Chorda tympani

The chorda tympani is a branch of the facial nerve that originates from the taste buds in the front of the tongue, runs through the middle ear, and carries taste messages to the brain. It joins the facial nerve (cranial nerve VII) inside the facial canal, at the level where the facial nerve exits the skull via the stylomastoid foramen, but exits through a different route.[1]

Chorda tympani
The right membrana tympani with the hammer and the chorda tympani, viewed from within, from behind, and from above.
Details
Fromfacial nerve
Identifiers
MeSHD002814
TAA14.2.01.084
A14.2.01.118
FMA53228
Anatomical terms of neuroanatomy

The chorda tympani is part of one of three cranial nerves that are involved in taste. The taste system involves a complicated feedback 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. 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, a branch of the mandibular nerve.

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

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

Special sensory (taste) fibers also extend from the chorda tympani to the anterior 2/3 of the tongue via the lingual nerve.

Function

The chorda tympani carries two types of nerve fibers from their origin with the facial nerve to the lingual nerve that carries them to their destinations:

Right chorda tympani nerve, viewed from lateral side

Taste

There are similarities between the tastes the chorda tympani picks up in sweeteners between mice and primates, but not rats. Relating research results to humans is therefore not always consistent.[2] Sodium chloride is detected and recognized most by the chorda tympani nerve.[2] The recognition and responses to sodium chloride in the chorda tympani is mediated by amiloride-sensitive sodium channels.[3] The chorda tympani has a relatively low response to quinine and varied responses to hydrochloride. The chorda tympani is less responsive to sucrose than is the greater superficial petrosal nerve.[4]

Chorda tympani transection

The chorda tympani nerve carries its information to the nucleus of the solitary tract, and shares this area with the greater superficial petrosal and glossopharyngeal nerves. 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.[5] The chorda tympani, as part of the peripheral nervous system, 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 taste buds are not likely to grow back to full strength.[6][7] 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.[2] 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.[3] The chorda tympani innervates the fungiform papillae on the tongue.[7] 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”.[8] When some of the other papillae grow back, they do so without a pore.[7]

Dysfunction

Injury to the chorda tympani nerve leads to loss or distortion of taste from anterior 2/3 of tongue.[9] 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
  • Mandible of human embryo 24 mm. long. Outer aspect.
  • Distribution of the maxillary and mandibular nerves, and the submaxillary ganglion.
  • Plan of the facial and intermediate nerves and their communication with other nerves.
  • The course and connections of the facial nerve in the temporal bone.
  • Sympathetic connections of the submaxillary and superior cervical ganglia.
  • View of the inner wall of the tympanum (enlarged.)
  • Dissection of chorda tympani nerve
  • Lateral head anatomy detail.Facial nerve dissection.
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References
  1. Kwong, Y; Yu, D; Shah, J (August 2012). "Fracture mimics on temporal bone CT: a guide for the radiologist". AJR. American Journal of Roentgenology. 199 (2): 428–34. doi:10.2214/ajr.11.8012. PMID 22826408.
  2. Golden, G. J.; Ishiwatari, Y.; Theodorides, M. L.; Bachmanov, A. A. (2011). "Effect of Chorda Tympani Nerve Transection on Salt Taste Perception in Mice". Chemical Senses. 36 (9): 811–9. doi:10.1093/chemse/bjr056. PMC 3195788. PMID 21743094.
  3. Sollars, Suzanne I.; Bernstein, Ilene L. (1994). "Amiloride sensitivity in the neonatal rat". Behavioral Neuroscience. 108 (5): 981–7. doi:10.1037/0735-7044.108.5.981. PMID 7826520.
  4. Sollars, Suzanne I.; Hill, David L. (2005). "In vivorecordings from rat geniculate ganglia: Taste response properties of individual greater superficial petrosal and chorda tympani neurones". The Journal of Physiology. 564 (3): 877–93. doi:10.1113/jphysiol.2005.083741. PMC 1464453. PMID 15746166.
  5. Corson, S. L.; Hill, D. L. (2011). "Chorda Tympani Nerve Terminal Field Maturation and Maintenance is Severely Altered Following Changes to Gustatory Nerve Input to the Nucleus of the Solitary Tract". Journal of Neuroscience. 31 (21): 7591–603. doi:10.1523/JNEUROSCI.0151-11.2011. PMC 3117282. PMID 21613473.
  6. Hosley, M. A.; Hughes, S. E.; Morton, L. L.; Oakley, B (1987). "A sensitive period for the neural induction of taste buds". The Journal of Neuroscience. 7 (7): 2075–80. doi:10.1523/JNEUROSCI.07-07-02075.1987. PMC 6568951. PMID 3612229.
  7. Sollars, Suzanne I. (2005). "Chorda tympani nerve transection at different developmental ages produces differential effects on taste bud volume and papillae morphology in the rat". Journal of Neurobiology. 64 (3): 310–20. doi:10.1002/neu.20140. PMC 4965235. PMID 15898061.
  8. Sollars, Suzanne I.; Smith, Peter C.; Hill, David L. (2002). "Time course of morphological alterations of fungiform papillae and taste buds following chorda tympani transection in neonatal rats". Journal of Neurobiology. 51 (3): 223–36. doi:10.1002/neu.10055. PMC 4965232. PMID 11984844.
  9. Cain, P.; Frank, M. E.; Barry, M. A. (1996). "Recovery of chorda tympani nerve function following injury". Experimental Neurology. 141 (2): 337–46. doi:10.1006/exnr.1996.0169. PMID 8812170.
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