Overview
This lecture explains the structure, function, and clinical relevance of the oculomotor nerve in eye movement and related symptoms of its damage.
Oculomotor Nerve Anatomy & Function
- The oculomotor nerve supplies four of six extraocular muscles: medial rectus, superior rectus, inferior rectus, and inferior oblique.
- The medial rectus moves the eye towards the nose.
- The superior rectus lifts the eye upward.
- The inferior rectus lowers the eye downward.
- The inferior oblique moves the eye up and out.
- The nerve also supplies the levator palpebrae superioris, which elevates the eyelid.
- It connects to the ciliary ganglion to control the pupillary sphincter (pupil constriction) and ciliary muscle (lens focus for near vision).
- Oculomotor nerve fibers originate from the oculomotor nucleus in the midbrain.
- Fibers travel to the orbit, separating into branches for the different muscles.
- Fibers for pupillary constriction and lens focusing originate in the Edinger-Westphal nucleus and travel with the oculomotor nerve.
Effects of Oculomotor Nerve Damage
- Damage to the oculomotor nerve causes deficits in the same side (ipsilateral) eye.
- The affected eye may deviate outward and downward due to unopposed action of other muscles.
- Medial (towards the nose) and vertical (up and down) eye movements are impaired.
- Double vision (diplopia) is a common symptom.
- Drooping eyelid (ptosis) occurs due to paralysis of the levator palpebrae superioris.
- Dilated pupil (mydriasis) can occur because the pupillary constrictor is affected.
- Difficulty focusing on close objects may result from paralysis of the ciliary muscle.
Key Terms & Definitions
- Oculomotor nerve — Cranial nerve (CN III) that controls most eye movements, eyelid elevation, and pupil/lens functions.
- Levator palpebrae superioris — Muscle that lifts the upper eyelid.
- Ciliary ganglion — Group of neurons involved in pupil constriction and lens focusing.
- Edinger-Westphal nucleus — Midbrain nucleus whose fibers control pupil and lens adjustments.
- Diplopia — Double vision.
- Ptosis — Drooping of the upper eyelid.
- Mydriasis — Abnormal dilation of the pupil.
Action Items / Next Steps
- Review drawings or diagrams of the oculomotor nerve pathways and the muscles it innervates.