Hypoglossal nerve

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Hypoglossal nerve
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Hypoglossal nerve, cervical plexus, and their branches.
Brain human normal inferior view with labels en.svg
The hypoglossal nerve arises as a series of rootlets, from the caudal brain stem, here seen from below.
Details
To Ansa cervicalis
Innervates Genioglossus, hyoglossus, styloglossus, geniohyoid, thyrohyoid, 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

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 the vagus nerve. [lower-alpha 1]

Contents

CN XII is a nerve with a sole motor function. The nerve arises from the hypoglossal nucleus in the medulla [1] [2] as a number of small rootlets, pass through the hypoglossal 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 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 was by Herophilos in the third century BC. The name hypoglossus springs from the fact that its passage is below the tongue, from hypo (Greek : "under") and glossa (Greek : "tongue").

Structure

The hypoglossal nerve arises as a number of small rootlets from the front of the medulla, the bottom part of the brainstem, [1] [2] in the anterolateral sulcus which separates the olive and the pyramid. [3] The nerve passes through the subarachnoid space and pierces the dura mater near the hypoglossal canal, an opening in the occipital 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 the anterior ramus of C1. It then travels close to the vagus nerve and spinal division of the accessory nerve, [2] spirals downwards behind the vagus nerve and passes between the internal carotid artery and internal jugular vein lying on the carotid sheath. [4]

At a point at the level of the angle of the mandible, the hypoglossal nerve emerges from behind the posterior belly of the digastric muscle. [4] It then loops around a branch of the occipital artery and travels forward into the region beneath the mandible. [4] The hypoglossal nerve moves forward lateral to the hyoglossus and medial to the stylohyoid muscles and lingual nerve. [5] It continues deep to the genioglossus 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 the hypoglossal nucleus near the bottom of the brain stem. [1] The hypoglossal nucleus receives input from both the motor 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 the lingual nerve which synapses on the trigeminal mesencephalic nucleus. [2]

Development

Neurons of the hypoglossal nucleus are derived from the basal plate of the embryonic medulla oblongata. [7] [8] The musculature they supply develops as the hypoglossal cord from the myotomes 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]

Function

Schematic image of the hypoglossal nerve and innervation targets. Lawrence 1960 17.26.png
Schematic image of the hypoglossal nerve and innervation targets.

The hypoglossal nerve provides motor control of the extrinsic muscles of the tongue: genioglossus, hyoglossus, styloglossus, and the intrinsic muscles of the tongue. [2] These represent all muscles of the tongue except the palatoglossus muscle, which is innervated by the vagus nerve. [2] The hypoglossal nerve is of a general 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 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. [2]

Clinical significance

Damage

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 following arteriosclerosis of the vertebrobasilar artery. Such a stroke may result in tight oral musculature, and difficulty speaking, eating and chewing. [2]

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 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]

Examination

An injured hypoglossal nerve will cause the tongue to waste away and the tongue will not be able to stick out straight. The injury here occurred because of branchial cyst surgery. Unilateral hypoglossal nerve injury.jpeg
An injured hypoglossal nerve will cause the tongue to waste away and the tongue will not be able to stick out straight. The injury here occurred because of branchial cyst surgery.

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 the genioglossus 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 that synapse with upper motor neurons at the hypoglossal nucleus. 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), 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 (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, [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]

Use in nerve repair

The hypoglossal nerve may be connected (anastomosed) to the facial 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]

Hypoglossal nerve stimulator implant

The hypoglossal nerve has also been clinically implicated in the treatment of obstructive sleep apnea. [23] [24] Certain patients with obstructive sleep apnea who are deemed eligible candidates (e.g., failed continuous positive airway pressure therapy, underwent appropriate testing with drug induced sleep endoscopy, and meet other criteria as outlined by the FDA) [25] may be offered the hypoglossal nerve stimulator as an alternative. The purpose of the hypoglossal nerve stimulator is to relieve tongue base obstruction during sleep by stimulating the tongue 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 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 Herophilos (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. [27]

Other animals

The hypoglossal nerve is one of twelve cranial nerves found in amniotes including reptiles, mammals and birds. [28] As with humans, damage to the nerve or nerve pathway will result in difficulties moving the tongue or lap ping 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 of primates, with reasoning that larger nerves would be associated with improvements in speech associated with evolutionary changes. This hypothesis has been refuted. [31]

See also

Related Research Articles

<span class="mw-page-title-main">Cranial nerves</span> Nerves that emerge directly from the brain and the brainstem

Cranial nerves are the nerves that emerge directly from the brain, of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck, including the special senses of vision, taste, smell, and hearing.

