Visual system

Question 1

Place the following anatomical parts of the eye in the right order: retina, retinal pigment epithelium, cornea, lens, pupil.

Answer 1

Cornea > pupil > lens >retina > retinal pigment epithelium.

Question 2

Where does the retina get thinner?

Answer 2

In the fovea.

Question 3

What are the mechanisms of the eye to adjust vision?

Answer 3

• Pupil size (modulating how much light enters the eye)

- Lens’ thickness (modulated by the ciliary muscles to adjust the refractive intensity of the eye)

Question 4

What vision problem occurs from the lens losing too much elasticity (i.e. accommodation power decreasing, often with age)?

Answer 4

Presbyopia.

Question 5

What is color blindness caused by?

Answer 5

Missing a cone type.

Question 6

What vision problem arises from spherical aberrations of the eye (creating multiple focal points)?

Answer 6

Astigmatism

Question 7

What is the difference between macula and fovea?

Answer 7

Macula is the part of the retina that is responsible for high resolution vision. Fovea is the central part of macula, responsible for the highest resolution vision.

Question 8

What are the 5 main neuronal cell types found in the eye?

Answer 8

• Photoreceptors (rods AND cones)
• Bipolar cells
• Ganglion cells
• Horizontal cells
• Amacrine cells

Question 9

True of false: rods vastly outnumber cones.

Answer 9

True.

Question 10

Give three reasons for which visual acuity is increased in the fovea.

Answer 10

1. Skinnier cones allow them to be densely packed.
2. Rest of retinal cells are pushed to the side (makes some space for light to hit directly the cones).
3. Avascular (no blood vessels), so light directly hits the cones of the fovea and there’s more room to stack them.

Question 11

Who (what cell), in the first place, generates the visual signal?

Answer 11

Photoreceptors.

Question 12

Photoreceptors fire graded potentials, not APs, and it is thought to still be efficient. Why?

Answer 12

Electrical impulses only have a very short distance to travel (against impulses travelling along long axons of PNS).

Question 13

What is counter-intuitive about the excitation of photoreceptors?

Answer 13

Photoreceptors stop (or drastically slow the rate of) releasing Glu in presence of light because they become hyperpolarized.

Question 14

Describe the movement of Na+, Ca++ and K+ when levels of cGMP are high and low in the membrane of a photoreceptor.

Answer 14

In the dark, levels of cGMP are high, opening the cGMP-gated channel for the inward flow of Na+ and Ca+: Na+ and Ca2+ are flowing IN, K+ is flowing OUT = membrane slightly depolarized

In the presence of light, cGMP levels are low, closing the cGMP-gated channel for the inward flow of Na+ and Ca+: Na+ and Ca2+ are NOT flowing IN, K+ is flowing OUT = membrane hyperpolarized

Question 15

Describe the phototransduction cascade in rods (from light hitting the photoreceptor to hyperpolarization).

Answer 15

Light hitting retinal molecules in membrane-attached opsin proteins -> retinal molecules go through a conformational change (11-cis to all-trans conf.) -> activation of rhodopsin channels -> transducin activated (G prot.) -> PDE (phosphodiesterase) activated -> cGMP hydrolyzation ->low [cGMP] -> closure of cGMP-gated channels ->Na+ and Ca++ no more flowing in -> photoreceptor membrane hyperpolarized.

Question 16

Provide the 5 steps of the retinoid cycle.

Answer 16

1. All-trans retinal dissociates from opsin and diffuses in cytosol
2. All-trans retinal converted to all-trans retinol
3. All-trans retinol shuttled to retinal pigment epithelium via IRBP
4. All-trans retinol converted to 11-cis retinal
5. 11-cis retinal shuttled back to opsin by IRBP

Question 17

What are the 3 cone types?

Answer 17

Short wavelength -> blue light
Medium wavelength ->green light
Long wavelength ->red light

Question 18

From what does colour arise?

Answer 18

Comparing contrasting responses of different cone types.

