The Anatomy of the Chorda Tympani Nerve

The chorda tympani is a branch of the facial nerve and, along with other nerves, is important for carrying information about taste and other sensations from your taste buds to your brain. It's also involved in salivary function and a process called inhibition, which means that it lessens signals from other nerves that have to do with both taste and pain.

While the cranial nerves themselves are part of the central nervous system, the chorda tympani functions as part of the peripheral nervous system. It's therefore considered a peripheral nerve.

Anatomy

The facial nerve, which has a branch named the chorda tympani, is the seventh of the 12 paired cranial nerves (nerves of the head). While they exist in pairs, they're usually referred to as a single nerve or, when necessary, as the right or left nerve. The non-cranial nerves branch off from the spinal cord, but the cranial nerves originate in the brain. The cranial nerves pass through small holes in your skull base (called foramen) to exit and travel to various head and neck regions.

The facial nerve is one of many that emerge from your brainstem, which connects the brain and spinal cord from its position low in the back of your brain. From there, it travels a relatively long and complex course with numerous branches performing many distinct functions.

The facial nerve consists of two parts:

1. Extracranial (outside the skull, traveling through the face and neck)
2. Intracranial (moving through the inside of the skull and brain)

The intracranial branch runs close to the inner ear and into the facial canal, then gives off three branches:

1. Greater petrosal nerve, which deals with the mucous and lacrimal glands
2. Nerve to stapedius, which sends motor fibers to the stapedius muscle in the middle ear
3. Chorda tympani

Structure and Location

After splitting off from the intracranial branch of the facial nerve, the chorda tympani enters the ear. Its association with the ear is what gives the chorda tympani its name. "Tympani" is a type of drum, and the eardrum is called the tympanic membrane.

The chorda tympani branches off from the facial nerve in its vertical segment of the temporal bone (the main skull bone that houses the inner ear). It passes behind the ear drum and between two tiny bones of the middle ear called the incus and malleus. While in the middle ear, the chorda tympani sends a branch to the eustachian tube.

The nerve then leaves the ear through the petrotympanic fissure, exits the cranial cavity (skull), and joins the pathway of the lingual nerve, which runs along the jaw and goes to the tongue and floor of the mouth.

In the jaw, the chorda tympani reaches a collection of nerve cells called the submandibular ganglion. It then sends and receives signals across a synapse (a gap between nerves) with postganglionic fibers connected to two salivary glands, the submandibular (lower jaw) and sublingual (under the tongue). It also sends a branch to the parotid gland in the cheek.

The chorda tympani also sends off specialized fibers that continue along the lingual nerve to the front two-thirds of your tongue, where it connects to your taste buds. The lingual branch of the glossopharyngeal nerve performs the same function for the back one-third of the tongue.

Anatomical Variations

Several possible variations of the chorda tympani's course have been noted by researchers. These include different points of exit from the facial canal and differences in the angles of the channels it travels through. Some research has also identified differences in how long it travels alongside the lingual nerve.

In some rare cases, the chorda tympani may lie between the skin and bone of the external auditory canal.

These variations don't typically lead to functional changes that people are aware of. However, it's important for healthcare providers—and especially those who perform dental, ear, or facial surgeries—to know about these possible variations so they can avoid injuring the nerve during procedures.

Anatomical variations can also make it more difficult for practitioners to identify and diagnose cases of nerve damage or entrapment.

Function

The chorda tympani plays a few highly specialized roles, one being a specialized sensory function and another dealing with motor function (movement). Its final function deals with something called inhibition, which involves lessening the signals of other nerves.

Special Sensory Function

Like most other sensory nerves, the chorda tympani provides information on general sensation, such as pain and temperature, from the tongue to the brain.

However, it's also highly specialized and deals with taste signals in the front two-thirds of your tongue. It doesn't detect all tastes, just certain flavors. You may have heard that different areas of the tongue detect different tastes, and this is why: It all depends on what the nerves in that area are designed for.

The chorda tympani nerve is known to detect sodium chloride (saltiness) more than any other taste-related nerves. Research on the chorda tympani and sweet flavors is less certain, however, as it appears to detect sweetness in mice and primates but not in rats, so not all of the animal research that's been performed can be applied to humans.

