Muscles and taste sensation of the tongue
The tongue is a muscular organ situated in the oral cavity, and an accessory digestive organ. Its main functions include sensation of taste, mastication (chewing), deglutition (swallowing), speech, and clearing the oral cavity.
The rich motor and sensory innervation of the tongue is carried by four cranial nerves
- The hypoglossal nerve (CN XII)
- The facial nerve (CN VII)
- The glossopharyngeal nerve (CN IX)
- The vagus nerve (CN X)
|Intrinsic muscles||Superior longitudinal, inferior longitudinal, transverse, and vertical muscles
Innervated by hypoglossal nerve (CN XII)
|Extrinsic muscles||Genioglossus, hyoglossus, styloglossus, and palatoglossus
Innervated by hypoglossal nerve (CN XII), except palatoglossus innervated by vagus nerve (CN X) (via branches of pharyngeal plexus)
Receptors: taste buds within fungiform, vallate, and foliate papillae
Anterior two-thirds: chorda tympani (branch of facial nerve CN VII)
Posterior one-third and vallate papillae: glossopharyngeal nerve (CN IX), superior laryngeal nerve (branch of vagus nerve CN X)
|Taste (gustatory) pathway||
Anterior two-thirds: facial nerve (CN VII) → chorda tympani → geniculate and otic ganglia → anterior solitary tract nucleus
Posterior third: superior laryngeal and glossopharyngeal (CN IX) nerves → inferior glossopharyngeal and inferior vagal ganglia → anterior solitary tract nucleus
From anterior solitary tract nucleus → central tegmental tract → thalamus → gustatory cortex
|Clinical relations||Unilateral and bilateral paralysis of hypoglossal nerve, paralysis of facial nerve, dysarthria, damage to taste sensation, papillae, tongue muscles|
This article will discuss the anatomy of the tongue, taste sensation, the gustatory pathway, as well as pathological conditions related to the tongue.
- Tongue muscles
- Taste sensation
- Clinical relations
The tongue is an entirely muscular organ. It is separated medially into two halves by a connective septum, the lingual septum. The muscles that make up the tongue are paired, with each muscle from a pair being related to one half of the tongue.
There are two groups of muscles associated with the tongue. One group are muscles that comprise the core of the tongue and are placed within it, and therefore are called the intrinsic muscles. Their main function is altering the shape of the tongue, and they are: the superior longitudinal, inferior longitudinal, transverse and vertical muscles.
The other group comprises the muscles that are outside the tongue but are functionally associated with it and help it perform its function. They are called the extrinsic muscles, and their main function is altering the position of the tongue. They are: the genioglossus, hyoglossus, styloglossus and the palatoglossus muscles.
Superior longitudinal muscle
Inferior longitudinal muscle
These muscles make up the inside of the tongue. Their fibers are placed in different directions, based on which they are named (i.e. transverse muscle has its fibers on the transverse plane), and their contractions define the shape of the tongue.
These muscles all work together to produce movements that are essential for mastication (chewing), speech, and deglutition (swallowing). These movements include elongating and retracting the tongue, elevating and lowering the apex of tongue and broadening and narrowing the surface of the tongue. They are all innervated by the hypoglossal nerve (CN XII).
Origin - submucosa of posterior tongue, lingual septum
Origin - root of tongue, body of hyoid bone
Insertion - apex of tongue
Innervation - hypoglossal nerve (CN XII)
Action - retracts and broadens tongue, lowers apex of tongue
Origin - lingual septum
Insertion - lateral margin of tongue
Innervation - hypoglossal nerve (CN XII)
Action - narrows and elongates tongue
Origin - root of tongue, genioglossus muscle
Insertion - lingual aponeurosis
Innervation - hypoglossal nerve (CN XII)
Action - broadens and elongates tongue
Extrinsic muscles insert into the tongue, while their origins are outside of it. There are four pairs of extrinsic muscles, which include the genioglossus, hyoglossus, styloglossus, and palatoglossus.
They produce those movements of the tongue which the intrinsic muscles can't, which are protrusion, retraction/retrusion, depression and elevation of the tongue. All of them are innervated by the hypoglossal nerve (CN XII), except for the palatoglossus muscle which is innervated by the vagus nerve (CN X) via branches of the pharyngeal plexus.
