visual system 1

What are the three layers surrounding the liquid sphere of the eye?

Retina, uveal layer, and sclera.

What is the main role of the retina?

It contains light-sensitive receptors.

What structures make up the uveal layer?

Choroid, ciliary body, and iris.

What does the choroid contain?

Capillaries and melanin.

What is the role of melanin in the choroid?

It is a light-absorbing pigment.

What is the function of the ciliary body?

Its muscles adjust the lens.

What is the iris?

The colored portion of the eye containing muscles that regulate pupil size.

What is the sclera?

White fibrous tissue that provides structural support to the eye to help move eyeball.

With light transparent cornea at the front

What is the pupil?

“hole” through which light enters (black circle)

• less light: pupil enlarges

• more light: pupil smaller

Lens / cornea relationship in bending and reason for bending?

• cornea bends light

• lens further bends light

Reason is so light reaches the fovea & sharp image

What is special about the anterior portion of the scleral outer layer?

It includes the light-transparent cornea at the front.

Approximately how thick is the retina?

About 100–200 µm.

Approximately how many output neurons are associated with the two retinas?

About 3 million total, approximately 1.5 million per eye.

What fluid fills the anterior chamber of the eye?

Aqueous humor.

What fluid fills most of the posterior interior of the eye?

Vitreous humor.

What fibers are attached to the lens and help control its shape?

Zonule fibers.

What structure exits the posterior eye and carries retinal output?

The optic nerve and retinal vessels

What retinal landmark lies near the optic nerve exit and lacks photoreceptors?

The optic disk.

What retinal region is specialized for high visual acuity?

The fovea, within the macular region.

Why can different species have different eye specializations?

Different species have different eyes because they have different survival demands.

What are examples of ways other species may have better visual systems than humans?

Better lenses, better muscle control for focus, nictitating membranes, more photodetectors, better-arranged blood vessels/neurons/axons, two foveae, and better acuity.

What is a nictitating membrane?

A third eyelid that provides more protection and moisture.

What advantage can two foveae provide in some species?

One can be binocular and one monocular, allowing better tracking.

Visible light represents what fraction of the electromagnetic spectrum?

Only a small part of the electromagnetic spectrum.

Approximately what wavelength range is visible light?

About 400–700 nm.

How are frequency and wavelength related for electromagnetic radiation?

Frequency = speed of light / wavelength; longer wavelength corresponds to lower frequency.

What is the visual field?

The amount of space viewed by the retina when the eye is fixated straight ahead.

Approximately how far does the normal monocular human visual field extend nasally from the vertical meridian?

About 60 degrees.

What does monocular mean?

Using one eye.

What does binocular mean?

Using two eyes.

What structures bend light to produce a focused retinal image?

The cornea and lens.

What is refraction in the eye?

Bending of light by the cornea and lens so that it focuses on the retina.

What is accommodation?

A change in the refractive power of the lens.

How does lens shape differ for distant versus near objects?

The lens is relatively flat for distant objects and becomes rounder for near objects.

What keeps the lens flat in the unaccommodated state?

Zonule fibers.

What muscles change lens refractive power and make the lens rounder?

Ciliary muscles.

Why must light be bent by the eye?

So that it focuses on the retina.

What are cataracts?

Opacities in the normally transparent lens.

According to the lecture, cataracts account for approximately what percentage of blindness?

50%.

How is the image projected onto the vertebrate retina oriented?

Upside down and curved.

Where do the top and bottom of an object project on the retina?

The top projects toward the lower retina, while the bottom projects toward the upper retina.

What is myopia?

Nearsightedness.

According to the lecture, approximately what percentage of the U.S. population has myopia?

About 50%.

What anatomical/refractive features can cause myopia?

A cornea that is too curved or an eyeball that is too long.

Where is the focal point in myopia?

In front of/before the retina.

• light focuses before reaching the retina

In myopia, are near or far objects seen better?

Near objects are seen better than far objects.

What possible environmental contribution to myopia is mentioned in the lecture?

Reading may interfere with proper growth of the eyeball; eyeball growth is described as light-mediated.

What is hyperopia?

Farsightedness.

What problem is associated with hyperopia in the lecture?

Refractory/refractive muscles are too weak.

Where is the focal point in hyperopia?

Behind the retina.

What are the macula lutea and fovea specialized for?

fovea: thing directly focused on, highest acuity

macula lutea: surrounds fovea and gives good vision but not same acuity of fovea itself

Why does the optic disk create a blind spot?

There are no photoreceptors where blood vessels enter and optic fibers leave the eye.

How does the brain handle missing visual information from the optic-disk blind spot?

Cortical mechanisms fill in the missing information.

