Movement of our tongue is important for speech and swallowing. Our tongue is a large muscle, and is affected in a number of diseases that affect the hypoglossal nerve. The hypoglossal nerve is the 12th and final cranial nerve. It is the only cranial nerve to emerge medial to the olive, and innervates the intrinsic muscles of the tongue.
The hypoglossal nerve is also known as cranial nerve 12 and is a purely somatic motor nerve. It arises from the hypoglossal nucleus of the brainstem, the ventromedial medulla to be precise. It is the only cranial nerve to emerge medial to the olive (an ovaloid structure responsible for pathways of motor learning and hearing) and emerges just medial to the pyramids (the descending corticospinal tract runs in the pyramids).
The nerve runs downward in the subarachnoid space, and runs behind the accessory and vagus nerves and leaves the skull. It runs between the internal carotid artery and internal jugular vein, and lies on the carotid sheath through the hypoglossal canal, an opening located within the depths of the foramen magnum. As the nerve leaves the cranium, it combines with the ventral rami of C1-2 from the cervical plexus, which do not merge with the hypoglossal nerve, but simply attach to it its sheath and travel with it. The section of the C1 spinal nerve that descends with the hypoglossal nerve before it separates and forms the superior root of the ansa cervicalis is called the descendens hypoglossi.
The hypoglossal nerve then passes deep to the posterior belly of digastric and runs to the submandibular region, where it will run on the lateral side of the hyoglossus muscle. It continues on its anterior course, running inferiorly to the lingual nerve. The lingual nerve is a sensory branch of the mandibular division of the trigeminal nerve, which also allows the chorda tympani branch of the facial nerve (taste from the anterior two thirds of the tongue) to hitch a ride back to the brainstem. The nerve will then pass inferior to the angle of the mandible, and then pass anteriorly to move toward the tongue. The nerve innervates three of the four extrinsic tongue muscles (genioglossus, hyoglossus and styloglossus, with palatoglossus being innervated by the vagus nerve), and all the intrinsic muscles of the tongue (inferior and superior longitudinal, vertical and transverse). All of these muscles function to move the tongue.
12th nerve palsy- Palsy of the nerve causes the tongue to deviate toward the affected side. It occurs following thrombotic changes to the brainstem, or atherosclerotic disease of the vertebrobasilar anastomosis. It may occur after neck surgery, as the nerve descends in the cervical region to reach the mouth.
Bulbar palsy- This disease refers to impairment of the 9th to 12th cranial nerves due to a lower motor neurone lesion i.e. the nerves themselves. This causes wasting and fasciculations of the tongue. A pseudobulbar palsy is damage to the nuclei of the above nerves, resulting in upper motor neurone signs of the tongue and palate (tremors, increased tone). As four cranial nerves are affected the signs are wide ranging, from dysphagia (glossopharyngeal and cranial accessory nerves), dysarthria and dysphonia (vagus and hypoglossal nerves) to nasal regurgitation. Contrasting to myaesthenia gravis, the eyes are spared in this disease.
Developmental precursor- Telencephalon (secondary vesicle of the developing nervous system)
Muscles- Extrinsic tongue muscles- genioglossus, hyoglossus and styloglossus, with palatoglossus being innervated by the vagus nerve
Intrinsic tongue muscles- inferior and superior longitudinal, vertical and transverse muscles
Hypo means under, and glosso means tongue
Remember that the nerve does not innervate the palatoglossus muscle
The hypoglossal nerve is the 12th and final cranial nerve. It is the only cranial nerve to emerge medial to the olive, and innervates the intrinsic muscles of the tongue.
1. Frank H.Netter MD: Atlas of Human Anatomy, 5th Edition, Elsevier Saunders, Chapter 1 Head and Neck
2. Chummy S.Sinnatamby: Last’s Anatomy Regional and Applied, 12th Edition, Churchill Livingstone Elsevier
3. Richard L. Drake, A. Wayne Vogl, Adam. W.M. Mitchell: Gray’s Anatomy for Students, 2nd Edition, Churchill Livingstone Elsevier
4. Elliiot L.Manchell: Gray's Clinical Neuroanatomy: The Anatomic Basis for Clinical Neuroscience
5. The Definitive Neurological Surgery Board Review
By Shawn P. Moore, 2005
6. Human Neuroanatomy
By James R. Augustine, 2008