Tongue

Anterior two thirds

The presulcal tongue includes the apex and body of the organ. It terminates at the sulcus terminalis; which can be seen extending laterally in an oblique direction from the foramen cecum towards the palatoglossal arch. The mucosa of the dorsal surface of the oral tongue is made up of vallate, filiform, and fungiform papillae. There is also a longitudinal midline groove running in an anteroposterior direction from the tip of the tongue to the foramen cecum. This marks the embryological point of fusion of the lateral lingual swellings that formed the oral tongue. It also represents the location of the median lingual (fibrous) septum of the tongue that inserts in the body of the hyoid bone. 

On the lateral surface of the oral tongue are foliate papillae arranged as a series of vertical folds. The ventral mucosa of the oral tongue is comparatively unremarkable. It is smooth and continuous with the mucosa of the floor of the mouth and the inferior gingiva. The lingual veins are relatively superficial and can be appreciated on either side of the lingual frenulum. Lateral to the lingual veins are pleated folds of mucosa known as the plica fimbriata. They are angled anteromedially toward the apex of the tongue.

Posterior third

The remainder of the tongue that lies posterior to the sulcus terminalis is made up by the base of the organ. It lies behind the palatoglossal folds and functions as the anterior wall of the oropharynx. Unlike the oral tongue, the pharyngeal tongue does not have any lingual papillae. Instead, its mucosa is populated by aggregates of lymphatic tissue known as the lingual tonsils. The mucosa is also continuous with the mucosa of the laterally located palatine tonsils, the lateral oropharyngeal walls, and the posterior epiglottis and glossoepiglottic folds.

Before moving on, review the tongue with our quiz!

Muscles

The tongue is chiefly a muscular organ with some amount of fatty and fibrous tissue distributed throughout its substance. All the muscles of the tongue are paired structures, with each copy being found on either side of the median fibrous septum. There are muscles that extend outside of the organ to anchor it to surrounding bony structures, known as extrinsic muscles. The other set of muscles are confined to each half of the organ and contribute to altering the shape of the organ; these are the intrinsic muscles.

Intrinsic tongue muscles

The intrinsic tongue muscles are responsible for adjusting the shape and orientation of the organ. It is made up of four paired muscles, which are discussed below in a dorsoventral manner. 

The superior longitudinal muscles are made up of a thin layer of muscle fibers traveling in a mixture of oblique and longitudinal axes just deep to the superior mucosal surface of the organ. These fibers arise from the median fibrous septum as well as the fibrous layer of submucosa from the level of the epiglottis. They eventually insert along the lateral and apical margins of the organ. These muscles are responsible for retracting and broadening the tongue, as well as elevating the tip of the tongue. The net effect of these muscles results in shortening of the organ.

Another set of muscles occupy the dorsoventral plane of the tongue deep to the superior longitudinal muscles. These are the vertical muscles that arise from the root of the organ and genioglossus muscle and insert into the median fibrous septum, along the entire length of the organ. These muscles facilitate flattening and widening of the tongue.

Deep to the ventral muscles is the layer of transverse muscles of the tongue. They take a lateral route, extending from either side of the medial lingual septum (origin) to the fibrous submucosa along the lateral margins of the tongue (insertion). As these muscles contract, they cause the tongue to narrow and elongate.

Finally, the inferior longitudinal muscles travel above the ventral submucosa of the tongue. These fibers travel between hyoglossus and genioglossus as it arises from the base of the tongue and body of the hyoid bone. The fibers end in the apex of the tongue; allowing the muscle to pull the tip of the tongue inferiorly and also shortening the organ.

The intrinsic tongue muscles can operate independently, or in combination with each other to give rise to numerous shapes. This is an important feature of the tongue as it facilitates molding of the food particles into a bolus in preparation for deglutition and speech. 

Extrinsic tongue muscles

While the shape of the tongue is determined by the intrinsic muscles of the tongue, movement of the organ within (and out of) the oral cavity is dependent on the extrinsic tongue muscles. There are four pairs of extrinsic muscles, which can be viewed as those arising from above the tongue, and those that originate from below the tongue. 

