| Hypoglossal nerve | |
|---|---|
 Hypoglossal nerve,cervical plexus, and their branches. | |
 The hypoglossal nerve arises as a series of rootlets, from the caudal brain stem, here seen from below. | |
| Details | |
| Innervates | Genioglossus,hyoglossus,styloglossus, intrinsic muscles of the tongue |
| Identifiers | |
| Latin | nervus hypoglossus |
| MeSH | D007002 |
| NeuroNames | 704 |
| TA98 | A14.2.01.191 |
| TA2 | 6357 |
| FMA | 50871 |
| Anatomical terms of neuroanatomy | |
| Cranial nerves |
|---|
|
Thehypoglossal nerve, also known as thetwelfth cranial nerve,cranial nerve XII, or simplyCN XII, is acranial nerve that innervates all theextrinsic and intrinsic muscles of thetongue except for thepalatoglossus, which is innervated by thevagus nerve.[a]
CN XII is anerve with a sole motor function. The nerve arises from thehypoglossal nucleus in themedulla[1][2] as a number of small rootlets, pass through thehypoglossal canal 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 andswallowing, 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, andmotor neuron disease. The first recorded description of the nerve was byHerophilos in the third century BC. The name hypoglossus springs from the fact that its passage is below thetongue, fromhypo (Greek:"under") andglossa (Greek:"tongue").
The hypoglossal nerve arises as a number of small rootlets from the front of themedulla, the bottom part of thebrainstem,[1][2] in theanterolateral sulcus which separates theolive and thepyramid.[3] The nerve passes through thesubarachnoid space and pierces thedura mater near thehypoglossal canal, an opening in theoccipital bone of the skull.[2][4]
After emerging from the hypoglossal canal, the hypoglossal nerve gives off a meningeal branch and picks up a branch from theanteriorramus ofC1. It then travels close to thevagus nerve and spinal division of theaccessory nerve,[2] spirals downwards behind the vagus nerve and passes between theinternal carotid artery andinternal jugular vein lying on thecarotid sheath.[4]
At a point at the level of theangle of the mandible, the hypoglossal nerve emerges from behind theposterior belly of thedigastric muscle.[4] It then loops around a branch of theoccipital artery and travels forward into the region beneath the mandible.[4] The hypoglossal nerve moves forward lateral to thehyoglossus and medial to thestylohyoid muscles andlingual nerve.[5] It continues deep to thegenioglossus muscle 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.[5]
The rootlets of the hypoglossal nerve arise from thehypoglossal nucleus near the bottom of thebrain stem.[1] The hypoglossal nucleus receives input from both themotor cortices but the contralateral input is dominant; innervation of the tongue is essentially lateralized.[6] Signals from muscle spindles on the tongue travel through the hypoglossal nerve, moving onto thelingual nerve which synapses on thetrigeminalmesencephalic nucleus.[2]
Neurons of the hypoglossal nucleus are derived from thebasal plate of the embryonicmedulla oblongata.[7][8] The musculature they supply develops as thehypoglossal cord from themyotomes of the first four pairs of occipital somites.[9][10] 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.[11][12]
The hypoglossal nerve provides motor control of the extrinsic muscles of the tongue:genioglossus,hyoglossus,styloglossus, and the intrinsic muscles of thetongue.[2] These represent all muscles of the tongue except thepalatoglossus muscle, which is innervated by thevagus nerve.[2] The hypoglossal nerve is of ageneral somatic efferent (GSE) type.[2]
These muscles are involved in moving and manipulating the tongue.[2] 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.[13] The hypoglossal nerve also supplies movements including clearing the mouth of saliva and other involuntary activities. The hypoglossal nucleus interacts with thereticular 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.[2]
Reports of damage to the hypoglossal nerve are rare.[14] The most common causes of injury in one case series were compression by tumours and gunshot wounds.[15] 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.[15][16] Damage can be on one or both sides, which will affect symptoms that the damage causes.[2] 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.[16] 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 followingarteriosclerosis of thevertebrobasilar artery. Such a stroke may result in tight oral musculature, and difficulty speaking, eating and chewing.[2]
Progressive bulbar palsy, a form ofmotor neuron disease, is associated with combined lesions of the hypoglossal nucleus andnucleus ambiguus with wasting (atrophy) of the motor nerves of thepons and medulla. This may cause difficulty with tongue movements, speech, chewing and swallowing caused by dysfunction of several cranial nerve nuclei.[2] Motor neuron disease is the most common disease affecting the hypoglossal nerve.[17]
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, due to thegenioglossus muscle receiving nerve signals on one side but not the other.[6][19] 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 approximants,dental stops,alveolar stops,velar nasals,rhotic consonants etc.).[17] Tongue strength may be tested by poking the tongue against the inside of their cheek, while an examiner feels or presses from the cheek.[6]
The hypoglossal nerve carries lower motor neurons thatsynapse with upper motor neurons at thehypoglossal nucleus. Symptoms related to damage will depend on the position of damage in this pathway. If the damage is to the nerve itself (alower motor neuron lesion), 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 .[19][20] If the damage is to the nerve pathway (anupper 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,[19] with speech difficulties more evident.[6] 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.[2]
The hypoglossal nerve may be connected (anastomosed) to thefacial nerve 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).[21][22]
The hypoglossal nerve has also been clinically implicated in the treatment ofobstructive sleep apnea.[23][24] Certain patients withobstructive sleep apnea who are deemed eligible candidates (e.g., failedcontinuous positive airway pressure therapy, underwent appropriate testing withdrug induced sleep endoscopy, and meet other criteria as outlined by theFDA)[25] may be offeredhypoglossal nerve stimulation as an alternative. The purpose of the hypoglossal nerve stimulator is to relievetongue base obstruction during sleep by stimulating thetongue 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.,genioglossus) via an incision in the neck.[26] A sensor lead is then placed in the chest between the ribs in the layer between theinternal intercostal muscles andexternal 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.
The first recorded description of the hypoglossal nerve was byHerophilos (335–280 BC), although it was not named at the time. The first use of the namehypoglossal in Latin asnervi hypoglossi externa was used byWinslow in 1733. This was followed though by several different namings includingnervi indeterminati,par lingual,par gustatorium,great sub-lingual by different authors, andgustatory nerve andlingual nerve (by Winslow). It was listed in 1778 asnerve hypoglossum magnum by Soemmering. It was then named as thegreat hypoglossal nerve byCuvier in 1800 as a translation of Winslow and finally named in English byKnox in 1832.[27]
The hypoglossal nerve is one of twelve cranial nerves found inamniotes includingreptiles,mammals and birds.[28] As with humans, damage to the nerve or nerve pathway will result in difficulties moving the tongue orlapping water, decreased tongue strength, and generally cause deviation away from the affected side initially, and then to the affected side as contractures develop.[29] The evolutionary origins of the nerve have been explored through studies of the nerve in rodents and reptiles.[30] The nerve is regarded as arising evolutionarily from nerves of the cervical spine,[2] which has been incorporated into a separate nerve over the course of evolution.[30]
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 ofprimates, with reasoning that larger nerves would be associated with improvements in speech associated with evolutionary changes. This hypothesis has been refuted.[31]
