BMS 302: Vision

Which photoreceptor is responsible for night (low acuity, low light threshold) vision?

Rods

Which photoreceptor is responsible for color (high acuity, high light threshold) vision?

Cones

Concentrations of photoreceptors:

• Fovea: mostly cones (highest concentration, no rods)

• Periphery: mostly rods, few cones

Why can light directly stimulate cones in the fovea?

The cones are arranged so that inner retinal layers are displaced, allowing light to hit cones directly. → (contributes to greatest visual acuity)

What gives the fovea the greatest acuity?

High cone density and direct light access (minimal scattering).

What causes the physiologic blind spot?

Optic disc – area where optic nerve exits; no photoreceptors present.

What two factors determine refractive power of a lens?

• Curvature of the lens surface

• Difference in refractive indices between the two media

Four components of the eye’s refractive system (mark the one with greatest refraction *):

• Cornea*

• Aqueous humor

• Lens

• Vitreous humor

Which component can change refractive power?

The lens (via accommodation).

What is the focal point of a lens?

Point where light rays converge after passing through the lens.

For clear vision, where must the focal point fall?

Directly on the retina

What is focal length?

Distance between the center of the lens and its focal point.

Convex lenses cause light rays to ___; concave lenses cause light rays to ___.

Convex → converge
Concave → diverge

Diopter sign convention:

Convex (converging) = (+) diopters; Concave (diverging) = (–) diopters

Formula for refractive power:

P = 1 / f (f in meters)

What is emmetropia?

Normal vision – focal point lands on retina with relaxed lens.

Eyeball length & refractive power for emmetropia:

22.6 mm to 23 mm length, +60 diopters

What are the three reflexes of the near response?

• Accommodation (lens curvature ↑)

• Pupillary constriction

• Convergence of eyes

What happens to lens refractive power during accommodation? Why?

Increases; to bend diverging light from near objects onto the retina.

Ciliary muscle and suspensory ligament roles:

• Ciliary muscle contracts → suspensory ligaments relax → lens thickens → ↑ refractive power

• Ciliary muscle relaxes → ligaments tighten → lens flattens → ↓ refractive power

ANS control of ciliary muscle:

• Contraction: ↑PANS

• Relaxation: ↑SANS

Type of muscle in ciliary muscle?

Single-unit smooth muscle

Light rays from distant source: parallel or diverging?

Parallel

Ciliary muscles for distant vision: contracted or relaxed?

Relaxed

Light rays from near source: parallel or diverging?

Diverging

To view near object, refractive power must:

Increase

What happens to lens elasticity with age? Condition name? When does it begin?

Decreases; Presbyopia; Around age 40

Presbyopia: Near point increases or decreases?

Increases (can’t focus on close objects)

Focal point location (uncorrected presbyopia)? Lens correction? 

Behind the retina; Convex (converging) lens

What is the pupillary reflex of the near response? Why does it help focus near images?

Constriction of pupils; reduces peripheral light rays → sharper image.

ANS control of pupillary reflex:

• Dilation: ↑SANS

• Constriction: ↑PANS

Type of muscle in iris?

Smooth muscle

How does convergence aid near vision?

Aligns both eyes on the same near object → prevents double vision.

Nervous system branch controlling ocular muscle contraction?

Somatic

Causes of hyperopia:

• Eyeball too short

• Refractive power too weak

Focal point location (hyperopia)?

Behind retina

Corrective lens type? Correction effect? (hyperopia)

Convex (converging); shortens focal length (brings rays forward)

Causes of myopia:

• Eyeball too long

• Refractive power too strong

Focal point location (myopia)?

In front of retina

Corrective lens type? Correction effect? (myopia)

Concave (diverging); extends focal length

What does Snellen’s eye chart test?

Visual acuity (sharpness of vision)

Meaning of visual acuity numbers: 20/20, 20/400, 20/10

• 20/20: Normal vision (reads at 20 ft what normal eye reads at 20 ft)

• 20/400: Poor vision (sees at 20 ft what normal sees at 400 ft)

• 20/10: Excellent vision (sees at 20 ft what normal sees at 10 ft)

What is astigmatism?

Irregular curvature of cornea or lens → light focuses unevenly.

Two causes of astigmatism:

• Uneven corneal surface

• Uneven lens curvature

How does astigmatism chart help?

Lines appear unevenly dark or blurry → indicates irregular refraction.

Difference between spherical and cylindrical lenses?

• Spherical: same curvature in all directions

• Cylindrical: curved in one plane only (used for astigmatism)

Why can hard but not soft contacts correct astigmatism?

Hard lenses maintain shape to correct irregular cornea; soft lenses conform to cornea’s shape.