Anatomy of Taste
Taste is a component of our daily life and gives our food character. The way we taste is through our nose and our oral cavity, which send chemical information from our taste buds and olfactory epithelium to our brain for processing.
The process of taste and smell are intertwined, but we will focus mainly on taste in this article. The function of the salivary glands and the way our brain perceives taste are also addressed.
We will discuss the relevant anatomical structures and their function in this article.
Taste is only partly conveyed by the tongue. The sense of smell also has a significant role to play. Odorant molecules are inhaled through the nose, and will make contact with the olfactory epithelium that is coated with a range of olfactory receptors. These will transduce the chemical signals into electrical ones, which then pass into the olfactory nerve.
The olfactory nerve (cranial nerve 1) enters the skull via the cribriform plate (a section of the ethmoid bone) and synapses in the olfactory bulb, where it forms the olfactory tract and passes posteriorly to widen into a triangular shape known as the olfactory trigone. These structures are connected to many areas of the brain including the anterior olfactory nucleus (an anterior section of the olfactory tract), the piriform cortex (the main area for smell detection located in the medial region of the temporal lobe), the medial nucleus of the amygdala (the danger detector which lies anterior to the hippocampus in the temporal lobe), and the entorhinal cortex (located in the medial temporal lobe).
Circumvallate papillae- These papillae lie in a V-shaped row immediately anterior to the sulcus terminalis, which divides the dorsum of the tongue into its anterior two-thirds and a posterior third. The shape of the papillae resembles a frustum (truncated cone). The base that attaches onto the tongue is the narrower end and the broader apex is coated with stratified squamous epithelium.
Surrounding each papilla is a depression in which von Ebner’s glands empty a serous secretion. It is believed that this circular depression acts as a moat around each papilla, and works to remove stimuli from the base of the circumvallate papillae, ensuring that they are always primed for new taste stimuli. These papillae send afferent taste sensation to the glossopharyngeal nerve (despite the fact that they lie anterior to the sulcus terminalis).
Filiform Papillae- These are the commonest papillae on the tongue, and cover the majority of the anterior two thirds of the tongue. They are small and conical, and do not contain taste buds. They comprise a core of irregular connective tissue, which is covered by a keratinized epithelium featuring secondary processes, which contain elastic fibers and are hence flexible. A whitish tint can be found on these numerous secondary processes. They are overlapped into a brush like dense layer of processes.
Fungiform Papillae- These are named fungiform as they resemble mushrooms. Lying at their core is connective tissue. They are located on the superior surface of the tongue, and are dotted around the surface. Lying on them are numerous taste buds that are able to discern sweetness, sourness, saltiness, bitterness and umami. These papillae are innervated by the facial nerve (cranial nerve 7), with nerve fibers leaving the tongue via the submandibular ganglion, lingual nerve, chorda tympani and geniculate ganglion in order to reach the solitary nucleus of the brainstem.
Foliate Papillae- These are vertical, relatively short and lie on either side and back of the tongue. They can be located anterior to the palatoglossal arch. They are coated with non-keratinized epithelium and hence are softer than other papillae. They have numerous taste buds.
Chorda Tympani- This is a special branch of the facial nerve (cranial nerve 7) that arises from the taste buds of the anterior two thirds of the tongue (anterior to the sulcus terminalis). The facial nerve leaves the cranial vault through the internal auditory meatus along with the vestibulocochlear nerve and runs in the facial canal. Next it passes between the malleus and the incus which are two of the ossicles found in the middle ear. This is reflected in the name of the nerve as it makes a chord with the tympanic membrane, hence chorda tympani. It emerges from the petrotympanic fissure, and then emerges from the cranial cavity to enter the infratemporal fossa. At this point, the nerve converges with a division of V3, the mandibular division of the trigeminal nerve in order to reach the submandibular ganglion as the lingual nerve. It is then distributed to the taste buds of the anterior two thirds of the tongue with the branches of the lingual nerve that provide general sensation to the surface of the tongue.
Glossopharyngeal nerve- Taste and sensation of the posterior one third of the tongue is provided by the glossopharyngeal nerve (cranial nerve 9). The nerve leaves the skull through the jugular foramen along with the vagus nerve (cranial nerve 10) and the accessory nerve (cranial nerve 11).
Vagus Nerve- The epiglottis also has a small amount of taste sensation, which is conveyed by the vagus nerve (cranial nerve 10).
The taste buds provide sensation, which is passed to the caudal and rostral regions of the nucleus tractus solitarius in the medulla. From the gustatory parts of the medulla, the information next projects to the ventroposterior medial thalamus. From this nucleus, the information is sent to the primary gustatory cortex that is located in the insular cortex.
The lingual artery perfuses the tongue, and is the second branch of the external carotid artery. The tongue is drained by the lingual veins, which drain into the internal jugular veins.
Saliva has an important role in chewing, swallowing and taste. The three glands that secrete saliva into the mouth are located around the face.
Submandibular- Under the angle of mandible, these glands are innervated by the facial nerve via the submandibular ganglion and are perfused by the submental arteries (branches of the facial artery). They secrete saliva in the sublingual space through the submandibular ducts or Wharton’s ducts.
Sublingual- They are innervated by the facial nerve and perfused by the sublingual (lingual artery) and submental (facial artery) arteries. They empty their secretions into the floor of the mouth via a collection of ducts referred to as ducts of Rivinus.
Parotid Gland- This is a large gland that sits on the side of the face, anterior and inferior to the ear. The motor branches of the facial nerve (temporal, zygomatic, buccal, mandibular and cervical) run through it. Innervation is from the glossopharyngeal nerve via the otic ganglion. The parotid duct (Stenson’s duct) enters the mouth by the cheek, opposite the second upper molar. The gland is perfused by branches of the external carotid artery.
Bell’s Palsy- A lower motor neurone lesion of the facial nerve results in a multitude of symptoms. These include hemiplegia of the face (motor branches affected), inability to close the eyes fully (orbicularis oculi affected), sounds causing pain as they are too loud (due to lack of innervation by the nerve to stapedius), and abnormal taste in the anterior two thirds of the tongue (chorda tympani affected).
Ageusia- This is an inability to taste. Causes include multiple sclerosis, and familial dysautonomia (an inherited condition affecting the autonomic system).
Squamous cell carcinoma of the tongue- This is the most common form of tongue cancer. Risk factors include alcohol, tobacco smoking and human papillomavirus infection of the oral cavity. Symptoms include the growing tumour itself, which will often be painless for the patient.