The trigeminal nerve is one of the three largest and thickest cranial nerves in mammals, the others being the optic (for vision) and vagus (for autonomic function for the gut, heart, and lungs).
The ophthalmic branch is mostly composed of sensory fibers and innervates the top half of the face, the cornea, tear duct, and part of the nasal cavity. The maxillary branch is also primarily composed of sensory fibers and innervates the nasal cavity and the upper jaw, including the roof of the mouth. The mandibular branch contains both sensory and motor fibers. The sensory fibers of the mandibular branch innervate the lower jaw, lips, tongue, and cheeks. The motor fibers of the mandibular branch innervate the muscles of the jaw responsible for chewing. Sensory trigeminal fibers originate in the trigeminal (or Gasserian) ganglia that are found in a depression in the bottom of the cranial cavity.
Chemesthesis
Chemesthesis is the stimulation of the somatosensory system by chemicals rather than by tactile stimulus. Most somatosensory nerves are protected from chemicals by a layer of keratinized skin. However, the trigeminal nerve innervates mucus membranes, which are not so protected, allowing for the stimulation of trigeminal nerve fibers. Chemesthetic chemicals are commonly used in cooking as spices. For example, capsaicin, a chemical found in chili peppers, stimulates temperature-sensitive pain fibers, creating the sensation of heat. Other spices that contain chemicals that stimulate the somatosensory system are garlic, basil, and mint. In the nasal cavity, chemesthetic stimulation is generally considered painful and results in reflexive changes in respiration rate to protect the airway. It is common for the sensations of chemesthesis and taste or smell to be confused with each other, but they are all distinct sensory modalities, mediated by entirely different cranial nerves.
Trigeminal Neuralgia
Trigeminal neuralgia is a pathological condition typified by reoccurring facial pain. The pain can occur unilaterally or bilaterally and is typically described as burning or stabbing. The sensation usually reoccurs in the same region of the face but has been reported to spread from the original location as time with the condition passes. In some cases, trigeminal neuralgia is caused by compression of the nerve by a tumor, cyst, or aneurysm. In these cases, treatment of the underlying condition can provide relief. However, in many cases, the exact cause of trigeminal neuralgia can be idiopathic, meaning the exact cause of the condition cannot be determined.
Speculative causes of this disease have included tension placed on the trigeminal nerve at points where it exits the skull, compression of trigeminal ganglia due to anomalies in local circulation, and demyelination of the nerve root. The demyelination hypothesis is supported by the occurrence of trigeminal neuralgia in many individuals with multiple sclerosis. Initial treatments for trigeminal neuralgia usually focus on alleviating the pain with a combination of narcotics, anti-inflammatories, and steroids. Anticonvulsants have been successfully used to prevent bouts of pain, but these treatments generally become less effective over time. Surgically relieving the vasculature pressure on the trigeminal ganglia can relieve the pain in some situations. In extreme cases, cutting the trigeminal ganglion or nerve can bring relief, but it carries the risk of significant side effects, including permanent facial numbness or paralyzation (complete loss of muscle function) of the muscles of mastication. This means that patients with such complications may not be able to chew on the side of mouth where the cut was made.
C. J. Saunders
See also: Central Nervous System; Cranial Nerves; Nerves; Somatosensory Cortex; Somatosensory System
Further Reading
Bryant, Bruce P., & Wayne L. Silver. (2000). Chemesthesis: The common chemical sense. In T. E. Finger, W. Silver, and D. Restrepo (Eds.), Neurobiology of taste and smell (pp. 73—100). New York: Wiley-Liss.
Prasad, Sashank, & Steven Galetta. (2009). Trigeminal neuralgia: Historical notes and current concepts. Neurologist, 15(2), 87—94.
Silver, Wayne L., Phillip Roe, & Cecil J. Saunders. (2010). Functional neuroanatomy of the upper airway in experimental animals. In J. B. Morris and D. Shusterman (Eds.), Toxicology of the nose and upper airways (pp. 45—64). New York, NY: Informa Healthcare.
Tizzano, Marco, Brian D. Gulbransen, Aurelie Vandenbeuch, Tod R. Clapp, Jake P. Herman, Hiruy M. Sibhatu, … Thomas E. Finger. (2010). Nasal chemosensory cells use bitter taste signaling to detect irritants and bacterial signals. Proceedings of the National Academy of Sciences USA, 107, 3210—3215.