Styloglossus and palatoglossus are the two muscles arising from above. Palatoglossus is anatomically a part of the pharyngeal group of muscles. However, its attachments to the tongue mean that it is also an extrinsic tongue muscle. It originates from the oral part of the aponeurosis of the soft palate. Here, and also at its insertion in the lateral margins of tongue, the muscle is wider than along its middle section. Its role as an extrinsic tongue muscle is to elevate the dorsal surface of the tongue, and (while working synergistically with the contralateral palatoglossus) to act as a sphincter at the oropharyngeal isthmus.

Styloglossus originates from the anterolateral surface of the styloid process. Not only does it contribute to the stylomandibular ligament, but it also assists in retraction of the tongue (moving it posterosuperiorly). It is the smallest and shortest of the three styloid muscles. At the lateral margin of the tongue, the muscle bifurcates into longitudinal and oblique components. The former pierces the tongue on the dorsolateral aspect and integrates with the inferior longitudinal muscle; while the latter crosses over and decussates with hyoglossus.

Genioglossus and hyoglossus arise from below. Genioglossus originates from a slender tendon that is attached to the superior genial tubercle found on the inner surface of the symphysis menti. This attachment prevents the tongue from falling backward and obstructing the airway when an individual is supine. The lower fibers of the muscle also have indirect attachments to the anterior part of the body of the hyoid bone via its slender aponeurosis. It is a triangular, midline structure that travels posterosuperiorly at which point the upper fibers of the muscle interdigitate with the intrinsic muscles, before attaching along the length of the inferior surface of the tongue (extending from the root to the tip).

Hyoglossus originates from the entire greater cornu of the hyoid bone as a slender, quadrilateral muscle. It is often accompanied by chondroglossus (may be considered as part of the hyoglossus), which arises from the base of the lesser cornu of the hyoid bone. Hyoglossus takes a vertical course cranially, where it pierces the inferolateral margins of the tongue and subsequently blends between the inferior longitudinal muscles and the styloglossus.

The extrinsic muscles play an important role in pressing and molding the food bolus in preparation for the initial phase of swallowing. Additionally, they are used to move the bolus posteriorly into the oropharyngeal inlet. Furthermore, the action of palatoglossus closes off the oropharyngeal isthmus in order to prevent food from moving cranially during swallowing. Although some of these muscles are able to act in isolation, it is the combined effect of all the intrinsic and extrinsic muscles that allows the tongue to have significant flexibility. 

Take our quiz on the muscles of the tongue, or check out our study unit for a deeper look:

The lingual papillae

There are four types of lingual papillae found on the surface of the human tongue. These include:

• Filiform papillae are the most abundant of the four types of papillae. They are stretched, conical, grey-white papillae that are covered in a heavy coat of keratinized squamous epithelium. By making the dorsal surface of the tongue rough, these papillae provide friction to allow movement of the food bolus during chewing. It should be noted that these papillae do not possess taste buds.
• Fungiform papillae are weakly keratinized and less abundant than the filiform papillae. However, they are scattered across the entire dorsal surface of the tongue. These highly vascular, mushroom-shaped papillae contain a few taste buds on the apical aspect. 
• Foliate papillae appear as bilaterally paired, parallel, longitudinal slits on the posterolateral margin of the tongue, near the sulcus terminalis. The mucosa is non-keratinized and the papillae are populated with numerous taste buds. 
• Circumvallate (Vallate) papillae are organized linearly, as a set of four to six large papillae anterior to each limb of the sulcus terminalis (i.e. eight to twelve papillae in total). In longitudinal section, the characteristic furrow found within the papillae can be appreciated. These moats facilitate the drainage of serous salivary von Ebner glands that empty into the structure. The persistent lubrication creates a favorable environment for gustatory particles to dissolve so that they can be detected by the taste buds.

Taste buds

While taste buds are distributed throughout the entire oral cavity, they are at higher concentrations on the tongue. Each taste bud is clear, oval and covered by stratified squamous epithelium. A combination of elongated taste (gustatory), supportive, and basal stem cells can be found within each taste bud. The gustatory cells have an apical taste pore surrounded by numerous microvilli that binds dissolved molecules and brings them closer to the receptors responsible for taste. However, these cells have a relatively high turnover rate, as their shelf life is roughly seven to ten days.

