Topic: Anatomy of cranial nerves
Question: Describe the extrinsic and intrinsic muscles of eyeball, along with their nerve supply and actions.
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The eyeball muscles are of two types – extrinsic muscles and intrinsic muscles.
Extrinsic eye muscles: These muscles are attached to the eyeball and control the movement of the eye. There are six extrinsic muscles:
- Superior rectus: Innervated by Cranial Nerve III. Elevates the eye and adducts it.
- Inferior rectus: Innervated by Cranial Nerve III. Depresses the eye and adducts it.
- Medial rectus: Innervated by Cranial Nerve III. Adducts the eye.
- Lateral rectus: Innervated by Cranial Nerve VI. Abducts the eye.
- Superior oblique: Innervated by Cranial Nerve IV. Depresses and abducts the eye.
- Inferior oblique: Innervated by Cranial Nerve III. Elevates and abducts the eye.
Intrinsic eye muscles: These muscles control the shape of the lens and size of the pupil. They include:
- Ciliary muscle: Innervated by parasympathetic fibers. Contracts to adjust the lens for near vision. Relaxes for distance vision.
- Sphincter pupillae: Innervated by parasympathetic fibers. Contracts to constrict the pupil.
- Dilator pupillae: Innervated by sympathetic fibers. Contracts to dilate the pupil.
The extrinsic muscles control the movement and position of the eyeball, while the intrinsic muscles control accommodation for vision and pupil size regulation. The nerves that control these muscles are the oculomotor (III), trochlear (IV) and abducens (VI) nerves for extrinsic muscles and parasympathetic and sympathetic nerves for the intrinsic muscles.
