Module 5

Question 1

Three Membranes of the Eye:
Outer Membrane:

Answer 1

Outer Membrane
Sclera:
Tough protective covering. Visible portion appears as the white of the eye
Cornea
(front transparent portion)

Question 2

Eye: Middle Membrane

Answer 2

Choroid:
Lines the interior of the sclera. Contains most of the blood vessels supplying the eye with oxygen and nutrients

Question 3

Eye: Inner Membrane

Answer 3

Retina:
Made up of neurons, including photoreceptors. Converts incoming light into neural signals. (Functions of the retina are discussed in later chapters)

Question 4

Cornea

Answer 4

Location and Structure:
Transparent membrane at the front of the eye
Function:
First structure that incoming light passes through. Performs
most of the eye’s focusing
by sharply refracting (
bending
) incoming light
Limitation:
Rigid and cannot change its curvature to adjust focus. Fine focusing adjustments are handled by the flexible lens

Question 5

Iris

Answer 5

Iris: Colored, doughnut-shaped
muscle. Controls the
size of the pupil
by contracting or relaxing. Responds primarily to light intensity

Question 6

Pupil:

Answer 6

Opening in the middle of the iris through which light enters. Diameter ranges approximately from 2 mm (constricted) to 8 mm (dilated).

Question 7

Pupillary Reflex

Answer 7

Automatic adjustment of pupil size in response to light intensity

In bright light: iris
contracts→ pupil constricts (smaller)

In dim light: iris relaxes → pupil dilates (larger)

Both eyes’ irises act in tandem: shining light in one eye causes both pupils to constrict simultaneously

Question 8

Three Chambers of the Eye
Spatial Organization

Answer 8

Anterior Chamber:
Located between the cornea and the iris. Filled with aqueous humor.
Posterior Chamber:
Located between the iris and the lens. Also filled with aqueous humor.
Vitreous Chamber:
The large interior space behind the lens. Filled with vitreous humor.

Question 9

Humors and Their Properties
Aqueous Humor

Answer 9

Clear,
thin fluid
(over 99% water). Occupies both anterior and posterior chambers.
Refracts light slightly
(but cannot be adjustably reshaped)
Vitreous Humor:
Clear,
gel-like fluid
(over 99% water). Fills the vitreous chamber. Also contributes
a fixed amount of refraction

Question 10

Focal Length: Flat vs Thicker lens

Answer 10

The distance from the lens at which parallel (distant) light rays converge to a focus. A weak lens (thin, relatively flat) has a
long focal length. A strong lens (thicker, more curved) has a short focal length.

Question 11

Accommodation

Answer 11

Ciliary Muscles: Tiny ring-shaped muscles located in the ciliary body, attached to the choroid and connected to the lens via zonule fibers (suspensory ligaments).

When relaxed, the zonule fibers are taut, pulling on the lens and flattening it — ideal for distant vision.

When the ciliary muscles contract, they move inward, releasing tension on the zonule fibers, allowing the lens to become more rounded — ideal for near vision (accommodation).

Question 12

The Retinal Image
Projection

Answer 12

Light rays from each point in the environment are refracted by the lens so that they converge at corresponding points on the retina.
Inversion: Due to refraction, the retinal image is upside down
(top ↔ bottom) and reversed left-to-right.
Analogy: Conceptualized like a movie frame on a screen, though the retina is soft, curved, and textured.

Question 13

Anatomy of the Retina

Answer 13

Location: Innermost of the three eye membranes (sclera, choroid, retina).
Photoreceptor layer: Closest to the choroid, containing outer and inner segments of rods and cones embedded in the pigment epithelium.

Question 14

Pigment epithelium

Answer 14

Pigment epithelium: A cell layer attached to the choroid that supports photoreceptor function.

Question 15

Retinal Layers: Outer segment layer

Answer 15

Outer segment layer: Photoreceptor “business ends” where light transduction occurs.

Question 16

Retinal Layers: Outer nuclear layer

Answer 16

Contains photoreceptor cell bodies (nuclei).

Question 17

Retinal Layers: Outer synaptic (plexiform) layer

Answer 17

Synapses among photoreceptors, bipolar cells, and horizontal cells.

Question 18

Retinal Layers: Inner nuclear layer

Answer 18

Inner nuclear layer:
Contains bipolar cells, horizontal cells, and amacrine cells.

Question 19

Retinal Layer: Inner synaptic (plexiform) layer

Answer 19

Inner synaptic (plexiform) layer:
Synapses among bipolar cells, amacrine cells, and retinal ganglion cells.

Question 20

Retinal Layer: Ganglion cell layer

Answer 20

Ganglion cell layer:
Contains retinal ganglion cell bodies.

Question 21

Retinal Neuron Classes and Functions: Photoreceptors

Answer 21

Photoreceptors
Rods: High sensitivity for black-and-white vision under dim light.
Cones: High-acuity color vision in bright light; three types based on wavelength sensitivity:
S-cones (short wavelengths)
Module Five - The Eye 8
M-cones (medium wavelengths)
L-cones (long wavelengths)

Question 22

Retinal Neuron Classes and Functions: Horizontal cells

Answer 22

Horizontal cells:
Lateral interneurons connecting photoreceptors and other horizontal cells, shaping spatial integration.

