Overview
This lecture covers the anatomy and functions of the oculomotor (III), trochlear (IV), and abducent (VI) nerves as they supply muscles within the orbit.
Extraocular Muscles and Their Innervation
- Seven muscles in the orbit: four rectus, two oblique, and the levator palpebrae superioris.
- Oculomotor nerve (III) supplies five muscles: superior rectus, medial rectus, inferior rectus, inferior oblique, and levator palpebrae superioris.
- Trochlear nerve (IV) supplies only the superior oblique muscle.
- Abducent nerve (VI) supplies only the lateral rectus muscle.
Anatomy and Branching of the Oculomotor Nerve
- Oculomotor nerve enters the orbit and divides into upper and lower branches.
- Upper branch supplies levator palpebrae superioris and superior rectus.
- Lower branch supplies medial rectus, inferior rectus, and inferior oblique.
- The inferior oblique muscle lies beneath the eyeball and is not visible in standard views.
Autonomic Function of the Oculomotor Nerve
- Oculomotor nerve, via short ciliary nerves, provides autonomic fibers to intra-ocular muscles.
- These autonomic fibers control pupil size and lens shape for focusing.
Trochlear and Abducent Nerve Pathways
- Trochlear nerve (IV) innervates the superior oblique muscle, which uses the trochlea (pulley).
- Abducent nerve (VI) innervates the lateral rectus muscle, responsible for abducting the eye.
Key Terms & Definitions
- Oculomotor nerve (III) — Cranial nerve supplying most extraocular muscles and some intra-ocular muscles.
- Trochlear nerve (IV) — Cranial nerve supplying the superior oblique muscle.
- Abducent nerve (VI) — Cranial nerve supplying the lateral rectus muscle.
- Rectus muscles — Four straight muscles (superior, inferior, medial, lateral) around the eyeball.
- Oblique muscles — Two angled muscles (superior and inferior) in the orbit.
- Levator palpebrae superioris — Muscle that lifts the upper eyelid.
- Short ciliary nerves — Nerves carrying autonomic fibers to the eye's internal muscles.
Action Items / Next Steps
- Review the anatomy and function of the trigeminal nerve (V) branches in the orbit for the next lesson.
- Study the pathways and muscle innervations of cranial nerves III, IV, and VI for diagram-based questions.