There are five gustatory sensations that are perceived by individuals. These are sweet, salty, sour, bitter, and umami. The microvilli found on the apical surface of the taste cells are equipped with various receptors that bind to varying molecules. The reaction generated from this compound-receptor interaction gives rise to varying action potentials that are subsequently perceived as taste. Saltiness is often associated with the cationic component of a compound (i.e. sodium ions), while sourness is related to the acidity (concentration of hydrogen ions) in the compound. Organic compounds such as carbohydrates or amino acids give rise to sweet taste, while bitterness is associated with long-chain organic compounds. The final taste - umami - also known as savoury, is related to compounds with the left-handed chiral isomer of glutamic acid.

Before moving on take a moment to test your new knowledge on the microstructure of the tongue.

Arteries

The vascular supply to the tongue muscles is provided by derivatives of the lingual artery. This is a branch of the external carotid artery that traverses the region between the middle pharyngeal constrictor and hyoglossus in order to access the floor of the mouth. It takes a sharp superior turn at the anterior border of hyoglossus as it travels alongside CN IX. Of note, the tongue has good collateral supply as the lingual artery also anastomosis with the contralateral vessel. The named branches of the lingual artery are as follows:

• The dorsal lingual arteries are relatively small derivatives of the lingual artery that arise medial to hyoglossus. In addition to supplying the dorsal mucosa of the tongue, it also gives branches to the palatoglossus, soft palate, palatine tonsils, and epiglottis. 
• Emerging at the anterior limit of the hyoglossus, the sublingual arteries course between the mylohyoid and genioglossus as it travels towards the sublingual glands in the floor of the oral cavity. As it arborizes, one of its branches anastomoses with the submental branches of the facial artery, while another traverses the gingiva of the mandible to anastomose with the analogous contralateral vessel. 
• As the lingual artery terminates near the lingual frenulum on the ventral surface of the tongue, it is referred to as the deep lingual artery.

The lingual artery is supported by other branches of the external carotid artery. The facial artery gives off the ascending palatine and tonsillar arteries that also supply the tongue. The ascending pharyngeal branch of the external carotid artery also supplies the organ.

Veins

The veins of the tongue are named similarly to the arteries that they accompany. They are formed from numerous venous tributaries that eventually coalesce. As the deep lingual vein forms adjacent to the apex of the tongue, it courses along the ventral surface of the tongue (deep to the mucosa).  As the deep lingual vein anastomosis with the sublingual vein, they become the vena comitans of hypoglossal nerve (CN XII). This venous network eventually drains to the lingual vein that later join the facial or the anterior division of the retromandibular veins. Here, they form the common facial vein, which is a tributary to the internal jugular vein. Alternatively, the venae comitantes may drain directly to the internal jugular vein.

The dorsal lingual veins are responsible for draining the lateral margins and dorsal surface of the tongue. They travel alongside the similarly named artery as they drain into the internal jugular vein.

Lymphatic drainage

When discussing the lymphatic drainage of the tongue, it helps to group them according to the region of the tongue that they drain. The marginal and central groups drain the anterior parts of the tongue, while the dorsal group drains lymph from the posterior third of the organ. It is not uncommon to see the central area of the tongue draining to both marginal and dorsal groups of lymph vessels.

The marginal lymph vessels will carry lymph to the submandibular nodes or to the jugulo-omohyoid nodes. It is not uncommon to see lymph vessels decussating to drain to contralateral lymph nodes. The vessels from the central region may go to the deep cervical nodes, with a particular preference for the jugulo-omohyoid or jugulodigastric nodes. The dorsal group of vessels also pass laterally on either side to eventually join the marginal vessels in their course to the jugulo-omohyoid and jugulodigastric vessels.

