Vision: The Eye

Lecture 4 – Vision: The Eye

The eye

Fluid-filled sphere surrounded by 3 tissue layers and with two distinct fluid environments

Retina

A tissue layer of the eye

• light sensitive receptors

• Neural portion of the eye that is part of the central nervous system

Uveal layer

A tissue layer of the eye which contains:

• choroid (capillaries and melanin, the lightabsorbing pigment)

• ciliary body (muscles to adjust lens)

• iris (colored portion with muscles that regulate size of pupil)

Sclera

A tissue layer of the eye

(tough white fibrous tissue, with lighttransparent cornea at front)

choroid

Part of the uveal layer of the eye

(capillaries and melanin, the lightabsorbing pigment)

ciliary body

Part of the uveal layer of the eye

(muscles to adjust lens)

iris

Part of the uveal layer of the eye

(colored portion with muscles that regulate size of pupil)

Aqueous humor

A distinct fluid environment of the eye

• Supplies nutrients to the cornea and lens

• Fluid is replaced ~ 12 times/day

• Glaucoma

• Failure of fluid drainage

• High levels of intraocular pressure reduce blood supply

Vitreous humor

A distinct fluid environment of the eye

• 80% volume of the eye

• Maintains shape of eye & contains phagocytic cells that remove blood & debris

• Floaters: collection of debris too large for consumption

refraction

Bending light to achieve a focused image on the retina

Accommodation

Dynamic changes in the shape of the lens

Ex. → Lens is flat to view distant objects and round to view near objects

• Zonule fibers hold the lens in place and keep the lens flat

• Ciliary muscle contraction reduces tension in zonule fibers and allows the elasticity of the lens to increase its curvature

Myopia

Nearsightedness

Unable to bring distant objects into clear focus

Cornea too curved, or eyeball too long

Focus point is in front of retina

Hyperopia

Farsightedness

Unable to focus on near objects

Refractory muscles too weak, or eyeball too short

Focus point is behind retina

Fovea

Image is upside down and curved since this area is curved [MAYBE EDIT]

Macula lutea and fovea

Region of high(est) visual acuity (the ability to resolve fine details)

→ 3 mm in diameter

Optic disk

No photoreceptors in area of retina where blood vessels enter and retinal axons leave the eye → “Blind spot”

→ cortical mechanisms “fill in” the missing info

Macular degeneration

Progressive loss of vision in the center of the visual field (the macula) because of damage to the retina.

• Difficult to read or recognize faces

• Peripheral vision remains

“dry” form Macular degeneration

debris between the retina and the choroid & thinning of macula

Gradual disappearance of the retinal pigment epithelium and loss of photoreceptors

“wet” form Macular degeneration

Abnormal blood vessel growth under macula that leaks fluid and blood into the retina and causes photoreceptor damage

Treated with laser coagulation and medication to destroy the leaky blood vessels

Five basic classes of neurons

• photoreceptors (rods, cones)

• bipolar cells

• ganglion cells

• horizontal cells

• amacrine cells

Three-neuron chain

(most direct path of information flow)

photoreceptor (rod or cone) → bipolar cell → ganglion cell

Axons of ganglion cells form optic nerve

Horizontal and amacrine cells

Enable lateral interactions

• Between photoreceptors and bipolar cells to maintain contrast over different light intensities (luminance)

• Between bipolar and ganglion cells

Rods vs cones

Distinguished by:

• Shape (gives them their name)

• Sensitivity to light

• Photopigment they contain

• Distribution across the retina

• Pattern of synaptic connection

Retinal pigment epithelium

• Reason why retina is inverted

• Photoreceptor disks have limited life span (~12 days)

• Epithelium discards “pinched off” disks

• Epithelium also regenerates photopigment molecules after they have been exposed to light

Phototransduction

Receptor potentials are graded (not all-or-none action potentials)

In the dark, receptor is depolarized

Light stimulation hyperpolarizes photoreceptors

The Dark current

In the dark, the receptor is in a depolarized state → membrane potential of roughly -40 mV

[REWRITE THIS ONE]

Outer segment:

→ High cGMP levels cGMP binds to Na+ permeable channels in the membrane, keeping them open and allowing sodium and other cations to enter

→ Inward current (Na+ , Ca2+; depolarization) through cGMP-gated channels → “dark current”

