Anatomy and Development of the Hypoglossal Nerve
- The hypoglossal nerve arises from the front of the medulla in the brainstem.
- It passes through the subarachnoid space and pierces the dura mater near the hypoglossal canal.
- After emerging from the hypoglossal canal, it gives off a meningeal branch and picks up a branch from the anterior ramus of C1.
- The nerve travels close to the vagus nerve and spinal division of the accessory nerve.
- It spirals downwards behind the vagus nerve and passes between the internal carotid artery and internal jugular vein.
- Neurons of the hypoglossal nucleus are derived from the basal plate of the embryonic medulla oblongata.
- The musculature they supply develops 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.
- By the fifth week, the roots have formed a single nerve and link to the tongue.
- Development of the hypoglossal nerve is essential for proper tongue function.

Function of the Hypoglossal Nerve
- The hypoglossal nerve provides motor control of the extrinsic and intrinsic muscles of the tongue.
- It innervates muscles involved in moving and manipulating the tongue.
- The genioglossus muscles are responsible for protruding the tongue.
- The hypoglossal nucleus interacts with the reticular formation, controlling reflexive motions.
- Corticonuclear fibers supply innervation aiding in unconscious movements related to speech and articulation.

Clinical Significance of the Hypoglossal Nerve
- Damage to the hypoglossal nerve is rare but can be caused by compression, trauma, or disease.
- Injuries to the nerve can result in difficulty with tongue movements, speech, chewing, and swallowing.
- Damage to the nerve can occur on one or both sides, leading to varying symptoms.
- The hypoglossal nerve is often affected in motor neuron diseases such as progressive bulbar palsy.
- Examination of the tongue and its movements can help diagnose hypoglossal nerve damage.

Use of the Hypoglossal Nerve in Nerve Repair
- The hypoglossal nerve can be connected to the facial nerve to restore function when the facial nerve is damaged.
- This connection, known as anastomosis, is used in cases of focal facial nerve damage.
- The connection of nerve fibers from the hypoglossal nerve to the facial nerve can be partial or complete.
- This procedure is often performed in cases of trauma or cancer affecting the facial nerve.
- The aim is to restore facial function by utilizing the intact hypoglossal nerve.

Other Aspects of the Hypoglossal Nerve
- The hypoglossal nerve is clinically implicated in the treatment of obstructive sleep apnea.
- Eligible candidates for the hypoglossal nerve stimulator implant are those who have failed continuous positive airway pressure therapy.
- The implant works by stimulating the tongue to protrude during inspiration, relieving tongue base obstruction during sleep.
- The hypoglossal nerve is found in amniotes, including reptiles, mammals, and birds.
- The evolutionary origins of the hypoglossal nerve have been studied in rodents and reptiles.