Accessory Nerve (CNXI)

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Accessory Nerve (CNXI)

Main Text

TERMINOLOGY

Abbreviations

• Accessory nerve (CNXI)

Synonyms

• 11th cranial nerve (CN)

Definitions

• CNXI: Pure motor CN, supplying sternocleidomastoid, trapezius muscles (through spinal component) and palatal, pharyngeal, laryngeal muscles (through cranial component)
• Comment: There is some controversy regarding fundamental anatomy of spinal accessory nerve; recently, some authors have argued that nerves that arise from caudal aspect of postolivary sulcus of medulla have variable interaction with adjacent CNs; these fibers may join vagus nerve directly

In some cases, these fibers remain as distinct nerves in cistern, ultimately merging with vagus nerve in jugular foramen or just below it to supply palate, larynx, and pharynx; in this circumstance, bulbar (cranial) components never actually merge with spinal component of CNXI

IMAGING ANATOMY

Overview

• Motor cranial nerve only
• 4 CNXI segments are defined

Intraaxial, cisternal, skull base, and extracranial

Intraaxial Segment

• 2 distinct nuclear origins

Bulbar (cranial) motor fibers originate in lower nucleus ambiguus (medulla)

– Fibers course anterolaterally to exit lateral medulla in postolivary sulcus inferior to CNIX and CNX

Spinal motor fibers originate from spinal nucleus of accessory nerve (spinal cord)

– Narrow column of cells along lateral aspect of anterior horn cells from C1 to C5

– Nerve fibers emerge from lateral aspect of cervical spinal cord between anterior and posterior roots

– Fibers combine forming bundle that ascends entering skull base via foramen magnum

Cisternal Segment

• Bulbar portion travels anterolaterally through basal cistern along similar course as CNIX and CNX
• Spinal portion enters lower lateral basal cistern, exits thorough jugular foramen (JF)
• Bulbar root joins spinal component of accessory nerve either in lower cistern or within JF

Skull Base Segment

• Passes through posterior pars vascularis of JF

Pars vascularis also contains vagus nerve (CNX) and jugular bulb (and Arnold nerve)

Pars nervosa transmits glossopharyngeal nerve (CNIX), Jacobson nerve, and inferior petrosal sinus
• Bulbar and spinal portions remain together in JF

Extracranial Segment

• Fibers from bulbar portion, which arise within nucleus ambiguus, separate from main nerve and merge with vagus nerve in carotid space

Travel via CNX to supply muscles of palate, pharynx, and larynx

– Palate : Levator veli palatini, palatoglossus, palatopharyngeus, and musculus uvulae

– Pharynx : Superior constrictor and soft palate via pharyngeal plexus

– Larynx : Except cricothyroid muscle via recurrent laryngeal nerve

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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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