→ Light reduces cGMP concentration → closes cGMP-gated channels + Reduction in the inward flow of Na+ and Ca2+ → hyperpolarization from inner segment dominates

Inner segment:

→ Outward current mediated by potassiumselective channels

Rod system

• extremely sensitive to light (requires only 1 photon for response)

• has very low spatial resolution (not great at details)

→ Saturate quickly, don’t adapt as efficiently to constant illumination

→ Show convergence (15-30 ___ per bipolar cell) → increases light sensitivity but reduces the spatial resolution of the system

Cone system

• Relatively insensitive to light (>100 photons)

• has high spatial resolution → connected 1:1 to bipolar cells

• responsible for perception of color

→ Adapt more efficiently to constant illumination

Distribution of Cones

(4.5 million)

• low density throughout the retina

• sharp peak in the center of the fovea (foveola) → highest visual acuity

• Individual cones become smaller

Distribution of Rods

(90 million)

• high density throughout retina

• sharp drop in the fovea

Trichromatic Vision

Humans normally have three kinds of cones, resulting in this

• L for long, responds to long wavelengths (peaking at red).

• M for medium, responds to medium wavelengths (peaking at green)

• S for short, responds to short wavelengths (blue) (5-10% of cones in the retina and absent from foveola)

Protanopia

loss of long (red) wavelength perception

Deuteranopia

loss of medium (green) wavelength perception

tritanopia

Third form of dichromacy (extremely rare) → deficiency in short wavelengths → blue-yellow color blindness

On-center ganglion cells

Increase firing when luminance increases in their receptive field center

bipolar cells respond to light increases

→ In response to glutamate release from the photoreceptor terminals (dark) → these bipolar cells have mGluR6 (G-protein-coupled metabotropic glutamate) receptors → that close Na+ channels, reducing inward current and → leading to hyperpolarization of the bipolar cell

Off-center ganglion cells

Increase firing when luminance decreases in their receptive field center

bipolar cells respond to light decreases

→ In response to glutamate release from the photoreceptor terminals (dark), these bipolar cells have ionotropic AMPA & kainate receptors that causes the bipolar cell to depolarize

“off” (dark) center

absence of light → depolarization of cone

sign-inverting: depolarization

→ more glutamate and more mGluR6 activity

→ increases inhibition (hyperpolarization) of oncenter bipolar cell

sign-conserving depolarization

→ more glutamate and more AMPA activity → increased excitation, (depolarization) of offcenter bipolar cell

Less glutamate release → fewer spikes in ganglion cell

More glutamate release → more spikes in ganglion cell

“on” (light) center

light → hyperpolarization of cone

sign-inverting: hyperpolarization

→ less glutamate release and less mGluR6 activity

→ reduction of inhibition, (depolarization) of oncenter bipolar cell

sign-conserving: hyperpolarization

→ less glutamate release and less AMPA activity

→ reduction of excitation, (leaving hyperpolarization) of off-center bipolar cell

more glutamate → more spikes in ganglion cell

less glutamate → fewer spikes in ganglion cell

Ishihara Test

Test for color blindness

A circle or circles with some colored circles in which a number/letter should stand out

Different light conditions for ON-center ganglion cells

• Move spot of light across on-center cell’s receptive field

→ Response decreases as move away from center

→ Completely peripheral in surround = inhibited

→ Out of receptive field → cell returns to resting potential

• Opposite for off-center cells

Horizontal cells

Antagonism of the surround area arises from lateral connections from ___ ___

Depolarized by glutamate from photoreceptors (“+”, sign-conserving synapse) and release GABA to hyperpolarize photoreceptors (“-”, sign-inverting synapse).

Net effect → inputs oppose changes in the photoreceptor that are induced by light events in the surround area

Addition of light to the surround → reduced glutamate from surround photoreceptors → strong hyperpolarization of ____ ____ → depolarizing effect on center photoreceptor → reduced light-evoked response

mis-engineered human eye

Other species have:

1. better lenses (zoom w. telephoto & macro functions)

2. better muscle control for focus

3. nictitating membranes (third eyelid) (more protection & moisture)

4. more photodetectors, e.g. 200K vs. 1M

5. better arranged blood vessels, neurons & axons

6. two fovea -- one binocular, one monocular (better tracking)

7. better acuity (20/2.5 vs. 20/20 [normal human]