|Nerve: Hypoglossal nerve|
Hypoglossal nerve, cervical plexus, and their branches.
The hypoglossal nerve arrises as a series of rootlets, from the caudal brain stem, here seen from below.
|Innervates||genioglossus, hyoglossus, styloglossus|
|Overview - Table|
The hypoglossal nerve is the twelfth cranial nerve XII, and innervates muscles of the tongue. The name hypoglossus springs from the fact that its passage is below the tongue, hypo meaning "under", and glossus meaning "tongue", both of which are from ancient greek. The nerve is involved in controlling tongue movements required for speech, food manipulation (i.e. formation of bolus), and swallowing.
The hypoglossal nerve arises from the hypoglossal nucleus of the caudal brain stem emerging from the ventromedial medulla oblongata from a number of smaller rootlets  in the preolivary sulcus separating the olive and the pyramid. After passing through the subarachnoid space the nerve then exits the skull-base of the posterior fossa  through the hypoglossal canal. On emerging from the hypoglossal canal, it gives off a small meningeal branch and picks up a branch from the anterior ramus of C1. Following in near proximity to the vagus and spinal division of the accessory nerve, it spirals behind the vagus nerve and passes between the internal carotid artery and internal jugular vein lying on the carotid sheath. After passing deep to the posterior belly of the digastric muscle, it passes to the submandibular region, passes lateral to the hyoglossus muscle, and inferior to the lingual nerve to reach and efferently innervate the tongue.
Development in rodents and reptiles may give some clues as to evolutionary origins and organization of the hypoglossal nerve. Nerves supplying lingual muscles, geniohyoid and infrahyoid (or hypobranchial muscle sheet) of common embryonic origin in reptiles arise from a sustained branch of neurons reaching between the caudal medulla and level with the third cervical spinal nerves outlet. This is relevant because caudal portions of the hypoglossal nucleus are intertwined with certain motor neurons of the cervical spinal cord through the supraspinal nucleus which additionally supplies the thyrohyoid through the first spinal nerve. An organization which may be present among humans, but has not been proven occurs in rodents where the first cervical nerve has also been shown to include fibres originating from the caudal hypoglossal nucleus which pass on to innervate certain intrinsic muscles of the tongue after joining with the hypoglossal nerve somewhere along the neck.
The hypoglossal nerve carries axons of general somatic efferent (GSE) type, providing motor control of the extrinsic muscles of the tongue; genioglossus, hyoglossus, styloglossus, and the intrinsic muscles of the tongue.
It supplies motor fibres to all muscles of the tongue, except the palatoglossus muscle, which is innervated by the vagus nerve or, according to some classifications by fibres from the glossopharyngeal nerve that "hitchhike" within the vagus, while some additional sources state it being innervated by the cranial division of the accessory nerve or cranial accessory nerve.
Swallowing to clear mouth of saliva and other involuntary activities completed by the tongue are controlled by the hypoglossal nerve; however, most functions are voluntary. Voluntary control requires conscious thought and nerve pathways occur in the corticobulbar region in the spinal cord. The hypoglossal nucleus is supplied with innervation from the reticular formation, by which it is involved in the control of several reflexive or automatic motions, and several corticonuclear originating fibers supply innervation aiding in unconscious movement required upon engaging in speech and articulation.
The function of the hypoglossal nerve in manipulation for speech contributes to learning languages. Many languages require specific and sometimes unusual uses to create the desired sounds, hence why adults learning a new language may have trouble adjusting to the new movements.
Damage or lesions affecting functions of the nerve may be supranuclear (as in cortical or rostrally to the hypoglossal nucleus in the brainstem), nuclear or infranuclear (as in below or distal to the nucleus), as well as unilateral or bilateral and will give symptoms which differ accordingly. Supranuclear injury of the brain stem affecting the hypoglossal nerve may give rise to crossed symptoms due to a majority of the supranuclear innervation to the hypoglossal nucleus being crossed. Symptoms often show deviation of the tongue towards the paralyzed side when it is stuck out. This is because of the weaker genioglossal muscle. This symptom often only manifests during the initial days, after this protrusion of the tongue will not see deviation towards the paralyzed side, though the nerve does not recover function.
