Visual system (Chapter 17)

Question 1

What is the general organization of the eye?

Answer 1

The eye has 3 concentric layers of tissue, 3 chambers, and a lens.

Question 2

Name and describe the three layers of tissue.

Answer 2

Sclera: Collagenous tissue, continuation of the dura, transitions to become cornea.
Uvea (or uveal tract): Mostly choroid, carries choroidal capillaries that supply the retina.
Retina: Innermost layer, actually 2 layers - retinal epithelium (outer) and neural retina (inner).

Question 3

Where in the eye are rods and cones found?

Answer 3

The neural retina contains retinal receptor cells: rods and cones.

Question 4

Name the three chambers.

Answer 4

Vitreous humor: Large chamber at the back of the eye; vitreous humor is not recycled, what you have at birth is all you get.
Anterior chamber: Filled with aqueous humor.
Posterior chamber: Contains ciliary body, which produces CSF (like choroid plexus)

Question 5

What happens if the retina and choroid split?

Answer 5

Retinal detachment = blind spot. The retina cannot function without blood perfusion of choroid.
Treatment is to “glue” retina back to choroid so it does not continue.

Question 6

How is the orb shape of the eye maintained?

Answer 6

Intraocular pressure:
Aqueous humor is (1) created in the ciliary body in the posterior chamber, (2) pushed through the pupil to the anterior chamber, and (3) reabsorbed into the chamber of Schlemm.

Question 7

What causes glaucoma?

Answer 7

Blockage of the scleral venous sinus (Canal of Schlemm), which creates pressure and damages the retina.

Question 8

What is the role of the cornea in focusing images on the retina?

Answer 8

The corneal surface (air/water interface) is where most refraction takes place.

Question 9

Which parts of the eye are involved in focusing images on the retina?

Answer 9

The cornea and lens focus images on the retina.
The shape of cornea can distort the way light enters the eye (e.g., why we need glasses).
The lens sets the focal point (the optimal point for clear image on retina).

Question 10

How does the lens adjust focus?

Answer 10

Zonules and the ciliary muscle work together to change the focal length of the lens. Zonules connect the lens to the globe. They keep it suspended and slightly flattened at baseline. Zonules are connected at their other end to the ciliary body. Contraction of the ciliary muscle (like a sphincter) relaxes tension on the zonules and causes the lens to bulge for near vision. Relaxation of the ciliary muscle increases tension on the zonules, causing the lens to flatten for distance vision.

Question 11

Why do we need bifocals as we age?

Answer 11

As we age, we lose the ability to contract the ciliary muscles to the same extent.

Question 12

What is the role of the iris in vision?

Answer 12

The iris functions like a camera aperture, mechanically adapting to modulate how much light is allowed in (the pupil is the actual opening). This affects the brightness and quality of the image on the retina.

Question 13

How is the size of the pupil controlled?

Answer 13

The size of the pupil is controlled by two muscles, the pupillary sphincter and pupillary dilator. The sphincter surrounds the pupil and can contract by about 80%.

Question 14

Name two situations in which the pupil would contract.

Answer 14

• Bright light (smaller pupil decreases light and improves optical performance by decreasing scatter, like a smaller camera aperture).
• Heightened attention (“something important is happening happening, need visual acuity”)

Question 15

Describe the organization of the retina, in terms of cell layers (do not need to be able to name all 10, but general layout).

Answer 15

Five cell types, three somata zones, ten layers:

[OUTERMOST]
- Pigment epithelium (outside of eye, just under choroid)
- Receptor layer (rods and cones)
- Layer with lots of mitochondria (outer limiting membrane)
- Rods and cones’ cell bodies (outer nuclear layer)
- Synaptic zone (receptors terminate on bipolar and ganglion cells - outer plexiform layer)
- Inner nuclear layer (cell bodies of retinal interneurons - horizontal, bipolar, and amacrine cells)
- Inner plexiform layer (bipolar cells synapse on amacrine and ganglion cells)
- Ganglion cell layer
- Nerve fiber layer
- Inner limiting membrane (separates end of cells and vitreous humor)
[INNERMOST]

Question 16

The further from the fovea, the ______ rods and ______ cones.

