Cranial Nerves Lecture: Oculomotor Nerve

Introduction

• 12 pairs of cranial nerves arising from the brain.
• Cranial nerve nuclei: Motor and Sensory types.
  • Motor nuclei classified into:
    • Somatic efferent: Supplying skeletal muscles.
    • Special visceral efferent: Supplying skeletal muscles derived from pharyngeal arches.
    • General visceral efferent: Supplying glands, smooth muscles of visceral organs, and cardiac muscles.
  • Sensory nuclei classified into:
    • General visceral afferent: Carry general sensations from the viscera (e.g., pain, reflex sensations).
    • General somatic afferent: Carry general sensations from the body wall (touch, pain, temperature).
    • Special somatic afferent: Carry senses like smell, vision, auditory, vestibular, and taste.

Third Cranial Nerve: Oculomotor Nerve

• Function: Responsible for most eye movements, pupil constriction, and accommodation reflexes.
• Identification:
  • Arises from the interpeduncular fossa.
• Functional Components:
  • General somatic efferent: Supplies extraocular muscles (excluding three).
  • General visceral efferent: Parasympathetic fibers to glands and muscles via Edinger-Westphal nucleus.
• Nuclear Complex:
  • Located around the Aqueduct of Sylvius.
  • Contains motor nucleus and Edinger-Westphal nucleus.

Course of Oculomotor Nerve

• Intraneural Course:
  • Inside midbrain, crosses red nucleus and substantia nigra.
  • Exits ventrally from midbrain.
• Intracranial Course:
  • Lies between posterior cerebral artery and superior cerebellar artery.
  • Enters the cavernous sinus and superior orbital fissure.
  • Divides into superior and inferior rami.
• Intraorbital Course:
  • Superior ramus: Supplies superior rectus and levator palpebrae superioris.
  • Inferior ramus: Supplies medial rectus, inferior rectus, and inferior oblique.

Ciliary Ganglion

• Parasympathetic ganglion located at the posterior orbit.
• Pathway:
  • Edinger-Westphal nucleus → oculomotor nerve → ciliary ganglion → short ciliary nerves → ciliary muscle and sphincter pupillae muscle.

Clinical Relevance

• Lesions:
  • Unilateral lesions can cause:
    • Ptosis (drooping eyelid due to levator palpebrae superioris paralysis).
    • External strabismus (squint due to unopposed lateral rectus and superior oblique muscles).
    • Diplopia (double vision).
    • Loss of accommodation.
    • Dilated and fixed pupil.