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Hypoglossal Nerve (Cranial nerve XII)

The hypoglossal nerve is the twelfth cranial nerve (CN XII), responsible primarily for motor innervation to the muscles of the tongue. It plays a critical role in voluntary tongue movements essential for speech, swallowing, and manipulation of food within the mouth. This nerve arises from the hypoglossal nucleus in the medulla oblongata, exits the skull via the hypoglossal canal, and travels down to supply intrinsic and extrinsic muscles of the tongue. Understanding its anatomy, imaging appearances, and clinical significance is crucial for diagnosing various neurological and head & neck disorders.

Synonyms

  • CN XII

  • 12th cranial nerve

  • Nervus hypoglossus

Function

  • Supplies motor innervation to all intrinsic and most extrinsic muscles of the tongue (except palatoglossus).

  • Facilitates tongue movements necessary for speech, swallowing, and mastication.

  • Assists in clearing oral cavity, articulation, and manipulation of food.

MRI Appearance

  • T1-weighted MRI:

    • The hypoglossal nerve appears as a small, low-signal (dark) linear structure running anterolaterally from the medulla oblongata.

    • It is best seen as it exits the hypoglossal canal, sometimes aided by contrast enhancement in pathological cases.

    • Difficult to visualize due to its small size, but the hypoglossal canal can be seen as a well-defined bony canal at the skull base.

  • 3D T2-weighted MRI (such as CISS or FIESTA sequences):

    • The nerve appears as a small, hypointense (dark) linear structure against the hyperintense (bright) CSF background.

    • 3D T2 sequences improve visualization of the nerve within the cisternal segment and as it exits the hypoglossal canal.

    • Allows differentiation from adjacent vessels and other cranial nerves.

CT Appearance

  • The hypoglossal nerve itself is not directly visualized due to its small size and similar density to soft tissue.

  • Key features:

    • The hypoglossal canal is visualized as a distinct, well-corticated bony channel in the occipital bone, lateral to the foramen magnum.

    • Pathological processes (fractures, masses, or bone erosion) involving the canal may indirectly indicate hypoglossal nerve involvement.

MRI images

Hypoglossal nerve (Cranial nerve XII) 3T MRI axial image 1

MRI images

Hypoglossal nerve (Cranial nerve XII) 3T MRI axial image

MRI images

Hypoglossal nerve (Cranial nerve XII) 3T MRI coronal image