Motor innervation

The muscles of the tongue arise from occipital myotomes that migrated to the floor of the pharyngeal apparatus during development. These primitive myocytes took the fibers of the hypoglossal nerve (CN XII) along with them during their journey. As a result, the hypoglossal nerve provides motor innervation to all the muscles of the tongue, except palatoglossus. As the hypoglossal nerve pierces the ventrolateral part of the pharyngeal tongue, it gives a branch to the geniohyoid muscle. Subsequently, it bifurcates into medial and lateral branches. The medial branch innervates the posterior part of the transverse and vertical muscles, as well as the medial part of the inferior longitudinal muscle, and the entire genioglossus. The lateral branch of the hypoglossal nerve innervates the lateral part of the inferior longitudinal, superior longitudinal, hyoglossus and styloglossus muscles.

While there is an agreement regarding the fact that the pharyngeal plexus brings motor fibers to the muscle, there is still some discrepancy regarding which component of the pharyngeal plexus (i.e. cranial part of accessory nerve [CN XI] or the vagus nerve [CN X]) that the fibers arise from. Some sources state that fibers of the accessory nerve piggyback along the vagus nerve to supply palatoglossus. However, other sources are adamant that there is no hitch-hiking, and that vagus nerve is the nerve which supplies the palatoglossus. One thing is certain, and it’s that nucleus ambiguus provides efferent fibers that innervate the skeletal muscles of the soft palate. Whether or not these fibers travel via the accessory or vagus nerve is still uncertain.

Tactile sensory innervation

The lingual nerve is a branch of the mandibular nerve (CN V3). It is responsible for conveying general somatic afferent impulses from the anterior two-thirds of the tongue. Additionally, it also carries sensory information from the oral mucosa beneath the ventral surface of the tongue as well as the gingival mucosa of the lingual side of the mandible. General afferent impulses from the circumvallate papillae, along with the posterior third of the tongue are carried by fibers of the glossopharyngeal nerve (CN IX).

Taste innervation

There are three cranial nerves responsible for conveying taste sensation from the tongue to the brain. These are facial (CN VII), glossopharyngeal (CN IX) and (to a lesser extent) vagus (CN X) nerves The region of the tongue covered by each nerve is dependent on the proximity of the developing taste bud (and lingual papilla) to the free nerve ending. The facial nerve mitigates special sensory signals from the anterior two-thirds of the tongue, as well as from the inferior part of the soft palate. Fibers of the facial nerve arise across the tongue, anterior to the terminal sulcus and are initially are carried via the lingual nerve, which in turn passes them onto the chorda tympani.

The postsulcal tongue, circumvallate papillae, palatoglossal arches, and oropharynx are governed by the glossopharyngeal nerve (CN IX). The vagus nerve (CN X) only provides supply to taste buds in the pharyngeal extremity of the tongue. These impulses are conveyed by the internal laryngeal branch of the vagus nerve, via the pharyngeal plexus.

The neurovasculature of the tongue can seem like a lot! The best way to learn is by applying what you've learned. Our quiz can help with that. Or if you need a little more revision, check out our study unit.

Anterior two thirds

The tongue is one of those structures derived from the pharyngeal apparatus. Towards the end of the fourth gestational week,  the mesenchyme of the ventromedial aspect of the first pharyngeal arches begins to proliferate in the floor of the primitive pharynx to form the median lingual swelling (tuberculum impar). This triangular protuberance is located cranial to the foramen cecum (opening of the thyroglossal duct) and is later joined by the two distal tongue buds (lateral lingual swellings). The lateral lingual swellings are oval enlargements arising on either side of the median lingual swelling. The rate of growth of the lateral lingual swellings exceeds that of the median lingual swelling.

By the 5th to 6th gestational week, they outgrow the median lingual swelling and fuse in the midline to form the oral part of the tongue. The fibrous lingual septum represents the point of fusion of the two lateral lingual swellings, which lies deep to the midline groove (a central groove along the longitudinal axis) of the tongue.  

Posterior third

Caudal to the foramen cecum, two mesenchymal derivatives arise from the ventromedial part of the second to fourth pharyngeal arches. The copula linguae are the product of the second pharyngeal arches, while the hypopharyngeal eminence – which develops below the copula – arises from the third and fourth pharyngeal arches. Like the lateral lingual swellings, the hypopharyngeal eminences grow at a faster rate than the copula. As a result, the copula regresses between the 4th and 5th week of development and the hypopharyngeal eminence becomes the pharyngeal part of the tongue. 

