Hypoglossal nerve

The hypoglossal nerve is the twelfth cranial nerve, and innervates all the extrinsic and intrinsic muscles of the tongue, except for the palatoglossus which is innervated by the vagus nerve. It is a nerve with a solely motor function. The nerve arises from the hypoglossal nucleus in the brain stem as a number of small rootlets, passes through the hypoglossal canal and down through the neck, and eventually passes up again over the tongue muscles it supplies into the tongue. There are two hypoglossal nerves in the body: one on the left, and one on the right. The nerve is involved in controlling tongue movements required for speech and swallowing, including sticking out the tongue and moving it from side to side. Damage to the nerve or the neural pathways which control it can affect the ability of the tongue to move and its appearance, with the most common sources of damage being injury from trauma or surgery, and motor neuron disease. The first recorded description of the nerve is by Herophilos in the third century BC. The name hypoglossus springs from the fact that its passage is below the tongue, from hypo (Greek: 'under') and glossa (Greek: 'tongue'). The hypoglossal nerve arises as a number of small rootlets from the front of the medulla, the bottom part of the brainstem, in the preolivary sulcus, which separates the olive and the pyramid. 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. After emerging from the hypoglossal canal, the hypoglossal nerve gives off a meningeal branch and picks up a branch from the anterior ramus of C1. It then travels close to the vagus nerve and spinal division of the accessory nerve, spirals downwards behind the vagus nerve and passes between the internal carotid artery and internal jugular vein lying on the carotid sheath. At a point at the level of the angle of the mandible, the hypoglossal nerve emerges from behind the posterior belly of the digastric muscle. It then loops around a branch of the occipital artery and travels forward into the region beneath the mandible. The hypoglossal nerve moves forward lateral to the hyoglossus and medial to the stylohyoid muscles and lingual nerve. It continues deep to the genioglossus muscle and continues forward to the tip of the tongue. It distributes branches to the intrinsic and extrinsic muscle of the tongue innervates as it passes in this direction, and supplies several muscles (hyoglossus, genioglossus and styloglossus) that it passes. The rootlets of the hypoglossal nerve arise from the hypoglossal nucleus near the bottom of the brain stem. The hypoglossal nucleus receives input from both the motor cortices but the contralateral input is dominant; innervation of the tongue is essentially lateralized. 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 hypoglossal nerve is derived from the first pair of occipital somites, collections of mesoderm that form next to the main axis of an embryo during development. The musculature it supplies develop 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. The hypoglossal nucleus is derived from the basal plate of the embryonic medulla oblongata.