The hypoglossal nerve, also known as the twelfth cranial nerve, cranial nerve XII, or simply CN XII, is a

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

that innervates all the extrinsic and intrinsic muscles of the

tongue The tongue is a Muscle, muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for chewing and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper s ...

except for the

palatoglossus The palatoglossal muscle is a muscle of the soft palate and an extrinsic muscle of the tongue. Its surface is covered by oral mucosa and forms the visible palatoglossal arch. Structure From its origin, it passes anteroinferiorly and laterally. I ...

, which is innervated by the

vagus nerve The vagus nerve, also known as the tenth cranial nerve (CN X), plays a crucial role in the autonomic nervous system, which is responsible for regulating involuntary functions within the human body. This nerve carries both sensory and motor fibe ...

. CN XII is a nerve with a sole motor function. The nerve arises from the

hypoglossal nucleus The hypoglossal nucleus is a cranial nerve nucleus, found within the medulla. Being a motor nucleus, it is close to the midline. In the open medulla, it is visible as what is known as the '' hypoglossal trigone'', a raised area (medial to the v ...

in the medulla as a number of small rootlets, pass through the

hypoglossal canal The hypoglossal canal is a foramen in the occipital bone of the skull. It is hidden medially and superiorly to each occipital condyle. It transmits the hypoglossal nerve. Structure The hypoglossal canal lies in the epiphyseal junction between ...

and down through the neck, and eventually passes up again over the tongue muscles it supplies into the tongue. The nerve is involved in controlling tongue movements required for speech and

swallowing Swallowing, also called deglutition or inglutition in scientific and medical contexts, is a physical process of an animal's digestive tract (e.g. that of a human body) that allows for an ingested substance (typically food) to pass from the mou ...

, including sticking out the tongue and moving it from side to side. Damage to the nerve or the neural pathways which control it can affect the ability of the tongue to move and its appearance, with the most common sources of damage being injury from trauma or surgery, and

motor neuron disease Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND) or—in the United States—Lou Gehrig's disease (LGD), is a rare, terminal neurodegenerative disorder that results in the progressive loss of both upper and low ...

. The first recorded description of the nerve was by

Herophilos Herophilos (; ; 335–280 BC), sometimes Latinised Herophilus, was a Greek physician regarded as one of the earliest anatomists. Born in Chalcedon, he spent the majority of his life in Alexandria. He was the first scientist to systematically p ...

in the third century BC. The name hypoglossus springs from the fact that its passage is below the

, from ''hypo'' () and ''glossa'' ().

Structure

The hypoglossal nerve arises as a number of small rootlets from the front of the medulla, the bottom part of the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

, in the anterolateral sulcus which separates the

olive The olive, botanical name ''Olea europaea'' ("European olive"), is a species of Subtropics, subtropical evergreen tree in the Family (biology), family Oleaceae. Originating in Anatolia, Asia Minor, it is abundant throughout the Mediterranean ...

and the

pyramid A pyramid () is a structure whose visible surfaces are triangular in broad outline and converge toward the top, making the appearance roughly a pyramid in the geometric sense. The base of a pyramid can be of any polygon shape, such as trian ...

. The nerve passes through the subarachnoid space and pierces the dura mater near the

, an opening in the

occipital bone The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lob ...

of the skull. After emerging from the hypoglossal canal, the hypoglossal nerve gives off a meningeal branch and picks up a branch from the

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

ramus of C1. It then travels close to the

and spinal division of the

accessory nerve The accessory nerve, also known as the eleventh cranial nerve, cranial nerve XI, or simply CN XI, is a cranial nerve that supplies the sternocleidomastoid and trapezius muscles. It is classified as the eleventh of twelve pairs of cranial nerv ...