Answer 16

The further from the fovea, the MORE rods and FEWER cones.

Question 17

How many synapses are there in the retina and what are they?

Answer 17

There are at least three synapses that occur:

1) Rods/cones > horizonal cells
2) Bipolar cells > ganglion cells
3) ganglion cells > axons that create the optic nerve

Question 18

What is the fovea?

Answer 18

The fovea is a small, central pit in the retina composed of densely packed cones in a direct line with the visual axis, specialized for high acuity vision.

Question 19

What is the blind spot?

Answer 19

The blind spot, or optic disk, is the location where the optic fiber bundle leaves through the optic nerve. There are no photoreceptor cells in this spot.

Question 20

What causes macular degeneration?

Answer 20

Macular degeneration = cones in the macula (spot with fovea at center) degrade, leading to blurry or no vision in the center of the visual field.

Question 21

What is the basis for highly detailed foveal vision?

Answer 21

Midget bipolar cells receive inputs from individual foveal cones, and synapse with individual midget ganglion cells.

Question 22

Rods and cones _______ without light, and ______ in the presence of light.

Answer 22

Rods and cones actually FIRE without light and STOP FIRING in the presence of light.

Question 23

On a very basic level, what are photopigments and how do they work?

Answer 23

Photopigments are G-coupled protein receptors that cause hyperpolarizing receptor potentials.

Question 24

Phototransduction happens the same in rods and cones, so why do rods function in dim light and cones function in bright light?

Answer 24

Opsins (receptor proteins) differ in terms of how they preferentially absorb light of different wavelengths. Rods and cones contain different opsins, with cones containing cone opsins and rods containing rhodopsin.

Question 25

What state are cation channels and photoreceptors in, in the (1) dark and (2) light?
(Hint: rods and cones fire without light and stop firing in the presence of light.)

Answer 25

In the dark:
    - Cation and K+ channels are open.
    - Photoreceptor is depolarized.
In the light:
    - Cation channels are closed
    - Photoreceptor is hyperpolarized as K+ cannot move out of the cell. 

remember: hyperpolarization inhibits firing of action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold

Question 26

What is the primary role of retinal neurons in vision, on the most basic level?

Answer 26

Retinal neurons translate patterns of light into patterns of contrast.

Question 27

Why do we need populations of cones to work together for our trichromatic vision?

Answer 27

There are 3 types of cones (red, blue, green) which each preferentially absorb long, middle, or short light wavelengths most efficiently.

Question 28

Why is colorblindness most common in males?

Answer 28

Red-green colorblindness (the most common kind) is more common in males because the genes for red and green cones are located next to each other on the X chromosome. Uneven crossing over during meiosis can result in one X chromosome having a missing or defective red or green gene.

Question 29

Describe the two types of ganglion cell receptor fields and their purpose in vision.

Answer 29

Ganglion cells are made up of two concentric circular areas. Illuminating the center causes an increase (ON-center) or decrease (OFF-center) in firing rate, while illuminating the surround causes the opposite. This allows us to detect contrast (light/dark, different colors).

Question 30

Describe the general pathway for vision, starting from ganglion cell axons.

Answer 30

Ganglion cell axons travel in the optic nerve to the optic chiasm, where they partially decussate and enter one of two optic tracts, terminating in the lateral geniculate nucleus of the thalamus. The LGN projects to the primary visual cortex (as the optic radiation).

Question 31

Vision involves the _______ geniculate nuclei, whereas hearing involves the _______ geniculate nuclei.

Answer 31

Lateral; medial

Question 32

How does each optic tract “look at” the contralateral visual field?

Answer 32

Information passing through the TEMPORAL side (of a vertical line through the fovea) enters the IPSILATERAL optic tract.
Information passing through the NASAL side (of a vertical line through the fovea) enters the CONTRALATERAL optic tract.