Question 23

Retinal Neuron Classes and Functions: Bipolar Cells

Answer 23

Bipolar cells:
Relay signals from photoreceptors to amacrine cells and ganglion cells.

Question 24

Retinal Neuron Classes and Functions: Amacrine Cells

Answer 24

Amacrine cells:
Complex interneurons interacting with bipolar cells, other amacrine cells, and ganglion cells; modulate
temporal aspects of vision.

Question 25

Retinal Neuron Classes and Functions: RGCs

Answer 25

Retinal ganglion cells (RGCs): Final retinal output neurons; receive inputs from bipolar and amacrine cells and send action potentials via the optic nerve to the brain.

Question 26

Optic Disk and Optic Nerve Optic disk (blind spot)

Answer 26

Region where ~1 million RGC axons exit the eye; contains no photoreceptors. Optic nerve: Bundle of RGC axons transmitting visual information to central targets.

Question 27

Fovea

Answer 27

Location & Optic Axis: The optic axis of the eye passes through the fovea centralis at the retinal center. Light from the point of gaze falls here. Photoreceptor Composition No rods: only cones are present. Very high cone density , optimized for detailed vision. Cone Packing & Geometry: Foveal cones are thinner than elsewhere, allowing a dense, hexagonal arrangement (most efficient for cylindrical cells). Layer Displacement: Ganglion-cell and inner-nuclear layers are pushed aside around the fovea, reducing light scatter and enhancing direct access to the cones. Functional Outcome: These specializations maximize high-acuity vision at the center of gaze.

Question 28

Phototransduction -> Through Pathway

Answer 28

Flow: Photoreceptors → Bipolar cells → Retinal ganglion cells (RGCs). Any bipolar cell connects exclusively to rods or to cones (Figure shows cones-only). Bipolar membrane-potential changes adjust their neurotransmitter release, which in turn modulates RGC firing rates .

Question 29

Phototransdution -> Lateral Pathway: Horizontal Cells:

Answer 29

Horizontal cells: Receive from photoreceptors (and other horizontals) and feed back to photoreceptors and horizontals.

Question 30

Phototransduction -> Lateral Pathway (Red Arrows): Amacrine Cells

Answer 30

Amacrine cells: Receive from bipolar cells (and other amacrines), feed back to bipolar/amacrine cells, and send signals to RGCs .

Question 31

Lateral Pathway function

Answer 31

Enables luminance-contrast processing—light at one location influences neighboring photoreceptor, bipolar, and RGC responses, crucial for edge and boundary detection.

Question 32

After Lateral and Through Pathways

Answer 32

RGC Output: Retinal ganglion cells integrate through- and lateral-pathway inputs and fire action potentials that travel via the optic nerve to the brain.

Question 33

Photopigments and Spectral Sensitivity

Answer 33

Rods: Single pigment with peak sensitivity at ~500 nm Cones: Three pigments with distinct peaks: S-cones : ~443 nm (short wavelengths) M-cones : ~543 nm (medium wavelengths) L-cones : ~574 nm (long wavelengths)

Question 34

Photoreceptor Anatomy

Answer 34

Outer segment: Stacks of membranous discs containing photopigments → site of photon absorption Inner segment: Contains organelles (mitochondria, protein synthesis machinery) → produces and ships photopigment molecules to outer segment Cell body (nucleus): Houses DNA Synaptic terminal: Releases neurotransmitter onto bipolar and horizontal cells

Question 35

Transduction Cycle

Answer 35

1. Photon absorption & photoisomerization: Photopigment in 11-cis form absorbs photon → converts to all-trans form 2. Biochemical cascade: Activates G-protein cascade → amplifies response (especially in rods) 3. Membrane hyperpolarization: Reduction in photoreceptor membrane potential 4.Neurotransmitter modulation: Decreased glutamate release at synapse alters activity of bipolar and horizontal cells. In the dark, the photoreceptors are depolarized, this depends on the influx of Na+, hold by the cGMP. The light bends the 11-cis molecule, activating a G-protein cascade, that activates an cGMP “killer” called PDE, thus, the influx of Na+ cannot be maintained anymore, hyperpolarizing the photoreceptor. Being hyperpolarized, the photoreceptor cannot stimulate the bipolar cells, who were inhibiting the ganglion cells, the inhibition is “inhibit”, activating the ganglion cells. 5. Signal propagation: Bipolar → amacrine → ganglion cells → optic nerve → brain 6. Photopigment regeneration: All-trans retinal converted back to 11-cis retinal (completing the cycle)

Question 36

Dark Adaptation

Answer 36

Two-Phase Dark Adaptation Curve Rapid "Cone" Phase (~0–3 min): Steep initial increase in sensitivity, mediated by cones. Cone Plateau (~3–7 min): Cone sensitivity levels off; no further significant gain. Rod–Cone Break (~7–10 min): Crossover point where rods surpass cones in sensitivity. Slow "Rod" Phase (~7–30 min): Gradual, continued increase in sensitivity driven by rods. Fully Dark-Adapted Sensitivity: Final sensitivity ∼100,000× greater than light-adapted state.