To easily remember their origins, just pay attention the their names. For example, the hyoglossus muscle: hyo- suggests the origin from the hyoid bone, while the -glossus is from the Greek ‘glossa’, meaning ‘tongue’, where this muscle inserts. ‘Stylo-’ refers to the styloid process and ‘palato-’ refers to the palate. ‘Genio-’ on the other hand references the Greek ‘geneion’ for ‘chin’.
Origin - Superior mental spine of mandible
Insertion - entire length of dorsum of tongue, lingual aponeurosis, body of hyoid bone
Innervation - hypoglossal nerve (CN XII)
Action - depresses and protrudes tongue (bilateral contraction); deviates tongue contralaterally (unilateral contraction)
Origin - body and greater horn of hyoid bone
Insertion - inferior/ventral parts of lateral tongue
Innervation - hypoglossal nerve (CN XII) Action - depresses and retracts tongue
Origin - anterolateral aspect of styloid process (of temporal bone), stylomandibular ligament
Insertion - blends with inferior longitudinal muscle (longitudinal part); blends with hyoglossus muscle (oblique part)
Innervation - hypoglossal nerve (CN XII)
Action - retracts and elevates lateral aspects of tongue
Origin - palatine aponeurosis of soft palate
Insertion - lateral margins of tongue, blends with intrinsic muscles of tongue
Innervation - vagus nerve (CN X) (via branches of pharyngeal plexus)
Action - elevates root of tongue, constricts isthmus of fauces
Genioglossus is a fan-shaped muscle that originates from the superior mental spine on the posterior mandibular surface. From that point, the inferior group of fibers inserts into the body of the hyoid bone, whereas the remaining fibers extend superiorly next to the entire length of the dorsum of tongue/lingual aponeurosis and blend with the intrinsic muscles. Bilateral contraction of this muscle depresses the central part of the tongue, creates a longitudinal furrow and protrudes the tongue outside of the mouth. Unilateral contraction on the other hand will deviate the tongue contralaterally.
Hyoglossus is a quadrangular muscle that originates from the body and greater horn of the hyoid bone. It passes through the oropharyngeal triangle (a space in the neck bounded by the superior pharyngeal constrictor, middle pharyngeal constrictor and mylohyoid muscles), and inserts into the inferior/ventral parts of the lateral tongue, right between the genioglossus (laterally) and styloglossus (medially). Its function is to depress the lateral aspects of the tongue, as well as retrude/retract the tongue. By being as wide as the greater horn of the hyoid bone, the hyoglossus muscle makes up a big part of the floor of the oral cavity. The lingual artery, the main artery that supplies the lingual muscles, passes between the hyoglossus and genioglossus. Additionally, the hypoglossal nerve (CN XII) and the lingual nerve pass over its external surface on their way to the tongue. This makes the hyoglossus muscle an important clinical landmark.
The styloglossus muscle is shaped as a thin stripe which originates from the anterolateral aspect of the styloid process of the temporal bone and the nearby stylomandibular ligament. To reach the tongue and insert into posterolateral aspects, it passes through the oropharyngeal triangle. The longitudinal part of the styloglossus will then blend with fibers of the inferior longitudinal muscle, and the oblique part of the styloglossus muscle will blend with fibers of the hyoglossus muscles. By contracting, the styloglossus muscle retrudes/retracts the tongue and elevates its lateral aspects.
The palatoglossus is a bow-shaped muscle associated both with the tongue and the soft palate. It originates from the palatine aponeurosis of the soft palate and arches laterally downward to insert into the dorsum of the tongue and blends with the intrinsic muscles. This muscle coordinates the actions of the tongue and the soft palate. It elevates the root of the tongue, and constricts the isthmus of fauces. All these movements are necessary for closing the isthmus of the oropharynx and pushing the bolus from the oral cavity into the pharynx. Note that this is the only muscle of the tongue that is not innervated by the hypoglossal nerve (CN XII) like all the others. Instead, it is innervated by the vagus nerve (CN X) via branches of the pharyngeal plexus like other muscles of the soft palate.
