Table of Contents
The most important ability of a human being that distinguishes it from all the other creations, i.e., animals, plants, insects, and everything else, is the ability to talk, the ability to describe and share his ideas. That’s why sometimes a human is also called a talking animal. The beautiful, highly complex, and intricate function of articulation after phonation is performed by a special organ in the oral cavity, the tongue. Although the larynx does voice production or phonation (voice box), modulation of this voice to the specific sounds, letters, and words is the function of the tongue along with teeth, gums, lips, etc. The musculature of the tongue is designed so that it can move the tongue in several directions easily and effectively. All this musculature is useless if the nerve supply from the brain control area is lost. This nerve supply to the tongue musculature is provided via the hypoglossal nerve. In the following words, we shall look at the origin, course, classification, and functional components of the hypoglossal nerve. We shall discuss the nucleus related to it and its functions. Last but not least, we shall have a look at the diseases caused by the damage to the hypoglossal nerve and the ways to check for its different lesions.
The sensory and motor nerve supply from the central nervous system to the structures present in the head and neck region of the body is exclusively provided via the cranial nerves. These nerves are named based on their direct origin in the brain. Most of them originate from the brain stem in particular. They are twenty-four in number in the form of twelve pairs. The hypoglossal nerve is the twelfth cranial nerve (CN XII). It is a motor nerve and supplies all the extrinsic and intrinsic muscles of the tongue except one, the palatoglossus muscle. It is so named because the terminal course of this nerve is under the tongue.
Origin and Course of the Hypoglossal Nerve
The hypoglossal nerve emerges from the brainstem at the level of decussation of medial lemnisci in the medulla oblongata between the olives and pyramids of the medulla. Olives are two anterolateral bulges of the medulla, each representing the inferior olivary nucleus, while pyramids are anteriorly located and represent one of the important descending motor tracts, called the pyramidal tract. It runs in the posterior cranial fossa for a while and leaves the cranial cavity via the hypoglossal canal to enter the neck. It descends down in the neck and then forward deep to the mylohyoid muscle lying on the lateral aspect of the hyoglossus muscle. Here, it gives branches to seven out of eight intrinsic and extrinsic muscles of the tongue.
Classification and Functional Components
The hypoglossal nerve is a purely motor nerve with no sensory counterpart. It contains general visceral efferents that provide motor information to the muscles of the tongue.
Nuclei of Hypoglossal Nerve
The nuclei of cranial nerves are the collection of cell bodies of neurons forming that cranial nerve present in the central nervous system. Note that outside the CNS, these collections are known as ganglions.
The nucleus of the hypoglossal nerve is located in the medulla oblongata near the center. It receives corticonuclear fibers from both the cerebral hemispheres except for the genioglossus muscle, for which there are only corticonuclear fibers from the opposite cerebral hemisphere.
The hypoglossal nerve supplies all the intrinsic muscles of the tongue. There are four intrinsic tongue muscles; 1) superior longitudinal, which causes the tongue to shorten when contracts. It makes the dorsum of the tongue concave. 2) inferior longitudinal, which contract to shorten the tongue, making its dorsum convex. 3) Transverse muscle, a sheet-like muscle, contractions of this muscle make the tongue narrow and the depth of the tongue increases. 4) Vertical muscle antagonizes the transverse one and makes the tongue broad and flat.
Extrinsic muscles are also four in number. 3 of them are supplied by the hypoglossal nerve. 1) Genioglossus, which forms the bulk of the tongue and acts to protrude the tongue. Also known as life-saving muscle. 2) Hyoglossus contracts to depress the tongue and also retracts the tongue. 3) Styloglossus also retracts the tongue and also helps to elevate it. The fourth intrinsic muscle of the tongue, the palatoglossus muscle, which elevates the tongue, is supplied by the accessory nerve (CN XI) via its cranial root through the pharyngeal plexus.
Diseases of the Hypoglossal Nerve
There are many diseases that affect the nervous system. Some of them are systemic, which affect the nervous system as a whole, and some are localized to particular nerves. We will limit our discussion to the diseases of the hypoglossal nerve and lesions of the brain stem that affect the nucleus of the hypoglossal nerve.
Damage to the hypoglossal nerve can be caused by neuritis, ischemic injury, trauma to the skull resulting in crush injury of the nerve, tumors (i.e., tumors of the brain in the posterior cranial fossa), stab wounds, gunshot wounds, exposure to toxins or drugs, multiple sclerosis, syringomyelia, and diabetic neuropathy, etc.
If the nerve is damaged, there will be a deviation of the tongue towards the paralyzed side when protruding. Muscle atrophy occurs, and tongue size gets reduced. Fasciculations may be present. Movements f the tongue will be impaired.
Lesions of The Brain Stem Affecting the Hypoglossal Nerve Nucleus
As the cranial nucleus of the hypoglossal nerve is present in the medulla oblongata, lesions of the lower part of the brain stem may involve these nuclei. This results in signs and symptoms the same as in the case of damage to the hypoglossal nerve along its course. Raised intracranial pressure (RIP), as in the case of tumors of the brain in the posterior cranial fossa, may cause herniation of the brain stem through the foramen magnum. Pressure over the nuclei and traction of the cranial nerves results in paralysis of these nerves. Mostly, the last four cranial nerves, glossopharyngeal, vagus, hypoglossal and hypoglossal nerves, all are involved in such lesions. Individual lesions are less common. The same happens in the case of Arnold-Chiari malformation, in which there is herniation of the cerebellar tonsils along with the medulla oblongata through the foramen magnum.
The hypoglossal nerve (CN XII) is the twelfth cranial nerve. It is a purely motor nerve and contains general efferents as its functional components. The hypoglossal nerve originates from the medulla oblongata between the olives and the pyramids. It runs in the posterior cranial fossa and reaches the hypoglossal canal, leaves the skull via this canal, and enters the neck. It descends down in the neck and then forward deep to the mylohyoid muscle lying on the lateral aspect of the hyoglossus muscle. Here, it gives branches to different muscles.
Motor functions of the hypoglossal nerve include innervation of all the intrinsic and extrinsic muscles of the tongue except the palatoglossus, which is supplied by the cranial root of the accessory nerve. These muscles do different actions such as protrusion, retraction, elevation, and depression of the tongue and changes in the tongue shape. Diseases related to accessory nerve lesions result in atrophy of the tongue musculature.
Damage to the hypoglossal nerve can be caused by neuritis, ischemic injury, trauma to the skull resulting in crush injury of the nerve, tumors (i.e., tumors of the brain in the posterior cranial fossa), stab wounds, gunshot wounds, exposure to toxins or drugs, multiple sclerosis, syringomyelia, and diabetic neuropathy, etc. Lesions of the brain stem (tumors of the brain stem, Arnold-Chiari malformation, etc.) involving the accessory nerve nucleus result in the same signs and symptoms. Treatment involves the solution of the underlying cause by using either medications or surgical interventions.
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Image Source: Hypoglossal Nerve