Neurology Explained - The Hypoglossal nerve (Cranial Nerve XII)
Hey everyone, this is final post on the cranial nerve, and I will be discussing the Hypoglossal nerve (cranial nerve 12). I took time to explain the cranial nerves 1 to 11, and in my last post, I gave a quick summary on them. Today, I will be talking about the origin of the hypoglossal nerve, the course of the nerve, the location it supplies, and the functions of the location or structure that it supplies. This post isn't going to be a long post at all, but I can tell it will be fun to read as you learn. Let's get the ball rolling.
The Hypoglossal Nerve Nucleus is found at the posterior part of the medulla. It is located close to the Dorsal Nucleus of Vagus which is close to the Nucleus of Tractus Solitarius. The hypoglossal nerve nucleus has at its anterior, the Medial longitudinal fasciculus, and close to this is the Tectospinal tract which is close to the Medial Lemniscus, and at the top of the Medulla is the Nucleus Gracilis. At the border part of the medulla is the Olivary Nuclear Complex which can be superior, or inferior. At the basal part of the medulla are fiber bundles known as pyramids where descending cortisol spinal pathways run through. It will be good to say that the Hypoglossal nerve Nucleus is located within the posterior medulla, anterior to the nucleus gracilis, medial to the dorsal nucleus of vagus, and posterior to the Medial longitudinal fasciculus.,.
From the Hypoglossal Nerve Nucleus, the nerve fiber moves through the reticular formation, reaching the medial longitudinal fasciculus, then go through the medial leminiscus. From the Medial Leminiscus, it goes to the Oliveri Nuclear complex, then to the pyramid, where it goes out of the medulla through the Dura Mater to the hypoglossal canal located within the Occipital Condyles. From the Hypoglossal canal, the fiber runs along with nerve fibers from the Jugular foramen. The hypoglossal nerve fiber runs through the external carotid artery then through the mylohyoid. The hypoglossal fiber supplies the hyoglossus, the genioglossus, and the Chondroglossus, then to the Intrinsic muscle of the tongue which includes the traverse lingual muscle of the tongue, vertical lingual muscle, and the superior and inferior longitudinal tongue muscle.,,,.
Nerves from C1 of the Cervical Plexus move out giving two branches, with one moving with the hypoglossal nerve to innervate the Geniohyoid muscle, and the Thyrohyoid membrane. The superior root of the C1 combines with C2 fibres and C3 fibers to form the inferior root of the ansa cervicalis which gives out branches to the Omohyoid muscle, sternothyroid muscle, sternohyoid muscle, and the Omohyoid in the inferior belly. The hypoglossal nerve supplies the extrinsic and intrinsic muscles of the tongue, they allow the tongue to move and form shapes. It utilizes the General Somatic Efferent Fibers, which helps the tongue to protract, retract, elevate, and depress the tongue., ,.
When there is an infarction of the upper motor neurons, which supplies the right side and part of the muscles of the left tongue (bilateral nerve supply), causes corticobulbar fiber lesions, it can cause complete dysfunction of the tongue, Atrophy of the tongue, and Spastic Dyspasthria. Tumours and Lesion in the nucleus (Nuclei lesion), also, if there is a lesion to the medial longitudinal fasciculus, it could cause medial medullary syndrome affecting the hypoglossal canal, the medullary pyramid, and medial lemniscus, leading to Atrophy, fasciculus, and tongue deviation.,,.
The hypoglossal nerve is a somatic motor nerve, that innervates the intrinsic and extrinsic muscles of the tongue but it doesn't innervate the muscle of the palatoglossus. It originates from the brain. It is important to know that virtually all cranial nerve nuclei come from the Pons, and Medulla of the brainstem. The nuclei are arranged in the brainstem with the First cranial nerve being at the superior part of the brainstem, and the twelfth cranial nerve nucleus being at the inferior part of the brainstem. The Axons from the hypoglossal nucleus leave the skull through the hypoglossal canal to innervate the tongue.,
You can read my previous post on the cranial nerve by clicking on any of the links below.