<span class="mw-page-title-main">Vagus nerve</span> Main nerve of the parasympathetic nervous system

The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that carries sensory fibers that create a pathway that interfaces with the parasympathetic control of the heart, lungs, and digestive tract.

Swallowing, also called deglutition or inglutition in scientific contexts, is the process in the body of a human that allows for a substance to pass from the mouth, to the pharynx, and into the esophagus, while shutting the epiglottis. Swallowing is an important part of eating and drinking. If the process fails and the material goes through the trachea, then choking or pulmonary aspiration can occur. In the human body the automatic temporary closing of the epiglottis is controlled by the swallowing reflex.

<span class="mw-page-title-main">Facial nerve</span> Cranial nerve VII, for the face and tasting

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 sensations from the anterior two-thirds of the tongue. The nerve typically travels from the pons through the facial canal in the temporal bone and exits the skull at the stylomastoid foramen. It arises from the brainstem from an area posterior to the cranial nerve VI and anterior to cranial nerve VIII.

<span class="mw-page-title-main">Brainstem</span> Posterior part of the brain, adjoining and structurally continuous

The brainstem 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 continuous with the thalamus of the diencephalon through the tentorial notch, and sometimes the diencephalon is included in the brainstem.

<span class="mw-page-title-main">Glossopharyngeal nerve</span> Cranial nerve IX, for the tongue and pharynx

The glossopharyngeal nerve, also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior to the vagus nerve. Being a mixed nerve (sensorimotor), it carries afferent sensory and efferent motor information. The motor division of the glossopharyngeal nerve is derived from the basal plate of the embryonic medulla oblongata, whereas the sensory division originates from the cranial neural crest.

<span class="mw-page-title-main">Accessory nerve</span> Cranial nerve XI, for head and shoulder movements

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 nerves because part of it was formerly believed to originate in the brain. The sternocleidomastoid muscle tilts and rotates the head, whereas the trapezius muscle, connecting to the scapula, acts to shrug the shoulder.

<span class="mw-page-title-main">Pyramidal tracts</span> The corticobulbar tract and the corticospinal tract

The pyramidal tracts include both the corticobulbar tract and the corticospinal tract. These are aggregations of efferent nerve fibers from the upper motor neurons that travel from the cerebral cortex and terminate either in the brainstem (corticobulbar) or spinal cord (corticospinal) and are involved in the control of motor functions of the body.

<span class="mw-page-title-main">Solitary nucleus</span> Sensory nuclei in medulla oblongata

The solitary nucleus(SN) (nucleus of the solitary tract, nucleus solitarius, or nucleus tractus solitarii) is a series of neurons whose cell bodies form a roughly vertical column of grey matter in the medulla oblongata of the brainstem. Their axons form the bulk of the enclosed solitary tract. The solitary nucleus can be divided into different parts including dorsomedial, dorsolateral, and ventrolateral subnuclei.

<span class="mw-page-title-main">Nucleus ambiguus</span> Group of motor neurons in the brain stem

The nucleus ambiguus 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 innervate the muscles of the soft palate, pharynx, and larynx which are associated with speech and swallowing. As well as motor neurons, the nucleus ambiguus contains preganglionic parasympathetic neurons which innervate postganglionic parasympathetic neurons in the heart.

<span class="mw-page-title-main">Corticobulbar tract</span> Motor pathway in the brain connecting the motor cortex to the medullary pyramids

The corticobulbartract is a two-neuron white matter motor pathway connecting the motor cortex in the cerebral cortex to the medullary pyramids, which are part of the brainstem's medulla oblongata region, and are primarily involved in carrying the motor function of the non-oculomotor cranial nerves, like muscles of the face, head and neck. The corticobulbar tract is one of the pyramidal tracts, the other being the corticospinal tract.

<span class="mw-page-title-main">Genioglossus</span> Muscle of the tongue

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 the tongue. It is innervated by the hypoglossal nerve. The genioglossus is the major muscle responsible for protruding the tongue.

<span class="mw-page-title-main">Hyoglossus</span> Muscle of the tongue

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. It acts to depress and retract the tongue.

