HYPOGLOSSAL NERVE: ORIGIN, COURSE, FUNCTIONS, PATHOLOGIES - ANATOMY AND PHYSIOLOGY - 2025

The hypoglossal nerve is a cranial nerve that supplies the muscles of the tongue. It is sometimes called the greater hypoglossal nerve or the XII cranial nerve. Its name comes from the Latin nervus hypoglossus. This term was embraced by Dr. Jacob Winslow (1669-1760) who was a specialist in anatomy.

If the term hypoglossal is broken down, it means hypo: down and glossa: tongue. The function of the hypoglossal nerve is purely motor, that is, it allows to execute the movements of the tongue in a voluntary and coordinated way.

Hypoglossal nerve location / patient with unilateral hypoglossal nerve palsy. Source: Wikipedia File: Gray794.png / Wikipedia File: Unilateral hypoglossal nerve injury.jpeg Read in another language

Whereas, other properties of the tongue such as taste are provided by other nerves. The tongue is a muscular organ. It is made up of the muscles: styloglossus, hyoglossus, genioglossus and palatoglossus.

This nerve supplies all the muscles of the tongue, both intrinsic and extrinsic, with the exception of the palatoglossus muscle, which is innervated by another nerve (vagus nerve). It should be noted that the styloglossus muscle, in addition to being innervated by the hypoglossus, also receives nerve fibers from the glossopharyngeal nerve.

Although sensory, motor, special and parasympathetic axons are known to most of the cranial nerves, only a general somatic efferent component (ESG) is known to the hypoglossal nerve.

Knowledge of the trajectory and function of the hypoglossal nerve is of great interest to many professionals, such as: dentists, plastic surgeons, neurologists, neurosurgeons, otorhinolaryngologists, maxillofacial surgeons, among others.

At the time of head and neck surgery, the three reference zones that delimit the position of the hypoglossal nerve must be taken into account, to avoid iatrogenesis (damage caused by a medical procedure).

The limits are: above it is the posterior belly of the digastric muscle, behind it adjoins the internal jugular vein and in front is the facial venous trunk and external carotid artery.

Origin

The hypoglossal nerve originates from the cranial medulla, specifically part of the dorsal bulbar region (lower part of the brainstem). There the motor nuclei of the hypoglossal rest.

The motor nuclei of the hypoglossal are quite close to the floor of the rhomboid fossa and it is surrounded by 3 nuclei called perihipoglossus.

Journey

The hypoglossal tract is quite complex and is divided into intracranial and extracranial paths. The intracranial path in turn is divided into three parts, called:

Intramedullary segment, cisternal segment and canalicular segment. While the extracranial path is divided into two segments.

- Intracranial

Intramedullary segment

The hypoglossal nerve arises from the cranial medulla (a point called the hypoglossal motor nucleus or XII cranial nerve). It arises from this as small and fine roots (efferent root fibers). They emerge from the bulb through the preolivar groove, that is, between the pyramid and the olive.

Cisternal segment

In their passage through the sub-arachnoid space, the reticular fibers of the hypoglossal are correlated with the vertebral artery and the posterior and inferior cerebellar artery, the latter also known as (PICA).

Canalicular segment

Another group of efferent reticular fibers, located more superficially, are directed towards the encephalic dura, perforating it just in the posterior cranial fossa.

There, the fibers fuse and it goes to the anterior condylar foramen, through the canalis hypoglossi (hypoglossal bone canal) located in the occipital bone, in order to exit the skull.

- Extracranial

From this moment on, the journey is extracranial. Outside the skull it is related to a nerve branch corresponding to the cervical plexus, comprised of cranial nerves IX, X, XI. Extra cranially, the hypoglossal nerve is studied in two segments.

The first segment of the hypoglossal nerve is located behind the internal carotid artery and above the inferior vagal ganglion. Right here, it attaches to a ventral branch of the first cervical nerve (C1).

The second segment, part of the bowing of the nerve, just between the internal carotid artery and the internal jugular vein.

