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Vision: Eye Anatomy
and Basic Physiology

Explore the structures of the human eye, understand how light is refracted and focused, and discover how photoreceptors convert light into neural signals.

Anatomy & Physiology

What Is Vision?

Vision is the process by which electromagnetic radiation (visible light, wavelengths \( \approx 380\text{–}700\;\text{nm} \)) enters the eye, is refracted and focused onto the retina, and is converted into electrical signals that the brain interprets as images.

Three Tunics of the Eye

Fibrous Tunic
Outer layer: the sclera (white, protective) and the cornea (transparent, refracts light).
Vascular Tunic
Middle layer: the choroid (blood supply), ciliary body (lens shape), and iris (controls pupil size).
Neural Tunic
Inner layer: the retina containing photoreceptors (rods and cones) that detect light.

Cross-Section of the Human Eye

Vitreous Humor Aqueous Humor Cornea Iris Lens Retina Fovea Optic Nerve Blind Spot Sclera Light

How Light Is Focused

Two structures refract (bend) light to focus it on the retina. The cornea provides about \( \frac{2}{3} \) of the eye's total refractive power, while the lens provides the remaining \( \frac{1}{3} \) and can change shape for fine focusing.

Cornea
Fixed refractive power of \( \approx 43 \) diopters. Responsible for most light bending.
Lens
Variable power: \( \approx 13\text{–}26 \) diopters. Changes shape via accommodation to focus near or far objects.

Accommodation: Fine-Tuning Focus

Distant Object Lens: Flat Ciliary muscle relaxed Zonules taut Retina Near Object Lens: Round Ciliary muscle contracted Zonules slack Retina

The Retina: Rods and Cones

The retina contains two types of photoreceptors that convert light into electrical signals via a process called phototransduction.

Rods
\( \approx 120 \) million per eye. Sensitive to dim light (scotopic vision). No color discrimination. Concentrated in the periphery.
Cones
\( \approx 6 \) million per eye. Function in bright light (photopic vision). Enable color vision and high acuity. Concentrated in the fovea.

Phototransduction: Light to Signal

1
Light strikes rhodopsin (in rods) or photopsins (in cones), causing retinal to change from cis to trans configuration.
2
Opsin activates transducin (a G-protein), which activates phosphodiesterase (PDE).
3
PDE breaks down \( \text{cGMP} \), causing \( \text{cGMP} \)-gated \(\text{Na}^{+}\) channels to close.
4
The photoreceptor hyperpolarizes, reducing glutamate release onto bipolar cells.
5
Bipolar cells relay the signal to ganglion cells, whose axons form the optic nerve.

Rods vs Cones: Key Differences

Feature Rods Cones
Number \( \approx 120 \) million \( \approx 6 \) million
Pigment Rhodopsin Photopsins (3 types)
Sensitivity Very high (dim light) Lower (bright light)
Color Vision No (monochromatic) Yes (trichromatic)
Acuity Low (high convergence) High (low convergence)
Location Peripheral retina Fovea (center)

The Visual Pathway

Photoreceptors Bipolar Cells Ganglion Cells Optic Nerve Optic Chiasm LGN (Thalamus) Primary Visual Cortex (V1) Retina

The Pupillary Light Reflex

The pupil diameter controls how much light reaches the retina. This is regulated by two muscles in the iris, controlled by the autonomic nervous system.

Sphincter Pupillae
Circular muscle. Parasympathetic (\( \text{CN III} \)) stimulation causes miosis (pupil constriction) in bright light.
Dilator Pupillae
Radial muscle. Sympathetic stimulation causes mydriasis (pupil dilation) in dim light or "fight-or-flight."

Color Vision: Trichromatic Theory

Wavelength (nm) Response 400 450 530 560 700 S (Blue) M (Green) L (Red)

How to Trace the Visual Path

1
Light enters through the cornea: the transparent anterior surface that refracts most of the light.
2
Passes through the pupil: the iris adjusts its diameter to control light intensity.
3
Lens fine-focuses the light: accommodation adjusts lens curvature for object distance.
4
Light traverses the vitreous humor and strikes the retina (inverted image).
5
Phototransduction in rods/cones: light energy is converted to electrical signals.
6
Signal travels: photoreceptors → bipolar cells → ganglion cells → optic nerve → LGN → visual cortex (\( V_1 \)).

Worked Example: Myopia (Nearsightedness)

Focal Point Retina Eyeball is too long: image focuses in front of retina

Worked Example: Hyperopia (Farsightedness)

Retina Focal Point Eyeball is too short: image focuses behind retina

Common Mistakes

✕ Confusing Rods and Cones
Rods are for dim light and peripheral vision, NOT color. Cones detect color and are concentrated at the fovea, NOT the periphery.
✕ Cornea vs Lens Roles
The cornea does most of the refracting. The lens only fine-tunes focus. Students often think the lens does all the work.
✕ Myopia vs Hyperopia Correction
Myopia (near) needs a concave (diverging) lens. Hyperopia (far) needs a convex (converging) lens. These are frequently swapped.

Summary: Eye Structures and Functions

Structure Function Key Detail
Cornea Refracts light \( \approx 43 \) diopters
Iris/Pupil Controls light entry Sphincter + Dilator muscles
Lens Fine-focus (accommodation) \( 13\text{–}26 \) diopters (variable)
Retina Phototransduction Rods + Cones
Fovea Highest acuity Cones only
Optic Nerve Transmits signals to brain Ganglion cell axons (\( \text{CN II} \))
Visual Cortex Processes visual info Occipital lobe (\( V_1 \))

Question 1 of 5

Which structure provides approximately two-thirds of the eye's total refractive power?

Question 2 of 5

During phototransduction, light causes the photoreceptor to:

Question 3 of 5

A patient with myopia has distant objects appearing blurry. What type of corrective lens is needed?

Question 4 of 5

Which photoreceptor type is concentrated in the fovea and responsible for high-acuity color vision?

Question 5 of 5

At the optic chiasm, which fibers cross to the opposite side?