<span class="mw-page-title-main">Inferior longitudinal muscle of tongue</span> Intrinsic muscle of the tongue

The inferior longitudinal muscle of tongue is an intrinsic muscle of the tongue. It is situated on the under surface of the tongue between the genioglossus and hyoglossus. It is innervated by the hypoglossal nerve. Its contraction shortens and thickens the tongue.

<span class="mw-page-title-main">Cranial nerve nucleus</span> Collection of neurons in the brain stem

A cranial nerve nucleus is a collection of neurons in the brain stem that is associated with one or more of the cranial nerves. Axons carrying information to and from the cranial nerves form a synapse first at these nuclei. Lesions occurring at these nuclei can lead to effects resembling those seen by the severing of nerve(s) they are associated with. All the nuclei except that of the trochlear nerve supply nerves of the same side of the body.

<span class="mw-page-title-main">Facial motor nucleus</span>

The facial motor nucleus is a collection of neurons in the brainstem that belong to the facial nerve. These lower motor neurons innervate the muscles of facial expression and the stapedius.

Progressive bulbar palsy (PBP) is a medical condition. It belongs to a group of disorders known as motor neuron diseases. PBP is a disease that attacks the nerves supplying the bulbar muscles. These disorders are characterized by the degeneration of motor neurons in the cerebral cortex, spinal cord, brain stem, and pyramidal tracts. This specifically involves the glossopharyngeal nerve (IX), vagus nerve (X), and hypoglossal nerve (XII).

Bulbar palsy refers to a range of different signs and symptoms linked to impairment of function of the glossopharyngeal nerve, the vagus nerve, the accessory nerve, and the hypoglossal nerve. It is caused by a lower motor neuron lesion in the medulla oblongata, or from lesions to these nerves outside the brainstem, and also botulism. This may be caused by any of a number of genetic, vascular, degenerative, inflammatory, and other underlying conditions. It can be differentiated from pseudobulbar palsy. When there is airway obstruction, intubation is used.

<span class="mw-page-title-main">Gustatory nucleus</span> Rostral part of the solitary nucleus located in the medulla

The gustatory nucleus is the rostral part of the solitary nucleus located in the medulla oblongata. The gustatory nucleus is associated with the sense of taste and has two sections, the rostral and lateral regions. A close association between the gustatory nucleus and visceral information exists for this function in the gustatory system, assisting in homeostasis - via the identification of food that might be possibly poisonous or harmful for the body. There are many gustatory nuclei in the brain stem. Each of these nuclei corresponds to three cranial nerves, the facial nerve (VII), the glossopharyngeal nerve (IX), and the vagus nerve (X) and GABA is the primary inhibitory neurotransmitter involved in its functionality. All visceral afferents in the vagus and glossopharyngeal nerves first arrive in the nucleus of the solitary tract and information from the gustatory system can then be relayed to the thalamus and cortex.

Alternating hemiplegia is a form of hemiplegia that has an ipsilateral cranial nerve palsies and contralateral hemiplegia or hemiparesis of extremities of the body. The disorder is characterized by recurrent episodes of paralysis on one side of the body. There are multiple forms of alternating hemiplegia, Weber's syndrome, middle alternating hemiplegia, and inferior alternating hemiplegia. This type of syndrome can result from a unilateral lesion in the brainstem affecting both upper motor neurons and lower motor neurons. The muscles that would receive signals from these damaged upper motor neurons result in spastic paralysis. With a lesion in the brainstem, this affects the majority of limb and trunk muscles on the contralateral side due to the upper motor neurons decussation after the brainstem. The cranial nerves and cranial nerve nuclei are also located in the brainstem making them susceptible to damage from a brainstem lesion. Cranial nerves III (Oculomotor), VI (Abducens), and XII (Hypoglossal) are most often associated with this syndrome given their close proximity with the pyramidal tract, the location which upper motor neurons are in on their way to the spinal cord. Damages to these structures produce the ipsilateral presentation of paralysis or palsy due to the lack of cranial nerve decussation before innervating their target muscles. The paralysis may be brief or it may last for several days, many times the episodes will resolve after sleep. Some common symptoms of alternating hemiplegia are mental impairment, gait and balance difficulties, excessive sweating and changes in body temperature.

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Sources

Notes

  1. ↑ These are the genioglossus, hyoglossus, styloglossus, and intrinsic muscles of the tongue.
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