Question 19

Name 4 differences between OFF and ON bipolar cells of the cones.

Answer 19

OFF:

• Ionotropic
• Depolarized when photoreceptors fire in the dark
• Connect to OFF ganglion cells
• Stratify in upper layers of IPL (above 2-3)

ON:

• Metabotropic
• Depolarized when photoreceptors fire in the light
• Connect to ON ganglion cells
• Stratify in deeper layers of IPL (under 3)

Question 20

Bipolar cells fire APs or graded potentials?

Answer 20

Graded potentials.

Question 21

True or false: what happens in the photoreceptors inversly happens in the ON bipolar cells (in terms of graded potentials).

Answer 21

True

Question 22

In proportion to photoreceptors, the distribution of bipolar cells is specially low in one emplacement of the retina. What emplacement is that?

Answer 22

The fovea.

Question 23

How is the intensity of light conveyed in electrical signal?

Answer 23

By the graded signals frequency.

Question 24

True or false: rods only have ON bipolar cells.

Answer 24

True.

Question 25

True or false: rods have a direct pathway to ganglion cells.

Answer 25

False: rod signals hijack the cone bipolar system to reach its ganglion cells.

Question 26

OFF-center ganglion cells are ____ by light in their ____ receptive field and ____ by light in their ____ receptive field, whereas ON-center ganglion cells are ____ by light in their ____ receptive field and ____ by light in their ____ receptive field. Place: excited x2 inhibited x2 surround x2 center x2

Answer 26

OFF-center ganglion cells are excited by light in their surround receptive field and inhibited by light in their center receptive field, whereas ON-center ganglion cells are excited by light in their center receptive field and inhibited by light in their surround receptive field.

Question 27

What cells can provide lateral inhibition to photoreceptors, bipolar cells and ganglion cells?

Answer 27

Horizontal cells provide negative feedback to photorecetors. | Amacrine cells can provide lateral inhibition to bipolar cells and ganglion cells.

Question 28

How are center-surround receptive fields determined?

Answer 28

By the negative feedback of horizontal cells.

Question 29

Horizontal cells have the biggest receptive field, how does that correlate with their role?

Answer 29

They need to possess a large receptive field to impart the center-surround receptive fields to photoreceptors.

Question 30

How do relate luminance, light intensity and brightness?

Answer 30

Luminance is the physical property of light intensity and brightness is its perception.

Question 31

By what is imparted the direction selective character of some ganglion cells?

Answer 31

By their asymmetrical lateral inhibition caused by the Starburst amacrine cells.

Question 32

What are the three predominant ganglion cells in the human retina?

Answer 32

Parasol, midget and bistratified.

Question 33

What is a concrete example of colour opponency in the human eye?

Answer 33

Red-on-green-off midget ganglion cells.

Question 34

True or false: majority of the axons of the optic nerve cross over the midline.

Answer 34

True (60%).

Question 35

Link the following brain regions to the role they correspond to. Brain regions: - Hypothalamus - Superior colliculus - Primary visual cortex Roles: - Conscious vision - Head and eye movements to visual targets - Regulation of circadian rhythm

Answer 35

Regulation of circadian rhythm - Hypothalamus Head and eye movements to visual targets - Superior colliculus Conscious vision - Primary visual cortex

Question 36

Describe the pupillary light reflex pathway.

Answer 36

Some ganglion cells (responding to photorecpetors expressing melanopsin) send their axon to prectectum via optic nerve → projecting axons to EWN (Edinger-Westphal nuclei) in midbrain ipsilaterally and contralaterally → projecting axons via oculomotor nerve to ciliary ganglion cells → ciliary ganglion cells innervate constrictor muscle of iris (BOTH pupils respond to MONOcular visual stimulation)

Question 37

Describe the overall pathway of conscious vision (primary visual pathway).

Answer 37

Optic nerve -> LNG -> V1 (primary visual cortex)

Question 38

The optokinetic reflex is composed of two distinct eye movements. What are they? What ganglion cells are essential to this reflex?

