Bones of the Orbit and Extraocular Muscle Review Notes

Bones of the Orbit

• Superior Wall (Roof):     * Lesser wing of the Sphenoid bone.     * Orbital portion of the frontal bone.
• Inferior Wall (Floor):     * Orbital surfaces of the maxilla.     * Zygomatic bone.     * Palatine bone.
• Medial Wall:     * Orbital plate of the ethmoid bone.     * Body of the sphenoid bone.     * Frontal bone.     * Lacrimal bone.     * Maxilla.     * Clinical Significance: The medial wall is documented as being especially fragile. Because it contains the ethmoid bone, it represents a common site of fracture.
• Lateral Wall:     * Greater wing of the Sphenoid bone.     * Orbital plate of the frontal bone.     * Frontal process of the zygomatic bone.

Extraocular Muscles

• Muscle Functions (Action-Specific):     * Abducts the eye: Lateral rectus.     * Adducts the eye: Medial rectus.     * Depresses AND abducts the eye: Superior oblique.     * Elevate AND abducts the eye: Inferior oblique.     * Down and in motion: Inferior rectus.     * Responsible for eyelid elevation: Levator palpebrae superioris.
• Innervation of Extraocular Muscles:     * General Innervation: The majority of extraocular muscles are innervated by the Oculomotor nerve ($CN-III$).     * Local Pathway: This nerve passes through the Superior orbital fissure and the Common tendinous ring within the orbit.     * The Exceptions (Specific Innervations):         * Superior oblique ($SO_4$): Innervated by the Trochlear nerve ($CN-IV$).         * Lateral rectus ($LR_6$): Innervated by the Abducens nerve ($CN-VI$).

The Optic Canal

• Anatomical Location: The optic canal is located within the Sphenoid bone. It is specifically found at the posterior portion of the orbit.
• Contents:     * Optic nerve ($CN-II$).     * Ophthalmic artery.
• Clinical Case Study 1: Optic Nerve Compression:     * Scenario: A lesion compresses the optic nerve within the optic canal.     * Expected Deficit: Monocular vision loss (blindness in one eye).     * Explanation: The deficit affects only one eye because the compression occurs before the optic chiasm. At this point, all visual information from the optic nerve and the eye is grouped together for the single affected eye.
• Clinical Case Study 2: Visual Loss with Preserved Movement:     * Scenario: A patient presents with loss of vision in one eye but maintains normal eye movement.     * Structure Damaged: The Optic nerve ($CN-II$) is the likely site of damage.     * Explanation for Preservation: Extraocular movements are preserved because they are controlled by separate nerves, specifically $CN-III$, $CN-IV$, and $CN-VI$, which are distinct from the sensory optic nerve.

Internal Structures of the Eye

• Anterior and Posterior Segments:     * Anterior chamber of the eye.     * Posterior chamber of the eye.     * Lens.     * Zonular fibers.     * Iris.     * Ciliary body and Ciliary muscle.     * Cornea.     * Conjunctiva.
• Posterior Cavity and Retina:     * Vitreous body.     * Choroid.     * Sclera.     * Retina (neural part).     * Ora serrata (the serrated junction between the retina and the ciliary body).     * Fovea centralis.     * Optical axis.
• The Optic Disc (The Blind Spot):     * Definition: The point where ganglion (nerve) cells leave the eye to form the optic nerve.     * Reason for "Blind Spot" Designation: It does not contain any photoreceptors.     * Structures Present: The Optic nerve ($CN-II$) and retinal blood vessels.     * Functional Rationale: There are no photoreceptors at this location because this is the entry/exit point for the optic nerve cells and blood vessels.

Questions & Discussion

• Question: Which structure transmits nerves responsible for eye movement?     * Answer: Superior orbital fissure.
• Question: Which of the following is TRUE?     * Options:         * a) $CN\,II$ passes through the superior orbital fissure         * b) $CN\,III$ passes through the optic canal         * c) The optic canal is located in the sphenoid bone         * d) The superior orbital fissure transmits only one nerve     * Selected Answer: c) The optic canal is located in the sphenoid bone.
• Question: Which structure transmits only sensory visual information?     * Answer: Optic canal.
• Question: All of the following muscles are innervated by the oculomotor nerve ($CN\,III$) EXCEPT:     * Options:         * a) Inferior oblique         * b) Superior rectus         * c) Lateral rectus         * d) Medial rectus     * Selected Answer: c) Lateral rectus (Exception: $LR_6$).