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Oculomotor nerve (inferior branch)

The oculomotor nerve (inferior branch) is a crucial component of the third cranial nerve (CN III), responsible for controlling several muscles that move the eyeball. Originating from the oculomotor nucleus in the midbrain, this nerve divides into superior and inferior branches as it enters the orbit via the superior orbital fissure. The inferior branch is primarily responsible for innervating the medial rectus, inferior rectus, and inferior oblique muscles, and it also carries parasympathetic fibers to the ciliary ganglion, influencing pupil constriction and lens accommodation.

Synonyms

  • Inferior division of oculomotor nerve

  • Inferior ramus of cranial nerve III

  • Oculomotor nerve (CN III), inferior branch

Function

  • Motor innervation: Supplies the medial rectus, inferior rectus, and inferior oblique extraocular muscles

  • Parasympathetic function: Delivers preganglionic fibers to the ciliary ganglion, which controls pupil constriction (sphincter pupillae) and lens shape (ciliary muscle) for accommodation

  • Eye movement: Facilitates adduction (medial movement), depression, and extorsion (outward rotation) of the eye

  • Eyelid elevation: Although mainly a function of the superior branch (levator palpebrae superioris), the inferior branch plays an indirect role via ciliary muscle tone

MRI Appearance

  • T1-weighted images:

    • The oculomotor nerve (inferior branch) appears as a small, isointense or slightly hypointense linear structure within the orbit, best visualized with thin slices and high-resolution imaging.

  • T2-weighted images:

    • The nerve typically presents as a low signal (hypointense) structure surrounded by the higher signal (hyperintense) orbital fat. The nerve stands out against the bright background of orbital fat.

  • STIR (Short Tau Inversion Recovery) images:

    • The nerve maintains a low signal, while the surrounding fat is suppressed (appears dark), which may enhance the conspicuity of the nerve if edema or pathology is present (will appear hyperintense).

CT Appearance

  • The oculomotor nerve (inferior branch) is generally not directly visualized on routine CT due to its small size and soft tissue density.

MRI images

Oculomotor nerve (inferior branch) MRI image