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Cranial nerves · Motor

XII. Hypoglossal nerve

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The hypoglossal nerve (CN XII) is the motor nerve of the tongue, supplying all intrinsic muscles and all extrinsic muscles except palatoglossus (CN X). It also carries C1 fibres that travel with it to the geniohyoid and thyrohyoid.

Terminologia Anatomica (Latin): Nervus hypoglossus

Function

General somatic efferent to the muscles of the tongue (genioglossus, hyoglossus, styloglossus and the intrinsic muscles). Genioglossus protrudes the tongue, so its action on protrusion is the basis of the bedside test.

Nuclei

Hypoglossal nucleus in the medulla, in the floor of the fourth ventricle near the midline (hypoglossal trigone). Corticonuclear input is predominantly crossed for genioglossus, so a supranuclear lesion causes tongue deviation away from the side of the cortical lesion.

Exit from the skull

Rootlets emerge between the pyramid and the olive in the medulla, unite and leave the skull through the hypoglossal canal of the occipital bone. The nerve then descends between the internal carotid artery and the internal jugular vein, loops forward above the hyoid bone and enters the tongue.

Testing

Inspect the tongue at rest for wasting and fasciculations, then ask the patient to protrude the tongue and observe for deviation, and to push the tongue against each cheek against resistance. Assess speech and swallowing.

Signs of a lesion

In a lower motor neuron lesion the tongue deviates toward the side of the lesion on protrusion (the unopposed healthy genioglossus pushes it toward the weak side), with ipsilateral wasting and fasciculations. In a supranuclear lesion there is no wasting and the tongue deviates away from the cortical lesion, toward the weak side of the body.

Clinical correlations

Causes of hypoglossal palsy include carotid dissection or surgery, skull base tumours and metastases, neck trauma or infection, and brainstem stroke (for example medial medullary syndrome, with contralateral hemiparesis). Bilateral lower motor neuron lesions in motor neuron disease cause dysarthria and dysphagia with a wasted, fasciculating tongue.

Common exam points

  • Supplies all tongue muscles except palatoglossus (CN X).
  • Leaves through the hypoglossal canal.
  • Tongue deviates toward the lesion in a lower motor neuron lesion.
  • Wasting and fasciculations indicate a lower motor neuron lesion.
  • Supranuclear lesion: no wasting; deviation away from the cortical lesion.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)
  4. Bickley LS. Bates' Guide to Physical Examination and History Taking (Bickley). Wolters Kluwer, 13th edition (2020) — Tier 3 (textbook)

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