The chorda tympani is a branch of the

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste ...

that originates from the

taste buds Taste buds contain the taste receptor cells, which are also known as gustatory cells. The taste receptors are located around the small structures known as papillae found on the upper surface of the tongue, soft palate, upper esophagus, the che ...

in the front of the tongue, runs through the middle ear, and carries

taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...

messages to the brain. It joins the

(cranial nerve VII) inside the facial canal, at the level where the facial nerve exits the skull via the stylomastoid foramen, but exits through the petrotympanic fissure and descends in the infratemporal fossa. The chorda tympani is part of one of three cranial nerves that are involved in

. The taste system involves a complicated

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...

loop, with each nerve acting to inhibit the signals of other nerves.

Structure

The chorda tympani exits the cranial cavity through the internal acoustic meatus along with the facial nerve, then it travels through the middle ear, where it runs from posterior to anterior across the

tympanic membrane In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit sound from the a ...

. It passes between the malleus and the incus, on the medial surface of the neck of the malleus. The nerve continues through the petrotympanic fissure, after which it emerges from the skull into the infratemporal fossa. It soon joins the pathway of the larger

lingual nerve The lingual nerve carries sensory innervation from the anterior two-thirds of the tongue. It contains fibres from both the mandibular division of the trigeminal nerve (CN V3 ) and from the facial nerve (CN VII). The fibres from the trigeminal nerv ...

, a branch of the

mandibular nerve In neuroanatomy, the mandibular nerve (V) is the largest of the three divisions of the trigeminal nerve, the fifth cranial nerve (CN V). Unlike the other divisions of the trigeminal nerve ( ophthalmic nerve, maxillary nerve) which contain only ...

. The fibers of the chorda tympani travel with the lingual nerve to the

submandibular ganglion The submandibular ganglion (or submaxillary ganglion in older texts) is part of the human autonomic nervous system. It is one of four parasympathetic ganglia of the head and neck. (The others are the otic ganglion, pterygopalatine ganglion, and ci ...

. Here, the preganglionic fibers of the chorda tympani synapse with postganglionic fibers which go on to innervate the submandibular and

sublingual Sublingual (abbreviated SL), from the Latin for "under the tongue", refers to the pharmacological route of administration by which substances diffuse into the blood through tissues under the tongue. The sublingual glands receive their primary ...

salivary glands. Special sensory (taste) fibers also extend from the chorda tympani to the anterior 2/3 of the

tongue The tongue is a muscular organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive process, and is the primary organ of taste. The tongue's upper surface (dorsum) is covered by taste ...

via the lingual nerve.

Function

The chorda tympani carries two types of nerve fibers from their origin with the facial nerve to the

that carries them to their destinations: *Special sensory fibers providing taste sensation from the anterior two-thirds of the

. *Presynaptic parasympathetic fibers to the

, providing secretomotor innervation to two salivary glands: the

submandibular gland The paired submandibular glands (historically known as submaxillary glands) are major salivary glands located beneath the floor of the mouth. They each weigh about 15 grams and contribute some 60–67% of unstimulated saliva secretion; on stimul ...

and

sublingual gland The paired sublingual glands are major salivary glands in the mouth. They are the smallest, most diffuse, and the only unencapsulated major salivary glands. They provide only 3-5% of the total salivary volume. There are also two other types of s ...

and to the vessels of the tongue, which when stimulated, cause a dilation of blood vessels of the tongue. Chorda tympani nerve

Taste

There are similarities between the tastes the chorda tympani picks up in sweeteners between mice and

primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...

, but not rats. Relating research results to humans is therefore not always consistent.

Sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...

is detected and recognized most by the chorda tympani nerve. The recognition and responses to sodium chloride in the chorda tympani is mediated by amiloride-sensitive sodium channels. The chorda tympani has a relatively low response to

quinine Quinine is a medication used to treat malaria and babesiosis. This includes the treatment of malaria due to '' Plasmodium falciparum'' that is resistant to chloroquine when artesunate is not available. While sometimes used for nocturnal leg ...

and varied responses to hydrochloride. The chorda tympani is less responsive to

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refine ...

than is the greater superficial petrosal nerve.

Chorda tympani transection

The chorda tympani nerve carries its information to the

nucleus of the solitary tract In the human brainstem, the solitary nucleus, also called nucleus of the solitary tract, nucleus solitarius, and nucleus tractus solitarii, (SN or NTS) is a series of purely sensory nuclei (clusters of nerve cell bodies) forming a vertical column ...

, and shares this area with the greater superficial petrosal and

glossopharyngeal nerve The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior (closer to the nose) to the vagus nerve. ...

s. When the greater superficial petrosal and glossopharyngeal nerves are cut, regardless of age, the chorda tympani nerve takes over the space in the terminal field. This takeover of space by the chorda tympani is believed to be the nerve reverting to its original state before competition and pruning. The chorda tympani, as part of the

peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brai ...

, is not as plastic in early ages. In a study done by Hosley et al. and a study done by Sollars, it has been shown that when the nerve is cut at a young age, the related

are not likely to grow back to full strength. In a bilateral transection of the chorda tympani in mice, the preference for sodium chloride increases compared to before the transection. Also avoidance of higher concentrations of sodium chloride is eliminated. The amiloride-sensitive channels responsible for salt recognition and response is functional in adult rats but not neonatal rats. This explains part of the change in preference of sodium chloride after a chorda tympani transection. The chorda tympani innervates the

fungiform papillae Lingual papillae (singular papilla) are small structures on the upper surface of the tongue that give it its characteristic rough texture. The four types of papillae on the human tongue have different structures and are accordingly classified as c ...

on the tongue. According to a study done by Sollars et al. in 2002, when the chorda tympani has been transected early in postnatal development some of the fungiform papillae undergo a structural change to become more “filliform-like”. When some of the other papillae grow back, they do so without a pore.

Dysfunction

Injury to the chorda tympani nerve leads to loss or distortion of taste from anterior 2/3 of tongue. However, taste from the posterior 1/3 of tongue (supplied by the glossopharyngeal nerve) remains intact. The chorda tympani appears to exert a particularly strong inhibitory influence on other taste nerves, as well as on pain fibers in the tongue. When the chorda tympani is damaged, its inhibitory function is disrupted, leading to less inhibited activity in the other nerves.

Additional images

File:Gray178.png, Mandible of human embryo 24 mm. long. Outer aspect. File:Gray778.png, Distribution of the maxillary and mandibular nerves, and the submaxillary ganglion. File:Gray788.png, Plan of the facial and intermediate nerves and their communication with other nerves. File:Gray789.png, The course and connections of the facial nerve in the temporal bone. File:Gray842.png, Sympathetic connections of the submaxillary and superior cervical ganglia. File:Gray911.png, View of the inner wall of the tympanum (enlarged.) File:Chorda tympani nerve 2.jpg, Dissection of chorda tympani nerve File:Slide1CAC.JPG, Lateral head anatomy detail.Facial nerve dissection.

References

External links

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Washington University Washington University in St. Louis (WashU or WUSTL) is a private research university with its main campus in St. Louis County, and Clayton, Missouri. Founded in 1853, the university is named after George Washington. Washington University is r ...

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