Bilateral supranuclear damage affecting the hypoglossal muscle often occurs in conjunction with damage to facial and trigeminal nerve dysfunction, often due to thrombotic damage to the brainstem upon vertebrobasilar arteriosclerosis. Symptoms may be those of tight oral musculature and dysfunctions of speech and without normal eating and chewing capacity.
Infranuclear lesions will lead to paralysis of the hypoglossal nerve leading to atrophy of muscles of the tongue.
Progressive bulbar palsy, a neuromuscular atrophy associated with combined lesions of the hypoglossal nucleus and nucleus ambiguous upon atrophy of motor nerves of the pons and medulla. The symptoms are those of dysfunctional movements of the tongue leading to speech and chewing impairments, as well as swallowing difficulties, symptoms which arise though dysfunction of several cranial nerve nuclei. Otherwise the symptoms are much like those of infranuclear lesions.
Testing function of the nerve is performed by asked the subject to stick their tongue straight out. If there is a loss of innervation to one side, the tongue will curve toward the affected side, due to unopposed action of the opposite genioglossus muscle. If this is the result of a lower motor neuron lesion, the tongue will be curved toward the damaged side, combined with the presence of fasciculations or atrophy. However, if the deficit is caused by an upper motor neuron lesion, the tongue will be curved away from the side of the cortical damage, without the presence of fasciculations or atrophy. Both atrophy and fasciculations are indicative of lower motor neuron damage to either the nerve or the nucleus. Unilateral atrophy of the muscle may be seen as a reduction in the size of the tongue on the affected side, but may also manifest as "wrinkling" of the tongue on the damaged side. Fasciculations may look similar to ordinary motion of the tongue, which may also be affected by tremor. For fasciculations to be indicative of neural damage they should also be present when the tongue is in a rested position, and are nonrythmic and localized, being likened to making the tongue look like a "bag of worms". 
Deviation is not always present upon damage to the nerve, and strength of the tongue can be tested by asking the subject to poke the inside of their cheek while feeling or administering counter-pressure on the outside of the cheek. Neither is it necessary to see pronounced weakness if the damage is in the upper motor neurons, when dysarticulation may be more evident.
Weakness of the tongue is displayed as a slurring of speech. The tongue may feel "thick", "heavy", or "clumsy." Lingual sounds (i.e., l's, t's, d's, n's, r's, etc.) are slurred and this is obvious in conversation. Damage may manifest differently, and testing of posterior and anterior function of the tongue may be done through assessing ability to make a "k" or "t" sound respectively.
Use in nerve repair
Facial nerve paralysis is a difficult situation to fix, but new cranial nerve substitution techniques allow for some usage to be restored, to include hypoglossal-facial anastomosis.
This procedure is considered the standard for reanimating the face when the proximal end of the facial nerve is not available, but the peripheral system is still viable. There are two options:
- Hypoglossal nerve completely transected and connected to facial nerve.
- Hypoglossal nerve partially transected and connected to facial nerve. This may be accomplished with interposition cable grafts or jump grafts. An advantage of partial transection is minimizing tongue weakness and purported decrease in synkinesis. There are disadvantages though since there are then fewer nerve cells to drive the movement of features in the face.
This article uses anatomical terminology; for an overview, see anatomical terminology.
The hypoglossal nerve is visible at the bottom on its way and passing though the hypoglossal canal.
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- Mtui, M.J. Turlough FitzGerald, Gregory Gruener, Estomih (2012). Clinical neuroanatomy and neuroscience (6th ed. ed.). [Edinburgh?]: Saunders/Elsevier. p. 216. ISBN 978-0-7020-3738-2.
- Kandel, Eric R. (2013). Principles of neural science (5. ed. ed.). Appleton and Lange: McGraw Hill. pp. 1541–1542. ISBN 978-0-07-139011-8.
- Ho, Tang. "Facial Nerve Repair Treatment". WebMDLLC. Retrieved 9 December 2011.
|Wikimedia Commons has media related to Nervus hypoglossus.|
|Wikimedia Commons has media related to Hypoglossal nerve.|
- NeuroNames hier-701
- MedEd at Loyola GrossAnatomy/h_n/cn/cn1/cn12.htm
- cranialnerves at The Anatomy Lesson by Wesley Norman (Georgetown University) (XII)
- Notes on Hypoglossal nerve