, spirals downwards behind the vagus nerve and passes between the

internal carotid artery The internal carotid artery is an artery in the neck which supplies the anterior cerebral artery, anterior and middle cerebral artery, middle cerebral circulation. In human anatomy, the internal and external carotid artery, external carotid ari ...

and

internal jugular vein The internal jugular vein is a paired jugular vein that collects blood from the brain and the superficial parts of the face and neck. This vein runs in the carotid sheath with the common carotid artery and vagus nerve. It begins in the posteri ...

lying on the

carotid sheath The carotid sheath is a condensation of the deep cervical fascia enveloping multiple vital neurovascular structures of the neck, including the common and internal carotid arteries, the internal jugular vein, the vagus nerve (CN X), and ansa c ...

. At a point at the level of the

angle of the mandible __NOTOC__ The angle of the mandible (a.k.a. gonial angle, Masseteric Tuberosity, and Masseteric Insertion) is located at the posterior border at the junction of the lower border of the ramus of the mandible. The angle of the mandible, which may ...

, the hypoglossal nerve emerges from behind the posterior belly of the

digastric muscle The digastric muscle (also digastricus) (named ''digastric'' as it has two 'bellies') is a bilaterally paired suprahyoid muscle located under the jaw. Its posterior belly is attached to the mastoid notch of temporal bone, and its anterior belly ...

. It then loops around a branch of the

occipital artery The occipital artery is a branch of the external carotid artery that provides arterial supply to the back of the scalp, sternocleidomastoid muscles, and deep muscles of the back and neck. Structure Origin The occipital artery arises from (the ...

and travels forward into the region beneath the mandible. The hypoglossal nerve moves forward lateral to the

hyoglossus The hyoglossus is a thin and quadrilateral extrinsic muscle of the tongue. It originates from the hyoid bone; it inserts onto the side of the tongue. It is innervated by the hypoglossal nerve (cranial nerve XII). It acts to depress and retract th ...

and medial to the

stylohyoid muscle The stylohyoid muscle is one of the suprahyoid muscles. Its originates from the styloid process of the temporal bone; it inserts onto hyoid bone. It is innervated by a branch of the facial nerve. It acts draw the hyoid bone upwards and backwards. ...

s and

lingual nerve The lingual nerve carries sensory innervation from the anterior two-thirds of the tongue. It contains fibres from both the mandibular division of the trigeminal nerve (CN V) and from the facial nerve (CN VII). The fibres from the trigeminal nerve ...

. It continues deep to the

genioglossus muscle The genioglossus is one of the paired extrinsic muscles of the tongue. It is a fan-shaped muscle that comprises the bulk of the body of the tongue. It arises from the mental spine of the mandible; it inserts onto the hyoid bone, and the bottom of ...

and continues forward to the tip of the tongue. It distributes branches to the intrinsic and extrinsic muscle of the tongue innervates as it passes in this direction, and supplies several muscles (hyoglossus, genioglossus and styloglossus) that it passes. The rootlets of the hypoglossal nerve arise from the

near the bottom of the

brain stem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is co ...

. The hypoglossal nucleus receives input from both the motor cortices but the contralateral input is dominant; innervation of the tongue is essentially lateralized. Signals from muscle spindles on the tongue travel through the hypoglossal nerve, moving onto the

which synapses on the

trigeminal In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and chewing ...

mesencephalic nucleus The mesencephalic nucleus of trigeminal nerve is one of the sensory nuclei of the trigeminal nerve (cranial nerve V). It is located in the brainstem. It receives proprioceptive sensory information from the muscles of mastication and other muscles ...

. File:Human brainstem anterior view description.JPG, The hypoglossal nerve emerges as several rootlets (labelled here as number 12) from the olives of the medulla (labelled 13), part of the

. File:Base of skull 19.jpg, The hypoglossal nerve leaves the skull through the

, which is situated near the large opening for the spinal cord, the

foramen magnum The foramen magnum () is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblongata, passes thro ...

