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Cranial nerve 3

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Cranial Nerve III - The Oculomotor Nerve

In-Situ Anatomy (Real Image)

This image from Neuroanatomy through Clinical Cases shows CN III viewed from above with the forebrain removed, demonstrating its relationship to surrounding structures:
Oculomotor Nerve CN III in situ - showing nuclei, exiting fascicles, relationship to posterior communicating artery, cavernous sinus, and cerebral peduncle
Fig. 13.2 - Oculomotor Nerves (CN III) In Situ. View from above with the forebrain removed, showing the oculomotor nuclei in the rostral midbrain and CN III in relation to adjacent structures. - Neuroanatomy through Clinical Cases, 3rd Ed.

Orbital Course (Real Image)

This image from Gray's Anatomy for Students shows the CN III branches inside the orbit:
CN III orbital muscles - Superior oblique, Medial rectus, Superior rectus, Levator palpebrae superioris as seen in the orbit
Fig. 8.106 - CN III branches in the orbit. - Gray's Anatomy for Students

Key Facts: CN III (Oculomotor Nerve)

Fiber Types

  • General Somatic Efferent (GSE) - innervates most extraocular muscles
  • General Visceral Efferent (GVE) - preganglionic parasympathetic fibers

Course

  1. Origin: Nucleus in the rostral midbrain at the level of the superior colliculus
  2. Exits: Anterior surface of the brainstem (between midbrain and pons), as exiting fascicles
  3. Subarachnoid space: Passes between the posterior cerebral artery (above) and superior cerebellar artery (below); close relationship to the posterior communicating artery - the parasympathetic fibers run superficially here, making them vulnerable to aneurysm compression
  4. Cavernous sinus: Travels in the lateral wall
  5. Entry to orbit: Through the superior orbital fissure, within the common tendinous ring, dividing into superior and inferior branches

Muscles Innervated

BranchMuscles
SuperiorSuperior rectus, Levator palpebrae superioris
InferiorMedial rectus, Inferior rectus, Inferior oblique
Parasympathetic (via ciliary ganglion)Sphincter pupillae (pupil constriction), Ciliary muscle (accommodation)

Edinger-Westphal Nucleus

The parasympathetic preganglionic fibers arise from the Edinger-Westphal nucleus (forms a V-shape over the dorsal oculomotor nucleus). These fibers run in the superficial and medial portion of CN III in the subarachnoid space - directly at risk from posterior communicating artery aneurysms ("blown pupil").

CN III Palsy - Classic Signs

  • Ptosis (levator palpebrae superioris failure)
  • "Down and out" eye position (unopposed CN IV + CN VI)
  • Mydriasis (dilated, unreactive pupil) - if parasympathetics involved
  • Surgical CN III palsy (e.g., PComm aneurysm) = pupil involved
  • Medical CN III palsy (e.g., diabetes) = pupil spared (ischemia spares superficial parasympathetic fibers)
Sources: Gray's Anatomy for Students; Neuroanatomy through Clinical Cases, 3rd Ed.; Gray's Anatomy for Students, p. 1083
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