Hypoglossal Nerve (CNXII)

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Hypoglossal Nerve (CNXII)

Main Text

TERMINOLOGY

Abbreviations

• Hypoglossal nerve (CNXII)

Synonyms

• 12th cranial nerve

Definitions

• CNXII: Motor nerve supplying intrinsic and extrinsic tongue muscles

IMAGING ANATOMY

Overview

• Motor cranial nerve to intrinsic and extrinsic tongue muscles

Only extrinsic muscle not innervated by CNXII is palatoglossus muscle (by CNX)
• Hypoglossal nerve anatomic segments

Intraaxial segment

Cisternal segment

Skull base segment

Extracranial segment

Intraaxial Segment

• Hypoglossal nucleus

In dorsal medulla between dorsal vagal nucleus and midline

Long, thin nucleus that is approximately same length as ventrolateral olive (15- to 18-mm craniocaudal dimension)

Extends from level of hypoglossal eminence (trigone) in floor of 4th ventricle just inferior to medullary striae of 4th ventricle to proximal medulla

In axial section, hypoglossal nucleus is located in dorsal medulla, medial to dorsal vagal nucleus
• Hypoglossal intraaxial axonal course

Efferent fibers from hypoglossal nucleus extend ventrally through medulla, lateral to medial lemniscus

Efferent fibers exit between olivary nucleus and pyramid (root exit zone) at ventrolateral sulcus also called preolivary sulcus

Cisternal Segment

• Efferent fibers coalesce to form multiple (6-14) rootlets

In premedullary cistern, course between posterior inferior cerebellar artery and vertebral artery
• Rootlets fuse into hypoglossal nerve (2-4 trunks) just as it exits skull base through hypoglossal canal
• Hypoglossal filaments may merge with vagal fibers
• Total length of cisternal segment ranges from 8-15 mm and mean width from 0.3-0.6 mm

Skull Base Segment

• Hypoglossal nerve exits occipital bone via hypoglossal canal, surrounded by venous plexus

Canal is in occipital bone caudal to jugular foramen

– “Empties” into medial nasopharyngeal carotid space

– Osseous septa may bisect hypoglossal canal

Mean length of hypoglossal canal reported to range from 9.5-16.0 mm and mean width from 1.3-3.0 mm

Variant anatomy of hypoglossal canal

– Osseous septa may bisect hypoglossal canal

– Rare persistent primitive hypoglossal artery arises from cervical internal carotid artery C1-C2 level and passes through hypoglossal canal into posterior fossa; anastomoses with vertebrobasilar system

Extracranial Segment

• Carotid space component of CNXII

Hypoglossal canal “empties” into medial nasopharyngeal carotid space

Hypoglossal nerve immediately gives off dural branches after exiting hypoglossal canal

Descends in posterior carotid space, closely apposed with CNX

Exits carotid space anteriorly between jugular vein and internal carotid artery, crosses lateral surface of external carotid artery at inferior margin of posterior belly of digastric muscle
• Transspatial component of CNXII

From carotid space, nerve runs anteroinferiorly toward hyoid bone, lateral to carotid bifurcation

At level of occipital artery base, nerve turns anterior, continuing as muscular branch below posterior belly of digastric muscle, medial to submandibular gland

Gives off superior root of ansa cervicalis from horizontal segment of nerve to anastomose with lower root

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Tags: Imaging Anatomy Brain and Spine

Mar 13, 2021 | Posted by admin in NUCLEAR MEDICINE | Comments Off

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