The oral and pharyngeal part of the tongue eventually fuses, forming a V-shaped junction known as the sulcus terminalis. While the connective tissue and vascular supply of the tongue mostly originate from the preceding pharyngeal arches, the intrinsic and extrinsic muscles are myoblast derivatives originating from the occipital myotomes. 

Are you surprised by how much there is to learn about the tongue? Be sure to remember the importance of active recall when learning this topic! 

Also, the tongue and the hard palate develop simultaneously. It contributes to molding the palate into the arched structure that it eventually becomes during postnatal life. Most of the morphological changes of the tongue occur during the first trimester of pregnancy. By the second trimester and into extrauterine life, the rest of the changes are related to elongation and repositioning of the organ. Both the presulcal and postsulcal parts of the tongue are within the oral cavity proper at birth. Caudal migration of the posterior third of the tongue is not completed until the age of 4 years.

Taste buds and papillae

The coarse texture of the dorsal surface of the tongue can be attributed to the numerous lingual papillae that are found on its surface. As the 8th gestational week draws to a close, foliate and vallate papillae are the first of the four papillae to develop. These are followed by the appearance of fungiform papillae. By the 10th – 11th week of gestation, the thread-like filiform papillae can be observed on the dorsal surface of the tongue. Each type of papillae has a particular role in tongue physiology, and as such, has a unique innervation based on the nerve endings they developed closest to.

The development of taste buds begins as the last of the papillae are formed in the 11th week of gestation. Their formation is influenced by the invading special sensory nerve fibers, as well as inductive factors from the surrounding epithelium. Although the majority of these specialized gustatory receptors will develop on the dorsum of the tongue, they also arise on the hard and soft palates, dorsum of the epiglottis, palatoglossal arches, and the posterior oropharyngeal wall. Taste bud development is concluded around the 13th gestational week.

Nerve supply

Owing to the fact that each pharyngeal arch has its own neurovascular bundle, the derivatives of these pouches will also take their nerve supply from these sources as well. The mandibular division of the trigeminal nerve (CN V3) carries sensory impulses arising from the derivatives of the first pharyngeal arch. Since this arch gave rise to the tuberculum impar and lateral lingual swellings (which forms the oral part of the tongue), the mucosa of this entire area sends afferent impulses via the lingual branch of CN V3.

Even though the second pharyngeal arch does not contribute to a visible structure on the surface of the tongue, a part of its nerve supply – the chorda tympani; a branch of the facial nerve (CN VII) – carries special afferent signals from the taste buds (excluding vallate papillae) of the presulcal part of the tongue. The vallate papillae, along with the mucosa of the postsulcal part of the tongue, pass afferent stimuli to the glossopharyngeal nerve (CN IX); which is the nerve of the third pharyngeal arch. This innervation is also supplemented by the nerve of the fourth pharyngeal arch – the vagus nerve (CN X) – which carries sensory stimuli from the base of the tongue that is anterior to the epiglottis.

Recall that both intrinsic and extrinsic muscles of the tongue arise from occipital myotomes that migrated ventrally to populate the developing tongue. As a result, the migrating myocytes took the hypoglossal nerve (CN XII) along with it. Therefore, CN XII provides efferent stimuli to all the muscles of the tongue. The exception to this rule is palatoglossus, which derives its motor supply from the pharyngeal plexus. 

To recap what was just discussed: 

• Development of the tongue begins toward the end of the fourth gestational week.
• The anterior two-thirds of the organ is known as the presulcal (oral) part, and the posterior third is the postsulcal (pharyngeal) part.
• The presulcal tongue originates from the first pharyngeal arch, while the postsulcal part arises from the third and fourth pharyngeal arches.
• Neither the tuberculum impar (from 1st pharyngeal arch) nor the copula (2nd pharyngeal arch) contributes to structures of the adult tongue. 
• The presulcal tongue has lingual papillae and taste buds, while the postsulcal part has lingual tonsils and taste buds. 
• Innervation of the tongue is dependent on the pharyngeal arch that the area was derived from.