Question 33

Describe the two types of layers that make up the LGN and what they contribute to visual processing.

Answer 33

Magnocellular: Large ganglion cells – pulling info from a large area of retina (lots of rods/cones) = sensitive to movement and contrast. Layers 1-2.

Parvocellular: Small ganglion cells – pulling info from small areas of retina and cones = sensitive to color & form/fine detail. Layers 3-6.

Question 34

What is the function of the dorsal stream?

Answer 34

• Dorsal stream: “Where” – motion of self and environment

Question 35

What is the function of the ventral stream?

Answer 35

• Ventral stream: “What” – form/color/shapes/faces

Question 36

What is the function of the third stream (superior temporal sulcus)?

Answer 36

• Third stream (STS): “Integration” – fusiform face area; see/hear a dog that is barking (combo of visual and auditory); polysensory neurons; V1 to superior temporal sulcus

Question 37

Why does light directed into one eye cause the pupil of the other eye to constrict?

Answer 37

The pupillary light reflex:

• Optic tract fibers synapse on the pretectal area (just dorsal to the superior colliculi).
• Pretectal neurons project bilaterally to the Edinger-Westphal nucleus, which then project to the ciliary sphincter muscle via the oculomotor nerve.

Light directed in one eye causes the same amount of activity in the Edinger-Westphal nucleus on each side, because each optic tract contains ganglion cell axons from both eyes and because each pretectal area projects bilaterally to the E-W nucleus.

Question 38

How do both eyes automatically accommodate for near vision?

Answer 38

The near reflex occurs when a person directs their eyes from a distant (>30 ft. away) object to a nearby object:

• Eyes converge so that image falls on both retinas
• Lens thickens (accommodates) to focus the image on the retina
• Pupils constrict.
• Involves the cerebral cortex, though not fully understood.
• As normal, optic tract synapses on lateral geniculates, projecting to the visual cortex.
• From visual cortex to superior colliculus/pretectal area.
• Cortical efferents to the superior colliculi and the Edinger-Westphal nucleus then project via the occulomotor nerve to the medial rectus muscle (moves eyes inward) and pupillary sphincter (constricts pupil).

Question 39

Other than the primary visual cortex, where else do fibers of the optic tract terminate?

Answer 39

Some fibers of the optic tract terminate in the superior colliculus and superchiasmatic nucleus.

Question 40

What is the importance of the superior colliculi pathway?

Answer 40

This pathway is very good at orienting (head/eye movements) & tracking motion - important for survival. Superior colliculi receive somatosensory and auditory inputs in addition to visual.

Question 41

What is the importance of the superchiasmatic nucleus pathway?

Answer 41

Inputs to the superchiasmatic nucleus control circadian rhythms via cyclic release of hormones.

Question 42

Information from the retina is sent to the LGN via the _____ > ______ > ______.

Answer 42

Information from the retina is sent to the LGN via the optic nerve > optic chiasm > optic tract.

Question 43

Information from the LGN is sent to the primary visual cortex/V1/BA17 via the ______.

Answer 43

Information from the LGN is sent to the primary visual cortex/V1/BA17 via the optic radiation.

Question 44

The dorsal pathway involves the following visual areas:

Answer 44

V2, V3, V5.

Question 45

The ventral pathway involves the following visual areas:

Answer 45

V2, V4.

Question 46

Lesion of the optic nerve would cause:

Answer 46

Vision loss on the same side as the lesion, only in that eye.

Question 47

Lesion at the optic chiasm would cause:

Answer 47

Vision loss in both eyes, but only the temporal side of visual field.

Question 48

Lesion of the optic tract would cause:

Answer 48

Vision loss on the contralateral side of both eyes (so if L lesion, vision loss on R side of both eyes).

Question 49

What does “parallel processing” in vision refer to?

Answer 49

Parallel processing in vision refers to the fact that although visual information (orientation, color, depth, motion, etc.) is separated and sent to different extrastriate areas for further processing, we perceive all of those features simultaneously.