. File:Sobo 1909 693.png, After leaving the skull, the hypoglossal nerve spirals around the

and then passes behind the deep belly of the

. File:Slide12ww.JPG, The hypoglossal nerve then travels

deep Deep or The Deep may refer to: Places United States * Deep Creek (Appomattox River tributary), Virginia * Deep Creek (Great Salt Lake), Idaho and Utah * Deep Creek (Mahantango Creek tributary), Pennsylvania * Deep Creek (Mojave River tributary ...

to the

hyoglossus muscle The hyoglossus is a thin and quadrilateral extrinsic muscle of the tongue. It originates from the hyoid bone; it inserts onto the side of the tongue. It is innervated by the hypoglossal nerve (cranial nerve XII). It acts to depress and retract th ...

, which it supplies. It then continues and supplies the

genioglossus The genioglossus is one of the paired extrinsic muscles of the tongue. It is a fan-shaped muscle that comprises the bulk of the body of the tongue. It arises from the mental spine of the mandible; it inserts onto the hyoid bone, and the bottom o ...

muscle, and towards the tip of the tongue, where it divides into branches supplying the tongue muscles.

Development

Neurons of the hypoglossal nucleus are derived from the basal plate of the embryonic

medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involun ...

. The musculature they supply develops as the

hypoglossal cord The hypoglossal nerve, also known as the twelfth cranial nerve, cranial nerve XII, or simply CN XII, is a cranial nerve that innervates all the extrinsic and intrinsic muscles of the tongue except for the palatoglossus, which is innervated by ...

from the myotomes of the first four pairs of occipital somites. The nerve is first visible as a series of roots in the fourth week of development, which have formed a single nerve and link to the tongue by the fifth week.

Function

The hypoglossal nerve provides motor control of the extrinsic muscles of the tongue:

styloglossus The styloglossus muscle is a bilaterally paired muscle of the tongue. It originates at the styloid process of the temporal bone. It inserts onto the side of the tongue. It acts to elevate and retract the tongue. It is innervated by the hypogloss ...

, and the intrinsic muscles of the

. These represent all muscles of the tongue except the

palatoglossus muscle The palatoglossal muscle is a muscle of the soft palate and an extrinsic muscle of the tongue. Its surface is covered by oral mucosa and forms the visible palatoglossal arch. Structure From its origin, it passes anteroinferiorly and laterally. I ...

, which is innervated by the

. The hypoglossal nerve is of a general somatic efferent (GSE) type. These muscles are involved in moving and manipulating the tongue. The left and right genioglossus muscles in particular are responsible for protruding the tongue. The muscles, attached to the underside of the top and back parts of the tongue, cause the tongue to protrude and deviate towards the opposite side. The hypoglossal nerve also supplies movements including clearing the mouth of saliva and other involuntary activities. The hypoglossal nucleus interacts with the

reticular formation The reticular formation is a set of interconnected nuclei in the brainstem that spans from the lower end of the medulla oblongata to the upper end of the midbrain. The neurons of the reticular formation make up a complex set of neural networks ...

, involved in the control of several reflexive or automatic motions, and several corticonuclear originating fibers supply innervation aiding in unconscious movements relating to speech and articulation.

Clinical significance

Damage

Reports of damage to the hypoglossal nerve are rare. The most common causes of injury in one case series were compression by tumours and gunshot wounds. A wide variety of other causes can lead to damage of the nerve. These include surgical damage, medullary stroke, multiple sclerosis, Guillain-Barre syndrome, infection, sarcoidosis, and presence of an ectatic vessel in the hypoglossal canal. Damage can be on one or both sides, which will affect symptoms that the damage causes. Because of the close proximity of the nerve to other structures including nerves, arteries, and veins, it is rare for the nerve to be damaged in isolation. For example, damage to the left and right hypoglossal nerves may occur with damage to the facial and trigeminal nerves as a result of damage from a clot following

arteriosclerosis Arteriosclerosis, literally meaning "hardening of the arteries", is an umbrella term for a vascular disorder characterized by abnormal thickening, hardening, and loss of elasticity of the walls of arteries; this process gradually restricts th ...

of the vertebrobasilar artery. Such a stroke may result in tight oral musculature, and difficulty speaking, eating and chewing. Progressive bulbar palsy, a form of

, is associated with combined lesions of the hypoglossal nucleus and

nucleus ambiguus The nucleus ambiguus ("ambiguous nucleus" in English) is a group of large motor neurons, situated deep in the medullary part of the reticular formation named by Jacob Clarke. The nucleus ambiguus contains the cell bodies of neurons that innerva ...

with wasting (

atrophy Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), malnutrition, poor nourishment, poor circulatory system, circulation, loss of hormone, ...

