Hypoglossal Nerve Anatomy

Also written as CN XII, XIIth cranial nerve.

The hypoglossal nerve (CN XII) supplies motor fibers to all intrinsic and most extrinsic tongue muscles, enabling speech, swallowing and airway patency. Its course from the medulla to the tongue makes it vulnerable to lesions at the skull base, neck, and oral cavity.

Overview

The hypoglossal nerve is a purely motor cranial nerve that originates from the hypoglossal nucleus in the medulla and exits the skull through the hypoglossal canal.

• Motor to tongue → speech, chewing, swallowing
• No sensory or parasympathetic components

Location and structure

After leaving the medulla, the nerve descends between the vertebral artery and the anterior spinal artery, passes through the hypoglossal canal, and travels deep to the posterior belly of the digastric before turning anteriorly to the tongue.

• Runs lateral to the internal carotid artery in the carotid sheath
• Crosses the hyoglossus muscle before entering the tongue

Relations

In the neck, the hypoglossal nerve lies superficial to the internal jugular vein and deep to the posterior belly of the digastric, making it susceptible during carotid endarterectomy and neck dissection.

• Close to the lingual artery (branch of external carotid)
• Adjacent to the ansa cervicalis loop

Blood supply

Arterial supply follows the lingual artery, a branch of the external carotid, which runs alongside the nerve in the submandibular region.

• Lingual artery provides vasa nervorum

Nerve supply

The hypoglossal nerve receives no parasympathetic fibers; it is purely somatic motor. It receives proprioceptive feedback via the trigeminal nerve (V) for tongue position.

• Proprioceptive fibers travel with V3 to the mesencephalic nucleus

Function

Activation of the intrinsic tongue muscles changes tongue shape for articulation, while extrinsic muscles (genioglossus, hyoglossus, styloglossus) move the tongue in three dimensions for chewing and swallowing.

• Genio‑glossus protrudes tongue
• Hyoglossus depresses and retracts tongue
• Styloglossus retracts and elevates tongue

Clinical relevance and injuries

Lesions produce ipsilateral tongue weakness, atrophy, and fasciculations. The classic bedside sign is deviation of the tongue toward the side of the lesion when protruded.

• MRI brain and neck: Localises lesion; if unavailable, use CT scan or bedside ultrasound of neck vessels
• Electromyography (EMG) of tongue: Confirms denervation; if unavailable, rely on clinical exam

• Unilateral palsy → tongue deviates to weak side
• Bilateral palsy → central tongue depression, airway obstruction
• Common causes: neck trauma, carotid surgery, skull base tumors, tuberculous meningitis

Exam pearls

At the bedside, ask the patient to stick out the tongue and observe direction, strength, and presence of fasciculations. Test palate elevation (CN X) to differentiate central from peripheral causes of dysarthria.

• Tongue deviation = hypoglossal lesion
• No deviation but dysarthria = consider cortical stroke

Questions an examiner may ask

• A patient presents with unilateral tongue deviation and dysphagia after carotid endarterectomy — discuss the likely nerve injured and immediate management — identify hypoglossal nerve injury; secure airway, assess for bilateral involvement, arrange imaging.
• During a neurological exam you note tongue atrophy but normal palate elevation — what does this localise to and why? — Isolated hypoglossal lesion; palate supplied by CN X.
• A 45‑year‑old with tuberculous meningitis develops tongue fasciculations — which cranial nerve is affected and what is the pathophysiology? — CN XII involvement due to basal meningitis compressing the nerve.
• How would you differentiate a central (cortical) speech disorder from a peripheral hypoglossal palsy at the bedside? — Central lesions spare tongue deviation; peripheral shows deviation toward weak side.
• In a low‑resource setting you suspect hypoglossal nerve compression by a neck tumour but lack MRI — what alternative investigations can you use? — CT neck, bedside ultrasound, clinical localisation.
• What are the emergency priorities for bilateral hypoglossal palsy? — Protect airway, consider intubation, monitor for aspiration, treat underlying cause.
• Why is a lingual artery injury a concern during hypoglossal nerve surgery? — The artery runs alongside the nerve; injury can cause tongue hematoma and airway obstruction.
• A patient on antiretroviral therapy presents with tongue weakness — list tropical infections that can affect CN XII and the rationale for each. — TB meningitis, cryptococcal meningitis, syphilis; all can cause basal meningitis compressing the nerve.

When to seek help

• Progressive tongue weakness causing drooling and aspiration
• Bilateral hypoglossal palsy with airway compromise
• Rapidly expanding neck mass with nerve deficit

Quick questions

Why does the tongue deviate toward the weak side?

Unopposed pull of the intact genioglossus pushes the tongue toward the side of the lesion.

Can a stroke cause hypoglossal palsy?

Yes, a lateral medullary (Wallenberg) or ventral pontine infarct can involve the hypoglossal nucleus.

Is the hypoglossal nerve involved in taste?

No, taste on the anterior two‑thirds of the tongue is carried by the chorda tympani (CN VII).