From there it reaches the neck (nasopharyngeal carotid space), then passes close to the jaw, curves forward to enter the tongue together with the lingual artery. Subsequently, it enters through the lateral aspect of the genioglossus muscle on the floor of the mouth.

Finally, the nerve branches into 7 collateral branches, behind the lingual nerve. This is how all the muscles of the tongue (extrinsic and intrinsic) are innervated, except for the palatoglossus muscle.

Function

The function of the hypoglossal nerve is purely motor, that is, it makes the tongue move and indirectly contributes to the formation of the food bolus, swallowing and the articulation of sounds, since for these functions it is necessary for the tongue to execute certain movements.

Exploration

In case of suspected paralysis, the patient is asked to stick out his tongue. First, its shape, size, symmetry and surface texture are observed.

It can also be seen if there are folds or other injuries or if there is a fine tremor (fasciculations). Another important point is the position of the tongue, it should be noted if it stays in the middle area or if it goes to the side.

The patient is then asked to try to move the tongue up, down, and to each side of the mouth. The operation is repeated but now opposing the movement with a low tongue.

If the patient is paralyzed, half of the tongue will be atrophied. Therefore, you will not be able to perform these simple exercises and the tongue will usually be seen displaced to the paralyzed side.

Pathology

Paralysis or partial paralysis of the hypoglossal nerve

It is a rare clinical entity; most of the cases there is joint involvement of other cranial nerves. At any point along its entire path, the hypoglossal nerve could be affected.

This condition appears to have a wide range of possible causes, including: head trauma, neoplasms at the base of the skull, CNS infections, demyelinating diseases, multiple myeloma, Arnold Chiari malformation, Behçet and Wegener disease, or stroke.

Additionally, cases have been reported due to post-anesthesia neurological involvement of the cervical plexus and post-naso and tracheal oro-intubation, among others.

However, it sometimes occurs idiopathically (with no apparent or unknown cause). In the latter case, the prognosis is always very favorable, with a rapid and spontaneous recovery. It can occur at any age.

When the hypoglossal nerve is affected, the tongue becomes asymmetric, deviated towards the side of the involvement, and the longitudinal folds on the side of the paralysis are more noticeable. The patient has difficulty chewing, swallowing, and speaking (dysarthria).

To determine the cause, a comprehensive evaluation of the patient with multiple studies is necessary, such as: nuclear magnetic resonance, lumbar puncture, among others.

Bilateral hypoglossal nerve palsy

It is a serious affectation, since it produces the suffocation of the patient, the atonic tongue displaces the epiglottis, producing the closure of the aditum of the larynx.

Syndromes affecting the hypoglossal

There are several syndromes that affect the hypoglossal nerve, among them Reynold Révillod-Déjerine anterior bulbar syndrome, Jackson syndrome, Tapia dorsal bulbar syndrome, Babinski-Nageotte syndrome, Cestan-Chenais syndrome or Collet syndrome. -Sicard.

References

1. "Cranial Nerves" Wikipedia, The Free Encyclopedia. 23 Aug 2019, 19:37 UTC. 3 Sep 2019, 01:42 en.wikipedia.org/
2. «Hypoglossal nerve» Wikipedia, The Free Encyclopedia. 31 Jul 2019, 21:50 UTC. 3 Sep 2019, 01:45 es.wikipedia.org/
3. Robaina J, González A, Curutchet L, Gil A. Idiopathic paresis of the hypoglossal nerve. Neurology, 2019; 34 (2): 125-127. Available at: elsevier.es
4. Gallardo J, Contreras-Domínguez V, Chávez J, Aragón A. Neurological lesion of the hypoglossal nerve secondary to cervical plexus anesthesia under ultrasonography. About a clinical case. Rev Chil Anest, 2013; 42: 325-330. Available at: sachile.cl
5. Rivera Cardona G. Anatomical and clinical considerations of the hypoglossal nerve: review of the literature. Univ. Méd. 2015; 56 (3): 323-340. Available at: Users / Team