Intraoperative anatomy of the hypoglossal nerve

(A) Course of the hypoglossal nerve. CN XII arises from the hypoglossal nucleus in medulla oblongata, leaves the brainstem via the anterolateral sulcus, exits the cranium via the hypoglossal canal, then descends into the retrostyloid space and passes over the external carotid artery. It courses deep to the submandibular gland, mylohyoid muscle, and the posterior belly and tendon of the digastric muscle. It runs superficial to the hyoglossus muscle, and it arborizes into terminal branches at the anterior edge of the muscle. The extrinsic muscles include the following:

• Genioglossus horizontal and the genioglossus oblique muscles, which are innervated by the medial division of CN XII. The genioglossus horizontal muscle pulls the tongue forward while the genioglossus oblique pulls the tongue down.
• Styloglossus and hyoglossus muscles, which are innervated by the lateral division of CN XII. The hyoglossus retracts and depresses the tongue while the styloglossus it retracts and elevates the tongue.

Intrinsic muscles include the following:

• Inferior longitudinal medial (m-IL) muscle, which is innervated by the medial division of CN XII and shortens the tongue and curls the tip inferiorly.
• Transverse (T) muscle which is innervated by the medial division of CN XII and narrows the tongue.
• Vertical (V) muscle, which is innervated by the medial division of CN XII and flattens the tongue.
• Superior longitudinal (SL), which is innervated by the lateral division of CN XII and shortens the tongue and curls the tip superiorly.
• Inferior longitudinal lateral (l-IL) muscle, which is innervated by the lateral division of CN XII and shortens the tongue and curls the tip inferiorly

CN XII is joined by cervical 1/cervical 2 (C1/C2) rootlets extracranially. These innervate the geniohyoid and thyrohyoid muscles. The course of these fibers is variable.

Accompanying the CN XII is the ranine vein, with the dominant vessel typically found inferior to the nerve and the smaller branch of the vein is found superiorly. Crossing branches can frequently be found overlying the nerve.

(B) Intraoperative anatomy of the hypoglossal nerve. This image depicts the lateral division of the hypoglossal nerve innervating the styloglossus/hyoglossus muscle (l-XII SG/HG). The medial division of the nerve is also shown innervating the genioglossus horizontal (m-XII GGh) and genioglossus oblique (m-XII GGo) muscles. Their relationship to cervical nerve 1 (C1) and the ranine vein are also shown.