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 The tongue is a muscular organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive process, and is the primary organ of taste. The tongue's upper surface (dorsum) is covered by taste ...

except for the

palatoglossus The palatoglossus or palatoglossal muscle is a muscle of the soft palate and extrinsic muscle of the tongue. Its surface is covered by oral mucosa and forms the visible palatoglossal arch. Structure Palatoglossus arises from the palatine aponeu ...

, which is innervated by the vagus nerve. CN XII is a nerve with a solely 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 Medulla or Medullary may refer to: Science * Medulla oblongata, a part of the brain stem * Renal medulla, a part of the kidney * Adrenal medulla, a part of the adrenal gland * Medulla of ovary, a stroma in the center of the ovary * Medulla of t ...

as a number of small rootlets, passes 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, 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. The first recorded description of the nerve is by

Herophilos Herophilos (; grc-gre, Ἡρόφιλος; 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 sci ...

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

, from ''hypo'' ( el, "under") and ''glossa'' ( el, "tongue").

Structure

The hypoglossal nerve arises as a number of small rootlets from the front of the

, the bottom part of the brainstem, in the anterolateral sulcus which separates the olive and the

pyramid A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrilat ...

. The nerve passes through the subarachnoid space and pierces the

dura mater In neuroanatomy, dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous system. ...

near the

, an opening in the occipital bone 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 ramus of C1. It then travels close to the vagus nerve and spinal division of the accessory nerve, spirals downwards behind the vagus nerve and passes between the internal carotid artery 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 poste ...

lying on the carotid sheath. At a point at the level of the

angle of the mandible __NOTOC__ The angle of the mandible (gonial angle) 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 be either inverted or everted, is marked by rough, obliq ...

, the hypoglossal nerve emerges from behind the posterior belly of the digastric muscle. It then loops around a branch of the occipital artery and travels forward into the region beneath the mandible. The hypoglossal nerve moves forward lateral to the hyoglossus and medial to the

stylohyoid muscle The stylohyoid muscle is a slender muscle, lying anterior and superior of the posterior belly of the digastric muscle. It is one of the suprahyoid muscles. It shares this muscle's innervation by the facial nerve, and functions to draw the hyoid ...

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 V3 ) and from the facial nerve (CN VII). The fibres from the trigeminal nerv ...

. It continues deep to the

genioglossus muscle The genioglossus is one of the paired extrinsic muscles of the tongue. The genioglossus is the major muscle responsible for protruding (or sticking out) the tongue. Structure Genioglossus is the fan-shaped extrinsic tongue muscle that forms the ma ...

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

(labelled 13), part of the brainstem. 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. File:Sobo 1909 693.png, After leaving the skull, the hypoglossal nerve spirals around the vagus nerve and then passes behind the deep belly of the digastric muscle. File:Slide12ww.JPG, The hypoglossal nerve then travels deep to the hyoglossus muscle, which it supplies. It then continues on and supplies the genioglossus 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 musculature they supply develops as the hypoglossal cord 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: genioglossus, hyoglossus,

styloglossus The styloglossus, the shortest and smallest of the three styloid muscles, arises from the anterior and lateral surfaces of the styloid process near its apex, and from the stylomandibular ligament. Passing inferiorly and anteriorly between the int ...

, and the intrinsic muscles of the

. These represent all muscles of the tongue except the palatoglossus muscle. The hypoglossal nerve is of a

general somatic efferent The general (spinal) somatic efferent neurons (GSE, somatomotor, or somatic motor fibers), arise from motor neuron cell bodies in the ventral horns of the gray matter within the spinal cord. They exit the spinal cord through the ventral roots, carr ...

(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, 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 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 motor neuron disease, is associated with combined lesions of the hypoglossal nucleus and nucleus ambiguus with wasting ( atrophy) of the motor nerves of the

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds 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 Va ...

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) or wasting ( atrophy). 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. 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 ''Larr ...

s, dental stops, alveolar stops,

velar nasal The voiced velar nasal, also known as agma, from the Greek word for '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 ''Englis ...

s, rhotic consonants 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 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). ...

), 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) 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 taste ...

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).

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 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 amniotes including reptiles, mammals 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 are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...

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

Structure

Development

Function

Clinical significance

Damage

Examination

Use in nerve repair

History

Other animals

See also

References

Notes

External links