3A Anatomy Challenge 2026: Sight

Anatomy Challenge 2026

Theme: Perceptual Prism

Session 3A: Sight

Content Overview

1. Anatomy/Histology
      1. The Orbit
      2. The Eyeball/Globe
      3. The Adnexa
         i) Extraocular Muscles
         ii) Eyelid Muscles
         iii) Lacrimal System and Conjunctiva

2. Neuro-Ophthalmology
      1. Cranial Nerves
      2. Visual Pathway
      3. Visual Reflexes

3. Embryology

Anatomy of the Eye

The Orbit, Eyeball, and Adnexa

The Orbit

Margins, Walls, and Openings

The Orbit Margins

• Superior Margin: Frontal bone

• Medial Margin: Maxilla (frontal process)

• Inferior Margin: Maxilla, Zygomatic bone

• Lateral Margin: Zygomatic bone (frontal process), Frontal bone (zygomatic process)

Orbit Description

• A bony cavity (one for each eye)

• Contains eyes, muscles, vessels, nerves, fat, lacrimal system

• Apex: At the back

• Base: At the front

Structures Involved in the Orbit

• Frontal Bone

• Orbital Septum

• Orbicularis Oculi

• Superior Tarsus

• Conjunctiva

• Maxilla

• Inferior Oblique

• Tendon of Levator Palpebrae Superioris

• Transverse Ligament of Whitnall

• Tendon of Superior Oblique

• Orbital Fat and Fascial Septum

• Superior Rectus

• Levator Palpebrae Superioris

• Suspensory Ligament of Lockwood

• Inferior Rectus

• Optic Nerve

• Dura Mater

Orientation of the Orbit

• Axis of orbit marked

• Medial and lateral orbital walls included with angle measurements at 90°.

The Orbit Walls

• Superior (Roof): Frontal bone, Lesser sphenoid wing

• Medial Wall:
    - Maxilla
    - Lacrimal bone
    - Ethmoid bone
    - Sphenoid bone

• Inferior (Floor): Zygomatic, Palatine bones, Maxilla

• Lateral Wall: Zygomatic bone, Greater sphenoid wing

Major Openings in the Orbit

• Optic Canal

• Superior Orbital Fissure

• Inferior Orbital Fissure

• Infraorbital Foramen + Groove

• Ethmoidal Foramina

• Nasolacrimal Duct

Orbital Foramina Details

The Eyeball

Ant. and Post. Segments

The Eyeball/Globe

Layers of the Eyeball

1. Outer Layer:

• Sclera (posterior 5/6)

• Cornea (anterior 1/6)

1. Intermediate Layer:

• Choroid, stroma of ciliary body and iris

1. Inner Layer:

• Retina, epithelium of ciliary body and iris

Segmentation of Eyeball

• Anterior Segment: Everything in front of lens

• Posterior Segment: Everything behind lens

Cornea - Detailed Structure

Epithelial Layer

• Non-keratinized, stratified squamous epithelium

• High regeneration capacity, sensitive to touch

• Origin: derived from surface ectoderm

• Function: tight junctions create a barrier against infection

Bowman's Layer

• An acellular layer of dense collagen

• No regeneration capacity; serves structural support

Stroma

• Thickest layer, transparent

• Composed of approximately 80% water, regular collagen arrangement

• Contains flattened fibroblasts and sulfated glycosaminoglycans in ground substance

Descemet's Membrane

• Thick, elastic basement membrane supporting endothelium

Endothelium

• Simple squamous epithelium

• Limited regeneration capacity

• Maintains corneal dehydration through active ion transport

Functions of the Cornea

1. Light Refraction
      - Refracts majority of light, contributing 45 Diopters at the cornea-air interface, remaining 15 Diopters by lens.
      - High refractive index and convex surface contribute to this refraction.

2. Light Transmission
      - Maintains transparency (unlike sclera) due to regular collagen arrangement
      - Corneal dehydration maintained by tear evaporation and active ion transport.

3. Protection
      - Corneal blink reflex triggered by touch:
        - Afferent Pathway: sensory branch detected through Ophthalmic nerve (CN V1)
        - Efferent Pathway: motor signals from Facial nerve (CN 7) stimulate the Orbicularis Oculi causing eyelid closure.

Iris - Overview

• Heavily pigmented, gives eye its color

• Contains antagonistic smooth muscles:
    - Outer: Longitudinal dilator pupillae
    - Inner: Circular sphincter pupillae responsible for pupil constriction and dilation.