Question 37

Convergence in Retinal Circuits Degrees

Answer 37

Degrees of convergence: High convergence: Dozens–hundreds of photoreceptors → one RGC Moderate convergence: Few photoreceptors → one RGC No convergence: One photoreceptor → one RGC

Question 38

Convergence

Answer 38

Effects of Convergence Sensitivity to Dim Light: High convergence funnels weak signals from many photoreceptors, boosting RGC response under low illumination. Visual Acuity: Low/no convergence preserves one-to-one mapping, enabling precise localization of stimuli and fine spatial discrimination.

Question 39

Retinal Distribution of Convergence Fovea: High RGC density, minimal convergence → maximal acuity

Answer 39

High RGC density, minimal convergence → maximal acuity

Question 40

Diversity Across RGC Types

Answer 40

Midget RGCs: Small receptive fields at any eccentricity → fine-detail encoding. Parasol RGCs: Larger receptive fields → greater summation, faster responses. Tiling: Each RGC type’s fields overlap extensively to cover the retina with minimal gaps.

Question 41

Two major types of retinal ganglion cells (RGCs) based on their receptive field properties and FUNCTION

Answer 41

On-Center RGCs Light in center → RGC fires more Light in surround → RGC fires less (due to lateral inhibition) Uses ON bipolar cells (reverse the signal) Contrast sensitivity: Uniform illumination of center+surround → minimal response. Maximally responsive to luminance contrast (edges) Off-Center RGCs Dark in center → RGC fires more Light in surround → RGC fires less (due to lateral inhibition) Uses OFF bipolar cells (preserve the signal)

Question 42

Lateral Inhibition

Answer 42

Center pathway: Cones → Bipolar cells → Retinal ganglion cells (RGCs) (excitatory) Surround pathway: Cones → Horizontal cells → inhibitory feedback onto center cones

Question 43

Presbyopia:

Answer 43

Age-related loss of lens elasticity.

Question 44

Astigmatism:

Answer 44

Irregular/asymmetrical curvature of cornea or lens. Light from a single point fails to converge to one retinal point → blurred image at all distances

Question 45

Cataracts

Answer 45

Progressive clouding of the crystalline lens. Caused by UV exposure, diabetes, aging. Symptoms: Gradual vision dimming, glare sensitivity. Treatment: Surgical removal of lens, intraocular plastic lens implant (monofocal or multifocal).

Question 46

Glaucoma

Answer 46

Is caused by elevated intraocular pressure due to impaired aqueous humor drainage , leading to fluid buildup in the anterior and vitreous chambers and resulting in optic nerve and retinal ganglion cell damage. The condition carries the risk of progressive, often unnoticed peripheral vision loss that can lead to blindness. Diagnosis involves tonometry, visual field testing, and optic nerve evaluation. Treatment includes medications (eye drops), laser therapy, or surgery to improve fluid outflow.

Question 47

Floaters:

Answer 47

Debris in vitreous humor casting shadows on retina (spots, threads). Common with vitreous condensation, shrinkage, or minor retinal tears

Question 48

Phosphenes:

Answer 48

Brief flashes of light not caused by external stimuli. Triggered by mechanical, electrical, or spontaneous retinal cell activation

Question 49

Macular Degeneration (AMD):

Answer 49

Macular Degeneration (AMD): Damage to photoreceptors in macula (central retina) Forms: Dry: Retinal Pigment Epithelium cell degeneration → photoreceptor loss → central scotomas (blind spots). visual pigment recycling is impaired + photoreceptors lose structural support, nutrient supply, and waste removal → they die over time. Wet: Choroidal neovascularization → leakage, bleeding, scarring Epidemiology : ~10% of ages 66–74; ≥30% of ages 75–85 Treatment : Anti-VEGF drugs for wet AMD; no cure for dry AMD

Question 50

Retinitis Pigmentosa (RP):

Answer 50

Inherited, progressive disease characterized by photoreceptor degeneration . It typically begins with night blindness due to rod cell loss , followed by the development of mid-peripheral ring-shaped scotomas that gradually expand both centrally and peripherally, leading to tunnel vision and, in some cases, eventual total vision loss . Currently, there are no approved treatments, though experimental approaches such as gene therapy, retinal implants, and stem cell therapies are under investigation.

Question 51

Purkinje Shift:

Answer 51

The shift in peak spectral sensitivity from ~560 nm (cone vision) toward ~500 nm (rod vision) during dark adaptation, causing short-wavelength (blue/green) objects to appear relatively brighter at dusk

Question 52

Visual field routing

Answer 52

Left LGN ← right visual field (left temporal + right nasal retina) Right LGN ← left visual field (right temporal + left nasal retina)