Location of the Hypoglossal Canal
The canal lies within the occipital bone, specifically in the thickness of its lateral parts (Partes laterales).
- On the internal skull 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 opens onto the external skull base (Basis cranii externa).
Topography and Neighboring Structures: On the intracranial surface of the lateral parts of the occipital bone are the sigmoid 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 content of the canal is the hypoglossal nerve (Nervus hypoglossus, cranial nerve XII). It 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 evolutionarily corresponds 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 the pyramid, before entering the hypoglossal canal.
- After leaving the cranial cavity through the canal, the nerve trunk lies in the neck immediately adjacent to the vagus nerve (CN X), running in the lower neck between the internal jugular vein and the internal carotid artery.
Innervation Zones and Connections of the Emerging Nerve
The intrinsic fibers of CN XII provide motor innervation to the muscles of the tongue. These include:
- Styloglossus muscle
- Hyoglossus muscle
- Genioglossus muscle (m. genioglossus) — responsible for protruding the tongue anteriorly and inferiorly.
Reflex pathways from the reticular formation, the solitary tract nucleus, and the trigeminal sensory nuclei project to the hypoglossal nucleus. These coordinate reflex tongue movements during swallowing, chewing, suckling, and licking.
Ansa cervicalis: Transient fibers from the upper cervical spinal segments (C1–C3) temporarily join the course of the hypoglossal nerve. They do not form true functional connections with the hypoglossal nerve itself and soon branch off. The branches of the ansa cervicalis innervate the infrahyoid muscles of the neck:
- Thyrohyoid muscle (m. thyrohyoideus)
- Sternothyroid muscle (m. sternothyroideus)
- Sternohyoid muscle
- Omohyoid muscle (m. omohyoideus)
Clinical Significance and Nerve Lesions
To assess nerve function, clinicians evaluate speech articulation, inspect the tongue inside the mouth, and observe it upon protrusion past the dental arch, checking for deviation, muscle atrophy, and fasciculations.
- In an unilateral lesion of the nerve itself or its nucleus, dysarthria and atrophy of the ipsilateral half of the tongue occur. Upon protrusion, the tongue deviates toward the side of the lesion (toward the paretic muscles). This occurs because the unopposed action of the healthy contralateral genioglossus muscle pushes the tongue over to the weak side.
- In an unilateral supranuclear lesion of the corticobulbar tract, marked tongue mobility deficits do not typically occur. This is because the muscles of both halves of the tongue function as a single unit, and the nucleus receives bilateral cortical innervation: predominantly from the contralateral hemisphere, with some fibers also contributing from the ipsilateral side.