) of the motor nerves of the

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other mammals, lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of ...

and medulla. This may cause difficulty with tongue movements, speech, chewing and swallowing caused by dysfunction of several cranial nerve nuclei. Motor neuron disease is the most common disease affecting the hypoglossal nerve.

Examination

The hypoglossal nerve is tested by examining the tongue and its movements. At rest, if the nerve is injured a tongue may appear to have the appearance of a "bag of worms" (

fasciculations A fasciculation, or muscle twitch, is a spontaneous, involuntary muscle contraction and relaxation, involving fine muscle fibers. They are common, with as many as 70% of people experiencing them. They can be benign, or associated with more seriou ...

) or wasting (

). The nerve is then tested by sticking the tongue out. If there is damage to the nerve or its pathways, the tongue will usually but not always deviate to one side, due to the

muscle receiving nerve signals on one side but not the other. When the nerve is damaged, the tongue may feel "thick," "heavy," or "clumsy." Weakness of tongue muscles can result in slurred speech, affecting sounds particularly dependent on the tongue for generation (i.e.,

lateral approximant A lateral is a consonant in which the airstream proceeds along one or both of the sides of the tongue, but it is blocked by the tongue from going through the middle of the mouth. An example of a lateral consonant is the English ''L'', as in ''L ...

dental stop In phonetics and phonology, a dental stop is a type of consonantal sound, made with the tongue in contact with the upper teeth (hence dental), held tightly enough to block the passage of air (hence a stop consonant). Dental and alveolar stops are ...

alveolar stop In phonetics and phonology, an alveolar stop is a type of consonantal sound, made with the tongue in contact with the alveolar ridge located just behind the teeth (hence alveolar), held tightly enough to block the passage of air (hence a stop cons ...

velar nasal The voiced velar nasal, also known as eng, engma, or agma (from Greek 'fragment'), is a type of consonantal sound used in some spoken languages. It is the sound of ''ng'' in English ''sing'' as well as ''n'' before velar consonants as in ''E ...

rhotic consonant In phonetics, rhotic consonants, or "R-like" sounds, are liquid consonants that are traditionally represented orthography, orthographically by symbols derived from the Greek alphabet, Greek letter Rho (letter), rho (Ρ and ρ), including R, , i ...

s etc.). Tongue strength may be tested by poking the tongue against the inside of their cheek, while an examiner feels or presses from the cheek. The hypoglossal nerve carries lower motor neurons that

synapse In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...

with upper motor neurons at the

. Symptoms related to damage will depend on the position of damage in this pathway. If the damage is to the nerve itself (a

lower motor neuron lesion A lower motor neuron lesion is a lesion which affects nerve fibers traveling from the lower motor neuron(s) in the anterior horn/anterior grey column of the spinal cord, or in the motor nuclei of the cranial nerves, to the relevant muscle(s). O ...

), the tongue will curve toward the damaged side, owing to weakness of the genioglossus muscle of affected side which action is to deviate the tongue in the contralateral side . If the damage is to the nerve pathway (an

upper motor neuron lesion An upper motor neuron lesion (also known as pyramidal insufficiency) Is an injury or abnormality that occurs in the neural pathway above the anterior horn cell of the spinal cord or motor nuclei of the cranial nerves. Conversely, a lower motor ...

