17.2 The Human Tongue

The tongue has many roles including ingestion, deglutition (swallowing), chemical digestion (lingual lipase), sensation (taste, texture, temperature), swallowing, and speech. It is positioned over the floor of the oral cavity, attached to the mandible, the styloid processes of the temporal bones, and the hyoid bone.

The tongue is divided into two parts, an anterior oral part (body) and a posterior pharyngeal part (root). The separation is made by a V-shaped groove called terminal sulcus. The apex of the terminal sulcus points posteriorly and it is marked by a blind foramen, the foramen cecum, which is the remnant of a duct to an embryonic sac (thyroid diverticulum) that connected the tongue to the thyroid gland during early embryonic development.

The left and right sides of the tongue are separated along most of its length by a vertical section of fibrous tissue (lingual septum or median septum) that forms a groove (median sulcus) on the tongue's surface. An adult human tongue has a length of about 10 cm and a mass of about 65 g, mostly formed by skeletal muscle. The surface of the tongue is divided into dorsal (the upper, visible surface) and ventral (the inferior surface that faces the floor of the mouth) and it is formed by mucosa, a sheet of connective tissue + squamous epithelium that secretes mucus.

Muscles

Deep to the mucous membrane covering, the tongue is composed of four intrinsic muscles and four pairs of extrinsic muscles. The intrinsic tongue muscles insert into the tongue from origins within it, whereas the extrinsic tongue muscles insert from outside origins. The extrinsic muscles move the entire tongue in various directions, whereas the intrinsic muscles change the shape of the tongue (ex: curling or flattening it).

The intrinsic muscles are are the superior longitudinal muscle, the inferior longitudinal muscle, the vertical muscle, and the transverse muscle. The superior longitudinal muscle runs along the upper surface of the tongue under the mucous membrane, and elevates, assists in retraction of, or deviates the tip of the tongue. It originates near the epiglottis, at the hyoid bone, from the median fibrous septum.

The inferior longitudinal muscle lines the sides of the tongue, and is joined to the styloglossus muscle. The vertical muscle is located in the middle of the tongue and joins the superior and inferior longitudinal muscles. The transverse muscle divides the tongue at the middle and is attached to the mucous membranes that run along the sides.

The extrinsic muscles all include the word root glossus (glossus = “tongue”) and the muscle names are derived from where the muscle originates. The genioglossus (genio = “chin”) originates on the mandible and allows the tongue to move downward and forward. The styloglossus originates on the styloid bone, and allows upward and backward motion. The palatoglossus originates on the soft palate to elevate the back of the tongue, and the hyoglossus originates on the hyoid bone to move the tongue downward and flatten it.

Working in concert, these muscles perform three important digestive functions in the mouth: (1) position food for optimal chewing, (2) gather food into a bolus (rounded mass), and (3) position food so it can be swallowed.

Mucosa

The dorsal surface of the tongue is formed by the masticatory oral mucosa. It contains areolar connective tissue with a variety of lingual glands and a keratinized stratified squamous epithelium. Lingual glands in the connective tissue secrete saliva, mucus and a watery serous fluid that contains the enzyme lingual lipase, which plays a minor role in breaking down fat but does not begin working until it is activated in the stomach.

The dorsal mucosa contains a high density of four types of lingual papillae. Filiform papillae cover most of the dorsal surface of the anterior 2/3 of the tongue, with fungiform interspaced. Filiform papillae are small, long, thin and lack taste buds. They have touch receptors are keratinized to create an abrasive surface on the tongue. Fungiform papillae are mushroom shaped with taste buds on their upper surface. Foliate papillae are four or five short vertical folds and are present on each side at the back of the tongue. They lack keratin and bear many taste buds. Circumvallate papillae are dome-shaped. Humans have 8 to 12 aligned in a V-shape immediately anterior to the foramen cecum and sulcus terminalis. Each papilla consists of a projection of mucous membrane surrounded by a circular depression. The wall of the projection is called (vallum), and the circular sulcus is called fossa. Numerous taste buds are found in the vallum. Ducts of lingual salivary glands, known as Von Ebner's glands, empty a serous secretion into the fossa.