Scientists do know, though, that the chorda tympani doesn't respond as much to sucrose (sugar) as does the greater superficial petrosal nerve.

Because the chorda tympani passes through the same space as the inferior alveolar nerve, which transmits pain signals from the lower teeth, dental anesthesia can eliminate taste in the front portion of the tongue.

Parasympathetic Function

The chorda tympani's parasympathetic function is also specialized. It's involved in making some glands secrete saliva, which is called secretomotor function. These glands include:

• Submandibular salivary gland
• Sublingual salivary gland
• Parotid gland

In addition, it causes the blood vessels in the tongue to dilate (open wider), which is called vasomotor function.

The chorda tympani does all this through the presynaptic parasympathetic fibers it carries, which transmit signals across the submandibular ganglion in your jaw.

Inhibitory Function

Studies have demonstrated that the chorda tympani has an inhibitory (lessening) effect on taste signals from the glossopharyngeal nerve at the back of the tongue as well as on pain in the tongue. This has been proven by anesthetizing the chorda tympani, which increases pain sensation and the perception of certain flavors, especially salt.

Experts theorize that this kind of inhibition may help the brain accurately classify a broader range of tastes and other sensations.

Associated Conditions

Consequences of damage to the chorda tympani and other oral sensory nerves are unpredictable and can take many forms.

The chorda tympani takes a long and meandering path through the head, and because of that, it's considered particularly vulnerable to damage. Damage is most common in the middle ear but can happen anywhere along its course. Common sources of damage include:

• Ear infection (especially in children)
• Otologic (ear) surgery
• Dental anesthesia
• Wisdom teeth removal
• Surgical removal of tumors in the ear
• Stroke
• Head trauma
• Facial paralysis
• Herpes zoster infection
• Temporomandibular joint disorder (TMJ)
• Reconstructive surgery

Damage to the facial nerve can also impair the chorda tympani's function.

Chorda tympani damage may cause:

• Reduced saliva secretion on the affected side
• Loss of taste to the front two-thirds of the tongue
• Increased pain response in the tongue
• Increased taste-perception of salt
• Phantom tastes (tasting things that aren't there)
• Phantom sensations
• Changes in the mouth-feel of food and beverages

Because of its inhibitory effect on pain, the chorda tympani is believed to be involved in a poorly understood painful condition called burning mouth syndrome.

If the chorda tympani is cut in a child, it's likely that the taste buds it innervates will never operate at full strength and might be structurally different from healthy taste buds.

Greater Implications Suspected

Some researchers suspect that taste damage in general, which often includes chorda tympani dysfunction, may be involved in more problems than previously thought.

Taste is thought to help your gastrointestinal system prepare for the arrival of food, and some evidence suggests that taste damage may lead to digestive problems.

For example, pregnant people who throw up a lot have been shown to have a loss of taste function. Ginger, a common folk remedy for pregnancy-related nausea and vomiting, stimulates taste, suggesting a link between taste loss and nausea. Additionally, many cancer treatments damage taste, and nausea is a common side effect of those treatments. While it hasn't been studied, many people claim that ginger, lemon, or candy can help relieve this nausea as well.

Rehabilitation

Typically, oral sensory complaints such as phantom pains or sensations resolve on their own, but it can take several months, and recovery may never be complete. Taste damage, especially, can be long-lasting, most notably if it involves bitterness.

If you have damage to the chorda tympani, your healthcare provider may be able to help you find treatments that manage the symptoms.

In cases of burning mouth syndrome, treatment involves the same drugs as in other neuropathic pain syndromes, including:

• Tricyclic antidepressants: Elavil (amitriptyline), Pamelor (nortriptyline)
• Benzodiazepines: Klonopin (clonazepam), Librium (chlordiazepoxide)
• Anticonvulsants: Neurontin (gabapentin), Lyrica (pregabalin)

Other problems resulting from chorda tympani damage may be treated with anti-inflammatory drugs (i.e., steroids, ibuprofen) or surgery, depending on the cause and nature of the dysfunction.