Why is the fovea specialized for high visual acuity?

Retinal layers are displaced to allow light more direct access to densely packed cones.

Which photoreceptors are concentrated in the fovea?

Cones.

What type of vision are cones associated with?

Color vision.

What type of vision are rods associated with?

Black-and-white/dim-light vision.

What is macular degeneration?

Damage to the retina in the macular region causing loss of central visual-field vision (macula)

Approximately how many people in the U.S. are affected by macular degeneration according to the lecture?

About 6 million.

What vision is lost and what vision generally remains in macular degeneration?

Central vision is lost, while peripheral vision generally remains.

What everyday tasks become difficult with macular degeneration?

Reading and recognizing faces.

What percentage of macular degeneration cases are the dry form?

About 90%.

What characterizes dry macular degeneration?

Debris accumulates between the retina and choroid, leading to disappearance of the retinal pigment epithelium and loss of photoreceptors.

What characterizes wet macular degeneration?

Blood vessels grow from the choroid, and the retina can also become detached.

Which form of macular degeneration is more severe?

The wet form.

How is wet macular degeneration treated according to the lecture?

Laser treatment and medication to stop blood-vessel growth.

What is the simplified three-neuron chain of retinal circuitry?

Photoreceptor (rod or cone) → bipolar cell → ganglion cell.

What do photoreceptor cells convert light energy into?

Changes in membrane potential/neural signals.

Do photoreceptors normally generate action potentials?

No. Their receptor potentials are graded.

What type of electrical signal do ganglion cells generate?

Action potentials.

How do ganglion-cell action potentials reach the brain?

Axons actually leave the eye → optic nerve → brain

Which retinal cells form the optic nerve and provide the retina's output?

Ganglion cells.

What are the five classes of retinal neurons?

Photoreceptors, bipolar cells, horizontal cells, amacrine cells, and ganglion cells.

Why is retinal circuitry described as "backwards"?

Light enters from the inner retinal side and passes through neural layers before reaching photoreceptors at the outer retina.

What direction does vertical information flow through the simplified retina?

Photoreceptor → bipolar cell → ganglion cell.

Which retinal neurons provide lateral information flow?

Horizontal cells and amacrine cells.

What is the function of bipolar cells?

They transmit signals from photoreceptor cells to ganglion cells.

What is the function of horizontal cells?

They modify bipolar-cell responses and contribute to lateral interactions and surround antagonism.

What is the function of amacrine cells?

They modify ganglion-cell responses.

By what major characteristics are rods and cones distinguished?

Shape, light sensitivity, photopigment, retinal distribution, and pattern of synaptic connection.

What are the three major structural regions of a photoreceptor?

Outer segment, inner segment, and synaptic terminal.

What is contained in the photoreceptor outer segment?

Membrane/disks containing photopigment involved in detecting light.

What important organelles are shown in the photoreceptor inner segment?

Mitochondria and the nucleus.

What is located at the basal end of a photoreceptor?

The synaptic terminal, which communicates with downstream retinal neurons/bipolar

What is the membrane-potential state of a photoreceptor in darkness versus light?

It is depolarized in darkness and hyperpolarized in light.

Are photoreceptor receptor potentials all-or-none?

No. They are graded potentials.

How does stronger light affect the graded photoreceptor response?

More intense light produces a larger hyperpolarizing response.

What ions carry the inward dark current in the photoreceptor outer segment?

Na+ and Ca2+.

Through what channels does the outer-segment dark current flow?

cGMP-gated channels.

What effect does the outer-segment dark current have on membrane potential?

It depolarizes the photoreceptor.

What ion carries the outward current in the inner segment?

K+.

What effect does the inner-segment K+ current have on membrane potential?

It promotes hyperpolarization.

Why does a photoreceptor hyperpolarize in light?

Light reduces cGMP and closes cGMP-gated Na+/Ca2+ channels, reducing the inward depolarizing current so the K+ hyperpolarizing current dominates.

What is retinal coupled to in photopigment?

One of several opsins, which differ for different wavelengths.

What does a light photon do to 11-cis retinal?

Converts 11-cis retinal to all-trans retinal.

Phototransduction / closing cGMP channels

• light proton converts 11-cis retinal to all-trans retinal

• activates transducer (which is a G protein)

• activates PDE

• reduces levels of cGMP

• so now cGMP is no longer keeping the channels open. CHANNELS CLOSE
  -outward K+ current dominates → hyperpolarization

What enzyme is activated downstream of transducin?

PDE, or cGMP phosphodiesterase.

What does PDE do during phototransduction?

Reduces cGMP levels.

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What is rhodopsin composed of?

Opsin and retinal.