The mucosa of the ventral side of the tongue is mostly non-keratinized. A medial fold on it, the lingual frenulum, tethers the tongue to the floor of the mouth. People with the congenital anomaly ankyloglossia, also known by “tongue tie,” have a lingual frenulum that is too short or otherwise malformed. Severe ankyloglossia can impair speech but it can be resolved with a simple surgery.

Gustation

Gustation is the special sense associated with the tongue. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. The taste buds of the lingual papillae are round epithelial structures containing basal (stem) cells, gustatory receptor cells and supporting cells.

The gustatory receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.

Salty taste is simply the perception of sodium ions (Na+) in the saliva. When you eat something salty, the salt crystals dissociate into the component ions Na+ and Cl–, which dissolve into the saliva in your mouth. The Na+ concentration becomes high outside the gustatory cells, creating a strong concentration gradient that drives the diffusion of the ion into the cells. The entry of Na+ into these cells results in the depolarization of the cell membrane and the generation of a receptor potential.

Sour taste is the perception of H+ concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For example, orange juice—which contains citric acid—will taste sour because it has a pH value of approximately 3. Of course, it is often sweetened so that the sour taste is masked.

The first two tastes (salty and sour) are triggered by the cations Na+ and H+. The other tastes result from food molecules binding to a G protein–coupled receptor. A G protein signal transduction system ultimately leads to depolarization of the gustatory cell. The sweet taste is the sensitivity of gustatory cells to the presence of glucose dissolved in the saliva. Other monosaccharides such as fructose, or artificial sweeteners such as aspartame (NutraSweet™), saccharine, or sucralose (Splenda™) also activate the sweet receptors. The affinity for each of these molecules varies, and some will taste sweeter than glucose because they bind to the G protein–coupled receptor differently.

Bitter taste is similar to sweet in that food molecules bind to G protein–coupled receptors. However, there are a number of different ways in which this can happen because there are a large diversity of bitter-tasting molecules. Some bitter molecules depolarize gustatory cells, whereas others hyperpolarize gustatory cells. Likewise, some bitter molecules increase G protein activation within the gustatory cells, whereas other bitter molecules decrease G protein activation. The specific response depends on which molecule is binding to the receptor.

One major group of bitter-tasting molecules are alkaloids. Alkaloids are nitrogen containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbe infection and less attractive to herbivores.

Therefore, the function of bitter taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are normally ingested are often combined with a sweet component to make them more palatable (cream and sugar in coffee, for example). The highest concentration of bitter receptors appear to be in the posterior tongue, where a gag reflex could still spit out poisonous food.

The taste known as umami is often referred to as the savory taste. Like sweet and bitter, it is based on the activation of G protein–coupled receptors by a specific molecule. The molecule that activates this receptor is the amino acid L-glutamate. Therefore, the umami flavor is often perceived while eating protein-rich foods. Not surprisingly, dishes that contain meat are often described as savory.

Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness.

Development

The tongue begins to develop in the fourth week of embryogenesis. Its mucosa is formed with contributions from the first four pharyngeal arches. The first step is the development of a median swelling called tuberculum impar on the first pharyngeal arch.

In the fifth week a pair of lateral swellings, the lateral lingual swellings (distal tongue buds) one on the right side and one on the left, form on the first pharyngeal arch. These lingual swellings quickly expand, cover the tuberculum impar and continue to develop through prenatal development. They form the anterior part of the tongue that makes up two thirds of the length of the tongue. The line of their fusion is marked by the median sulcus.

