Hypoglossal nerve

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.

CN XII is a nerve with a sole motor function. The nerve arises from the hypoglossal nucleus in the medulla 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. Signals from muscle spindles on the tongue travel through the hypoglossal nerve, moving onto the lingual nerve which synapses on the trigeminal mesencephalic nucleus. 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

thumb|300px|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. These represent all muscles of the tongue except the palatoglossus muscle, which is innervated by the vagus nerve. 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.

|alt=Image of a tongue protruding from a mouth, wasted on the left, and pointing to the left.]]

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. 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.). Tongue strength may be tested by poking the tongue against the inside of their cheek, while an examiner feels or presses from the cheek. 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,

Hypoglossal nerve stimulator implant

The hypoglossal nerve has also been clinically implicated in the treatment of obstructive sleep apnea. 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) may be offered hypoglossal nerve stimulation 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. 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.

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 lap<nowiki/>ping 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,