Hypoglossal canal

Where is the hypoglossal canal located?

The canal lies within the occipital bone, specifically in the thickness of its lateral parts (Partes laterales).

• From the internal cranial base, it opens into the posterior cranial fossa (Fossa cranii posterior). This fossa is bounded anteriorly by the dorsum sellae (Dorsum sellae) and the superior border of the petrous part of the temporal bone (Margo superior partis petrosae), and posteriorly by the cruciate eminence (Eminentia cruciformis) and the sulcus for the transverse sinus (Sulcus sinus transversi).
• Externally, the canal is visible on the external cranial base proper (Basis cranii externa).

Topography and adjacent structures: On the intracranial surface of the lateral parts of the occipital bone are the sigmoid sinus sulcus (Sulcus sinus sigmoidei), the hypoglossal canal, and the jugular tubercle (Tuberculum jugulare). The lateral parts bear the occipital condyles (Condylus occipitalis), posterior to which a condylar fossa with an inconstant condylar canal (Canalis condylaris) may be located.

What passes through the canal?

The contents of the canal are the hypoglossal nerve (Nervus hypoglossus, XII cranial nerve). This is a somatic efferent, motor nerve.

• The nucleus of this nerve (nucleus nervi hypoglossi) lies in the lower medulla oblongata, near the floor of the fourth ventricle, and is evolutionarily analogous to the anterior horns of the spinal cord.
• Axons descend through the medulla oblongata and emerge as rootlets in the anterolateral sulcus, between the inferior olive and pyramid, after which they enter the hypoglossal canal.
• After leaving the cranial cavity through the canal, the nerve trunk lies in the neck in close proximity to the vagus nerve (X pair), passing in the lower cervical region between the jugular vein and the carotid artery.

Innervation zones and connections of the exiting nerve

The intrinsic fibers of the XII pair provide motor innervation to the muscles of the tongue. These include:

• Styloglossus muscle
• Hyoglossus muscle
• Genioglossus muscle (m. genioglossus) — responsible for protruding the tongue forward and downward.

Reflex pathways from the reticular formation, the nucleus of the solitary tract, and the trigeminal nerve nuclei approach the nerve nucleus. They provide reflex movements of the tongue during swallowing, chewing, sucking, and licking.

Ansa cervicalis (ansa cervicalis): Transit fibers from the upper cervical segments of the spinal cord (C1–C3) temporarily join the course of the hypoglossal nerve. They do not form connections with the hypoglossal nerve itself and soon separate. Branches of the ansa cervicalis innervate the neck muscles located below the hyoid bone:

• Thyrohyoid muscle (m. thyrohyoideus)
• Sternothyroid muscle (m. sternothyroideus)
• Sternohyoid muscle
• Omohyoid muscle (m. omohyoideus)

Clinical significance and nerve palsy

To test the function of the nerve, the doctor evaluates speech articulation, examines the tongue in the oral cavity and when protruded past the teeth, paying attention to deviation, muscle atrophy, and fasciculations.

• With an unilateral lesion of the nerve itself or its nucleus, dysarthria and atrophy of the ipsilateral half of the tongue occur. When protruded, the tongue deviates towards the lesion — toward the paretic muscles. This is because when the genioglossus muscle is weak on one side, the healthy muscle on the opposite side overpowers it and pushes the tongue toward the affected side.
• With an unilateral supranuclear lesion of the corticonuclear tract, pronounced impairment of tongue mobility does not occur. This is explained by the fact that the muscles of both halves of the tongue function as a single unit, and the nucleus receives bilateral cortical innervation: predominantly from the opposite hemisphere, but some fibers also come from its own side.

Class and exam questions

What else to study on this topic

• Occipital bone and its parts
• Posterior cranial fossa
• Hypoglossal nerve (XII CN)
• External cranial base

Other topics in the "Anatomical Structures" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.