To understand the anatomical aspect of taste sensation, let's briefly describe the sensory innervation of the tongue.Almost every organ in the human body, including the tongue, can detect general sensory stimulation (general somatic afferent fibers), that is temperature, touch, and pain. General sensory innervation of the tongue is enabled by the lingual nerve (a branch of the mandibular nerve CN V3) and glossopharyngeal nerve (CN IX), where the former innervates the anterior two-thirds of the tongue, while the latter supplies the posterior one-third of the tongue.
On the other hand, every organ specialized for a specific sensation, like the tongue is for taste, also has special sensory innervation. The senses of taste and olfaction (smell) are carried by special visceral afferent fibers in cranial nerves. The sense of taste is mediated by the facial (CN VII) and glossopharyngeal (CN IX) nerves. Even though taste receptors and their innervation will be discussed in the following sections of this article, right now, for the sake of the later recap, take the following two sentences for granted:
- Taste receptors are found within three out of four types of lingual papillae, and those from the anterior two-thirds of the tongue, except for the vallate papillae, are innervated by the chorda tympani (a branch of the facial nerve)
- All receptors from the posterior one-third of the tongue are innervated by the glossopharyngeal nerve
Note that the vagus nerve (CN X) via its superior laryngeal branch provides both general and special sensory innervation to the part of the tongue that is directly in front of the epiglottis.
The dorsal surface of the tongue is rough and covered with numerous papillae. These structures contain taste buds that themselves contain gustatory receptors for taste. There are four types of the lingual papillae: filiform, fungiform, vallate, and foliate; and all of them, except for the filiform contain taste buds.
Filiform papillae are the smallest and the most numerous. They are scattered all over the anterior two-thirds of the dorsal surface of the tongue in lines parallel to the terminal sulcus, and then transversely at the apex. They are thin and elongated, and have a purely mechanical role only, since they do not possess taste buds.
Fungiform papillae are shaped like mushrooms. They can be found all over the dorsal surface of the tongue, but their highest concentration is at the tip and margins of the tongue.
Vallate papillae are the largest. There are very few of them, precisely 8 - 12 in humans. They are shaped cylindrically and can be found only anterior to the terminal sulcus of the tongue, characteristically aligned in a V shape.
Foliate papillae are the linear folds of the tongue mucosa and are located at the edges of the posterior one-third of the tongue. They are poorly developed in humans since they degenerate in childhood. Fun fact is that the cats have foliate papillae very developed on their tongues. That's why their tongues are so rough.
As previously said, taste buds contain the gustatory receptors cells which allow us to detect taste. The receptors are found within the lingual papillae, and they allow us to detect five types of taste: sweet, salty, bitter, sour, and umami.
The specific distribution of the receptors enables us to map the tongue based on types of taste, so that the apex predominantly detects sweetness, the lateral margins detect saltiness, the posterior part is for bitterness and sourness, while umami is detected equally in all parts. Most taste buds are found on the dorsal surface of the tongue, but they are also present on the soft palate, pharynx, larynx, and epiglottis.
Taste buds are located in different places within each papillae. In foliate and vallate papillae they are in the epithelium of their lateral surface, whereas in the fungiform papillae they are present in their apical epithelium. On the other hand, not all taste buds are innervated by the same nerve. The buds on the anterior two-thirds of the tongue and the soft palate are innervated by the chorda tympani, a branch of the facial nerve (CN VII). Those within the posterior one-third of the tongue (including all vallate papillae) and the pharynx are innervated by the glossopharyngeal nerve (CN IX). The vagus nerve (CN X) innervates the remaining taste buds in the larynx and epiglottis.
Histology of taste buds
Taste buds appear as round bodies that extend through the thickness of the epithelium of the lingual papillae. The epithelial surface of the bud contains an opening called a taste pore which interacts with the content of the oral cavity. In this way, the taste pore is the window that allows the bud to detect the chemical stimuli from the food present.
On the other hand, the inferior pole of the bud synapses with the corresponding sensory nerve (facial (CN VII), glossopharyngeal (CN IX), or vagus (CN X) that carries information of the chemical stimuli to the central nervous system via the gustatory pathway to be interpreted and perceived as a specific taste sensation.
