Chorda Tympani Nerve
The chorda tympani is a nerve that relays information between portions of the tongue and the brain. There are two types of fibers contained within the chorda tympani—sensory fibers and autonomic fibers, which are motor fibers that control involuntary actions. The sensory fibers innervate taste buds on the front part of the tongue and the autonomic fibers provide stimulation to the salivary glands, which in turn produce saliva. Hence, the chorda tympani is involved in transmitting taste sensations to the brain as well as producing the saliva needed to eat and begin digesting food.
The chorda tympani is one of two nerves that transmit taste information to the brain from the taste buds on the tongue. The glossopharyngeal nerve, or cranial nerve IX, is the nerve that innervates taste buds on the back of the tongue while the chorda tympani innervates taste buds on the front of the tongue. In addition, different nerves innervate taste buds located in other structures throughout the oral cavity such as the palate on the roof of the mouth and the epiglottis in the throat. All of these taste nerves connect to the base of the brain in the brainstem. Here the taste nerves form a complicated circuit where each taste nerve inhibits the signals from other taste nerves. The chorda tympani in particular strongly inhibits the other taste nerves as well as the pain nerve fibers from the tongue. Damage to the chorda tympani nerve can cause a disruption of this inhibitory function, resulting in making the sensation of taste irregular and unpredictable.
Anatomy and Physiology
The chorda tympani nerve originates from cranial nerve VII, also called the facial nerve. As the facial nerve travels along the facial canal, the chorda tympani branches off and travels through the middle ear along the eardrum. The chorda tympani takes a winding path from the ear down into the neck and eventually emerges into the mouth near the base of the tongue. At this point, the autonomic fibers continue toward the salivary glands located under the tongue and the sensory fibers extend to the taste buds on the front of the tongue.
The taste information that the sensory fibers transmit to the brain begins when taste molecules stimulate the taste cells. Once taste cells are stimulated, they communicate this information by releasing chemicals onto the taste nerve fibers. This causes an action potential to travel along the nerve, which is ultimately transmitted to the brain. As the brain processes the incoming taste signals, several outgoing neural pathways are activated in order to digest the food. One of the beginning steps of the digestive process is the increased production of saliva.
Saliva is produced and secreted by the salivary glands. Most animals have three major pairs of salivary glands. The autonomic fibers of the chorda tympani stimulate two pairs of these glands—the sublingual glands located underneath the front of the tongue and the submaxillary glands located beneath the lower jaw. Together these glands are responsible for about 75 percent of saliva production. Under normal conditions, the human salivary glands in adults produce up to one and a half quarts, or six cups, of saliva per day.
Saliva is a clear liquid consisting largely of water but also containing small amounts of electrolytes, mucus, and enzymes. Saliva serves to moisten and break down food so that it can be swallowed easily. Because chemicals in food must first be dissolved in order to bind to the taste cells, saliva also assists in mammals being able to taste food. You can appreciate the fact that taste molecules need to be in solution and the role of saliva by using a clean paper towel to thoroughly dry your tongue and then eat dry food, such as pretzels or crackers. In general, the taste of food is decreased or even absent without saliva or other aqueous liquids.
In addition to direct injury to the nerves or salivary glands, certain diseases and medicines can affect how much saliva is produced. Without enough saliva, the mouth can become very dry. This condition is called dry mouth or xerostomia. Patients who suffer from this condition often have difficulty chewing, swallowing, and tasting food. A dry mouth also causes the gums and tongue to become swollen and uncomfortable, making it difficult to talk. Further, the risk of gum disease, tooth decay, and infections of the mouth increases as saliva normally clears food particles and cavity-causing bacteria from the teeth. While the benefits of saliva often go unnoticed, it plays important roles in oral health, the digestive process, and the enjoyment of food.
See also: Cranial Nerves; Glossopharyngeal Nerve; Nerves; Taste System
Bowen, Richard A. (2002). Salivary glands and saliva. Retrieved from http://www.vivo.colostate.edu/hbooks/pathphys/digestion/pregastric/salivary.html
National Institute on Deafness and Other Communication Disorders. (2009). NIDCD fact sheet taste disorders. Retrieved from http://www.nidcd.nih.gov/staticresources/health/smelltaste/TasteDisorders.pdf
Yale School of Medicine. (1998). Course of the chorda tympani. Retrieved from http://www.yale.edu/cnerves/cn7/cn7_18.html