) the tongue will curve away from the side of damage, due to action of the affected genioglossus muscle, and will occur without fasciculations or wasting, with speech difficulties more evident. Damage to the hypoglossal nucleus will lead to wasting of muscles of the tongue and deviation towards the affected side when it is stuck out. This is because of the weaker genioglossal muscle.

Use in nerve repair

The hypoglossal nerve may be connected ( anastomosed) to the

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of ta ...

to attempt to restore function when the facial nerve is damaged. Attempts at repair by either wholly or partially connecting nerve fibres from the hypoglossal nerve to the facial nerve may be used when there is focal facial nerve damage (for example, from trauma or cancer).

Hypoglossal nerve stimulator implant

The hypoglossal nerve has also been clinically implicated in the treatment of

obstructive sleep apnea Obstructive sleep apnea (OSA) is the most common sleep-related breathing disorder and is characterized by recurrent episodes of complete or partial airway obstruction, obstruction of the respiratory tract#Upper respiratory tract, upper airway lea ...

. Certain patients with

who are deemed eligible candidates (e.g., failed

continuous positive airway pressure Continuous positive airway pressure (CPAP) is a form of positive airway pressure (PAP) ventilation in which a constant level of pressure greater than atmospheric pressure is continuously applied to the upper respiratory tract of a person. The a ...

therapy, underwent appropriate testing with drug induced sleep endoscopy, and meet other criteria as outlined by the

FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...

) may be offered hypoglossal nerve stimulation as an alternative. The purpose of the hypoglossal nerve stimulator is to relieve

base obstruction during sleep by stimulating the

to protrude during inspiration (i.e., inhale). In this procedure, an electrical stimulator lead is placed around branches of the hypoglossal nerve that control tongue protrusion (e.g.,

) via an incision in the neck. A sensor lead is then placed in the chest between the ribs in the layer between the internal intercostal muscles and external intercostal muscles. The stimulator and sensory lead are then connected via a tunneled wire to an implantable pulse generator. When turned on during sleep, the sensory lead in the chest detects the respiratory cycle. During inspiration (i.e., inhale), an electrical signal is fired via the stimulator lead in the neck, stimulating the hypoglossal nerve, and causing the tongue to protrude, thereby alleviating obstruction.

History

The first recorded description of the hypoglossal nerve was by

(335–280 BC), although it was not named at the time. The first use of the name ''hypoglossal'' in Latin as ''nervi hypoglossi externa'' was used by Winslow in 1733. This was followed though by several different namings including ''nervi indeterminati'', ''par lingual'', ''par gustatorium'', ''great sub-lingual'' by different authors, and ''gustatory nerve'' and ''lingual nerve'' (by Winslow). It was listed in 1778 as ''nerve hypoglossum magnum'' by Soemmering. It was then named as the ''great hypoglossal nerve'' by

Cuvier Jean Léopold Nicolas Frédéric, baron Cuvier (23 August 1769 – 13 May 1832), known as Georges Cuvier (; ), was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuv ...

in 1800 as a translation of Winslow and finally named in English by Knox in 1832.

Other animals

The hypoglossal nerve is one of twelve cranial nerves found in

amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...

s including

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s and birds. As with humans, damage to the nerve or nerve pathway will result in difficulties moving the tongue or lapping water, decreased tongue strength, and generally cause deviation away from the affected side initially, and then to the affected side as contractures develop. The evolutionary origins of the nerve have been explored through studies of the nerve in rodents and reptiles. The nerve is regarded as arising evolutionarily from nerves of the cervical spine, which has been incorporated into a separate nerve over the course of evolution. The size of the hypoglossal nerve, as measured by the size of the hypoglossal canal, has been hypothesised to be associated with the progress of evolution of

primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...

s, with reasoning that larger nerves would be associated with improvements in speech associated with evolutionary changes. This hypothesis has been refuted.

References

;Sources *

Notes

External links

{{Authority control Cranial nerves Innervation of the tongue Motor system Otorhinolaryngology Human head and neck Nervous system Neurology Nerves of the head and neck