Lens - Characteristics and Structure

• Features: Elastic, biconvex shape, avascular, transparent, crystalline
     - Functionally, the lens has a refractive power of 15 Diopters
     - Suspended by zonular fibers from ciliary body

• Structure:
     - Lens capsule acts as a basement membrane
     - Anterior epithelium - simple cuboidal
     - Lens fibers are tightly packed into cortex and nucleus
     - Transparency maintained by dehydrated state and regular lens fiber arrangement

Aqueous Humor - Production and Drainage

• Fluid secreted by ciliary body flows from posterior to anterior chamber

Pathways of Aqueous Humor Drainage

1. Trabecular Network:
      - Schlemm's Canal → Episcleral vein → Superior optic vein → Cavernous sinus

2. Uveoscleral Outflow Pathway:
      - Ciliary body → Suprachoroidal space

Regulation of Intraocular Pressure

• Options include decreasing production or increasing outflow

Retina - Layers

• Ten layers described from inner limiting membrane to retinal pigment epithelium (RPE):
    1. Inner Limiting Membrane (Neatest vitreous)
    2. Retinal Nerve Fiber Layer (RNFL) → Axons of retinal ganglion cells (RGCs)
    3. Ganglion Cell Layer → Cell bodies of RGCs
    4. Inner Plexiform Layer → Synaptic connections
    5. Inner Nuclear Layer → Contains bipolar cells, etc.
    6. Outer Plexiform Layer → Synaptic connections
    7. Outer Nuclear Layer → Cell bodies of rods and cones
    8. Outer Limiting Membrane → Apical boundary of Muller cells
    9. Photoreceptor Layer → Contains rods and cones
    10. Retinal Pigment Epithelium (RPE) → Simple cuboidal epithelium adjacent to Bruch's membrane of choroid absorbing light

Retinal Cells and Pathways

• Vertical (Radial) Pathway:
    - Photoreceptors → Receive and transduce light
    - Bipolar Cells → Transmit information to ganglion cells
    - RGCs → Project information to the brainstem and thalamus via optic nerve

• Horizontal (Tangential) Pathway:
    - Horizontal cells → Modify input from photoreceptors → Outer Plexiform Layer (OPL)
    - Amacrine cells → Modulate input from bipolar cells → Inner Plexiform Layer (IPL)

Photoreceptors in Retina

• Rods:
    - Long, cylindrical shape; responsible for dim light, black & white night vision
    - Absent in the fovea; many rods connect to one bipolar cell.

• Cones:
    - Long, conical shape, concentrated in the fovea, responsible for bright light, detailed color vision
    - Each cone connects to a single bipolar cell.

• Photopigments:
    - Rhodopsin in rods
    - Iodopsin in cones with three subtypes (R, G, B) corresponding to red, green, and blue light detection

Retina - Fundus

Key Landmarks

• Optic Disc (Nasal): Site where axons of RGCs exit the eye via optic nerve causing the blind spot.

• Macula (Temporal): Area with the highest concentration of photoreceptors; features the fovea as a central depression.

• Retinal Blood Vessels: Arising from Central Retinal Artery (CRA), supplying the inner 2/3 of the retina.

Anterior/Posterior Chamber

• Anterior chamber contains the lens, iris, zonular fibers, and ciliary body.

• Posterior chamber includes structures such as the retina, vitreous body, and choroid.

Ocular Anatomy Overview

The Eye Orientation

• Right eye representation including optic disc location and axons of RGCs.

• RNFL extending to the optic disc and architecture of the retinal layers demonstrated.

Circulatory Supply to the Eye

• Ophthalmic Artery stemming from Internal Carotid Artery (ICA) moving into additional branches:
     - Ocular, Orbital, Extraorbital Branch, and Central Retinal Artery
     - Including Ciliary arteries providing blood supply to choroid, iris, and ciliary body.

Venous Drainage Patterns

• The central retinal vein serves the inner drainage, spreading into superior and inferior ophthalmic veins, leading to the cavernous sinus for potential retrograde infections.