In the fourth week a swelling appears from the second pharyngeal arch, in the midline, called the copula. During the fifth and sixth weeks the copula is overgrown by a swelling from the third and fourth arches (mainly from the third arch) called the hypopharyngeal eminence, and this develops into the posterior part of the tongue (the other third). The hypopharyngeal eminence develops mainly by the growth of endoderm from the third pharyngeal arch. The boundary between the two parts of the tongue, the anterior from the first arch and the posterior from the third arch is marked by the terminal sulcus. The terminal sulcus is shaped like a V with the tip of the V situated posteriorly. At the tip of the terminal sulcus is the foramen caecum, which is the point where the embryological thyroid begins to descend.

In contrast with the origin of the mucosa described above, the intrinsic and extrinsic muscles of the tongue do not originate from the pharyngeal arches. The are formed by a group of cells from the somites of the embryo. These cells migrate ventrally and anteriorly to enter the mouth and form the lingual musculature. They are innervated by the hypoglossal nerve which provides the motor control to the tongue.

Innervation

The tongue is innervated by motor fibers, special sensory fibers for taste, and general sensory fibers for sensation.

• • Motor supply for all intrinsic and extrinsic muscles of the tongue is supplied by the hypoglossal nerve (CN XII)

Innervation of taste and sensation is different for the anterior and posterior part of the tongue because they are derived from different embryological structures.

Anterior two thirds of tongue (anterior to the terminal sulcus):

• • • Touch: lingual branch of the trigeminal nerve (CN V from 1st arch)

    • Taste: chorda tympani branch of the facial nerve (CN VII from 2nd arch)

Posterior one third of tongue:

• • • Touch and taste: glossopharyngeal nerve (CN IX from 3rd arch)

Base of tongue

• • • Touch and taste: internal branch of the superior laryngeal nerve (itself a branch of the vagus nerve, CN X from 4th arch) 

Blood supply

The tongue receives its blood supply primarily from the lingual artery, a branch of the external carotid artery. The lingual veins drain into the internal jugular vein. The floor of the mouth also receives blood from the lingual artery. The tongue has secondary blood supplies from the tonsillar branch of the facial artery and the ascending pharyngeal artery.

Summary

The human tongue has four intrinsic and four extrinsic muscles, all innervated by the hyoglossal nerve. Its mucosa receives sensory innervation from four cranial muscles because it is formed with tissue contributions from four pharyngeal arches.

Key terms

Lingua, terminal sulcus, foramen cecum, thyroid diverticulum, thyroid gland, pharyngeal arch, lingual septum, median sulcus, intrinsic tongue muscles, superior longitudinal, lingual muscle, inferior longitudinal lingual muscle, vertical lingual muscle, transverse lingual muscle, extrinsic tongue muscles, palatoglossus muscle, hyoglossus muscle, styloglossus muscle, genioglossus muscle, filifom papilla, fungiform papilla, foliate papilla, circumvallate papilla, taste bud, von Ebner's gland, lingual frenulum, tongue-tie, ankyloglossia, gustatory receptor cell, supporting cell, basal cell, tuberculum impar, lateral lingual swellings, copula, hypopharyngeal eminence, hypoglossal nerve, trigeminal nerve, facial nerve, chorda tympani, glossopharyngeal nerve, superior laryngeal nerve, vagus nerve, lingual artery, external carotid artery, lingual vein, internal jugular vein, tonsillar artery, ascending pharyngeal artery.

Figure credits

Figure 1 by Dr. Johannes Sobotta - Sobotta's Atlas and Text-book of Human Anatomy 1906, Public Domain, https://commons.wikimedia.org/w/index.php?curid=29847524

Figure 2 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 1020, Public Domain, https://commons.wikimedia.org/w/index.php?curid=552454

Figure 3 by OpenStax - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30131683

Figure 4 by Mahdiabbasinv - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=47480806

Figure 5 by OpenStax - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30147989

Figure 6 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 979, Public Domain, https://commons.wikimedia.org/w/index.php?curid=566916

Figure 7 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 559, Public Domain, https://commons.wikimedia.org/w/index.php?curid=522464