There are three types of cells found within taste buds:
- Neuroepithelial cells – sensory cells that extend from the basal lamina to the taste pore. Their microvilli project into the pore and detect chemical stimuli (taste) from food. They synapse with the facial, glossopharyngeal, or vagus nerve to send taste information to the central nervous system
- Supporting cells – also extend from the basal lamina to the taste pore. They provide mechanical support to the taste bud and neuroepithelial cells
- Basal cells – line the basal lamina of the taste bud. They are short (do not reach the pore) stem cells for the neuroepithelial and basal cells.
Since sensory pathways conduct information from the periphery (receptors) to the central nervous system, we’ll review the gustatory pathway in that direction.When the taste buds from the anterior two-thirds of the tongue and soft palate are stimulated, the special visceral afferent fibers of the facial nerve (CN VII) receive that information and leave the oral cavity together with the lingual nerve. Then, in the infratemporal fossa, these special visceral afferent fibers emerge from the facial nerve and form the chorda tympani. The chorda tympani then relays sensory input to the otic and geniculate ganglia. Postganglionic fibers emerge from these ganglia and project to the brainstem and enter at the pontomedullary junction. Then they synapse within the rostral part of the nucleus of the solitary tract in the posteroinferior part of the medulla oblongata.
When it comes to the posterior one-third of the tongue and pharynx, the special visceral afferent fibers from both the superior laryngeal nerve (branch of the vagus nerve (CN X)) and the glossopharyngeal nerve (CN IX) synapse with the inferior ganglion of the vagus nerve and the inferior ganglion of the glossopharyngeal nerve (also known as the petrosal ganglion). From the inferior ganglion, the postganglionic fibers exit and course towards the same spot as the chorda tympani (the nucleus of the solitary tract) after entering the brainstem at the rostral medulla oblongata.The fibers end within the rostral segment of the of anterior (ventral) solitary nucleus. The neurons of this nucleus send fibers that join the ipsilateral central tegmental tract, and then end within parvocellular division of the ventral posteromedial nucleus of the thalamus. The fibers exiting from the thalamus form central thalamic radiations, which pass through the posterior limb of the internal capsule. These fibers end within the primary gustatory cortical areas of the cerebral cortex, found within insula and frontal operculum.
Learn everything about the gustatory pathway in a more fun and engaging way through our study unit:
Damage of the hypoglossal nerve
Since all muscles of the tongue are innervated by the hypoglossal nerve (CN XII) (except for the palatoglossus), lesions within this nerve will result in a functional impairment of the tongue. The hypoglossal nerve does not decussate, so in cases where one of the two hypoglossal nerves are damaged, the damage will manifest on the same side of the tongue. Unilateral injuries of the hypoglossal nerve will result in muscular atrophy of the same side of the tongue, but because the opposite hypoglossal nerve is not damaged, impairments during mastication and speaking will not be severe.
On the other hand, if both hypoglossal nerves are damaged, it will result in complete paralysis of the tongue and present as difficult or unclear articulation, a condition called dysarthria. The tongue’s support in mastication will also be lost.
Damage of the gustatory pathway
When it comes to sensory innervation, patients may not notice that taste sensation is damaged, since the olfactory system may compensate and mask the loss of taste perception. If there are lesions within the facial nerve (CN VII), they will be noticed mostly because of a dysfunction of the facial muscles rather than because of a loss in taste sensation.
Papillae may be damaged in a variety of situations, from something common like burning the tongue with hot tea, to a serious condition like tongue carcinoma. Damage of the taste buds leads to an inability to sense taste from the affected part of the tongue. The loss of taste sensation is usually reversible if caused by drinking hot beverages. As soon as basal cells regenerate the neuroepithelial cells in the taste bud, taste sensation is recovered too.
Damage of the tongue muscles
Damage to the tongue muscles may also result from traumatic injuries and will alter speech and mastication capability. This inability is different than one caused by a hypoglossal nerve injury. In nerve injury, denervation usually leads to irreversible muscle atrophy and degeneration that worsens over time. On the other hand, something like a penetrating injury of the tongue muscles (like piercings) leads to a temporary disability of movements which get better over time as the tissue regenerates.