Color Blindness

• Etymology for terms related:
    - Tritan: 3rd (lowest wavelength)
    - Deuter: 2nd (middle wavelength)
    - Protan: 1st (largest wavelength)
    - Anomaly: indicating fault
    - Anopia: inability to see specific colors
  

Types of Anomalous Vision:

• Anomalous Trichromatism: Shifted peak responses to colors due to cone malfunctioning.
    - Tritanomaly: Malfunction S-cones (blue)
    - Deuteranomaly: Common condition shifting green-cone peak towards red
    - Protanomaly: Malfunctioning L-cones (red)

• Dichromatism: Loss of one cone type.
    - Tritanopia: Missing S-cone
    - Deuteranopia: Missing M-cone
    - Protanopia: Missing L-cone

• Monochromatism includes options of:
    - Rod Monochromatism: only rods present
    - Blue Cone Monochromatism: only blue cones present

The Adnexa

Extraocular Muscles (EOMs)

Origin and Insertion

1. Inferior Oblique: Originates from anterior maxilla

2. Superior Oblique: Originates from sphenoid body, traverses the trochlear tendon.

3. Other Rectus Muscles: All originate from the Annulus of Zinn and insert at the anterior sclera.

EOM Functions

EOM Testing Overview

• Terminology in usage of eye movements coded by specific abbreviations:
    - LR6 (Lateral Rectus CN 6), MR3 (Medial Rectus CN 3), SR3 (Superior Rectus) as part of test procedures.

EOM Eye Movement Terminology

• Key Eye Movement Types:
    1. Convergence/Divergence: Alignment for viewing nearby/far objects
    2. Saccade: Fast conjugate movement
    3. Smooth Pursuit: Slow tracking movement
    4. Optokinetic Reflex: Combination of smooth pursuit and saccade
    5. Vestibular-Ocular Reflex: Smooth movement maintaining image stability in response to head movement

Eyelid Anatomy

• Focus on muscles relating to eyelid function and associated nerve innervation affecting pathologies like ptosis.
    - Superior Tarsal Muscle: smooth muscle innervation from sympathetic system for passive elevation.
    - Orbicularis Oculi: skeletal muscle innervation from CN 7 for active closure of eyelid.

Lacrimal System Overview

• Functions of Lacrimal System: [Functionality aspects summarized: secure moisture and antimicrobial effect on the eye].
    - Components include: Lacrimal Gland, canaliculi, sac, and main duct.

• Tear Film Composition: [Describes the three layers of mucus, aqueous, and lipid contributing to surface stability].

• Functions of Tear Film:
    1. Lubrication
    2. Smooth refractive surface
    3. Nutrition to cornea
    4. Immune defense
    5. Waste removal

Neuro-Ophthalmology

Cranial Nerves & Their Functions

• Optic Nerve (CN II): Sensory

• Oculomotor Nerve (CN III): Motor and parasympathetic

• Trochlear Nerve (CN IV): Motor

• Abducens Nerve (CN VI): Motor

Visual Pathway Description

• Pathway:
     - Retina → Optic nerve → Optic chiasm → Optic tract → Lateral geniculate nucleus → Optic radiations → Visual cortex.

• Note on structure: More contralateral nasal fibers (~53%) than ipsilateral temporal fibers (~47%) present.

Visual Reflexes Explanation

1. Pupillary Light Reflex: Involves the optic tract, midbrain structures leading to sphincter muscles for pupil constriction.

2. Accommodation-Convergence Reflex: Contraction of ciliary muscle and medial rectus for lens thickening and eye convergence on close objects.

3. Pupillary Dilation: Neural pathways linked to emotions and pain response activate dilator muscles.

4. Corneal Reflexes: Blink reflex and lacrimation triggered by sensory and motor components of CN V and CN 7.

5. Vestibulo-Ocular Reflex: Adjusts eye movement to correlate with head motion using inputs from semicircular canals.

6. Horizontal Gaze Mechanism: Complex triggering specific muscles to allow for lateral movement in conjunctive gaze.

Internuclear Ophthalmoplegia (INO)

• Partial INO: Resulting from demyelination, limiting adduction of the ipsilateral eye.

• Complete INO: Derived from medial pons lesions, leading to failure to adduct ipsilateral eye, demonstrated by overshooting movements in the contralateral eye, creating nystagmus.

Embryology Summary

• Overview of eye structure development highlighting origins:

1. Lens: Surface ectoderm

2. Retina: Neuroectoderm

3. Iris: Derived from optic cup

4. Ciliary Body: Mesoderm and neuroectoderm combinations

5. Cornea: Various origins from ectoderm to mesenchyme

6. Vitreous Body: Sections invaginating from mesenchyme

7. Eyeballs: Development following optic stalks’ morphogenetic influences and subsequent vasculature for blood supply.

Conclusion

• Thanks!

• Aedann See

• @aedannsee