Answer 38

Saccades and smooth pursuits. | Direction selective ganglion cells.

Question 39

Temporal retina is situated ____ while nasal retina is situated ____. Nasal retina processes principally the ____ visual field while temporal retina processes principally the ____ visual field. Words to place: proximally central distally lateral

Answer 39

Temporal retina is situated distally while nasal retina is situated proximally. Nasal retina processes principally the lateral visual field while central retina processes principally the central visual field.

Question 40

True or false: images entering the eye are inverted on two axes.

Answer 40

True (vertically and horizontally).

Question 41

True or false: monocular vision exists in the human eye.

Answer 41

True: in the extremities of the visual field.

Question 42

True or false: inferior retinal quadrants and superior retinal quadrants take the same path from LGN to V1.

Answer 42

False: different paths.

Question 43

For every layer of LGN (and interlayer), describes its ganglion cell type, what visual property it is responsible for and where it projects in V1.

Answer 43

Ventral layers (1-2): - Magnocellular type - Motion sensitive - Projet to layer 4Cα in V1 Dorsal layers (3-4-5-6): - Parvocellular type - Color sensitive - Project to layer 4Cβ in V1 Between every layer of LGN: - Koniocellular type - Unclear function - Project to patches in layer 2/3 of V1

Question 44

Which layer of the primary visual cortex is the main input layer for vision?

Answer 44

Layer 4C.

Question 45

What are the cortical divisions of V1?

Answer 45

``` Layers... 1 2/3 4A 4B 4Cα 4Cβ 5 6 ```

Question 46

What is the main difference between superficially and deeply located pyramidal cells in V1?

Answer 46

Superficially located pyramidal cells project to other cortical areas. Deeply located pyramidal cells project to subcortical targets (LGN and superior colliculus).

Question 47

What is an an orientation column in V1?

Answer 47

Cells immediately above or below one another in different cortex layers taht process the SAME location in the visual field and share similar selective responses.

Question 48

What are orientation pinwheels in V1?

Answer 48

In every receptive field composing the visual field, all orientation-selective cells are present, forming a repeating "pinwheel" pattern in V1.

Question 49

What is stereopsis? Where does it anatomically arise?

Answer 49

Perception of depth driven by reception of visual information deriving from both eyes in the brain. Arises in V1 (not at the eye level)

Question 50

List 4 levels of organization in the visual system.

Answer 50

- Retinotopic maps - Orientation pinwheels - Ocular dominance columns - Color blobs

Question 51

What are the ventral and dorsal pathways (their steps)? What are the key words we can assign to them?

Answer 51

Ventral: V1 → V2 → V4 → temporal lobe "What" pathway Dorsal: V1 → V2 → MT → parietal lobe "Where" pathway

Question 52

What are the two types of strabismus?

Answer 52

Esotropia and exotropia.

Question 53

What condition can result from untreated strabismus?

Answer 53

Amblyopia.

Question 54

What is MT in the dorsal stream known for? What is V4 in the ventral stream known for?

Answer 54

MT is known for its sensibility to motion. | V4 is known for its high percentage of color selective cells.

Question 55

What is the aperture problem concerning V1? How is it overcame?

Answer 55

The visual receptive field of V1 cells is small, making it likely to provide misleading information about direction of movement (only integrates a part of the movement). The problem is overcame by the integration in MT of the V1 neurons' inputs.

Question 56

What differentiates cataract, glaucoma, macular degeneration, retinitis pigmentosa, diabetic retinopathy from blindsight, hemispatial neglect, cerebral akinetopsia and prosopagnosia?

Answer 56

The first conditions are attributable to eye problems, while the others are attributable to higher order processing deficiencies (in the brain).

Question 57

What is the first leading cause of blindness?

Answer 57

Cataract

Question 58

What is the cause of glaucoma?

Answer 58

Diseases damaging optic nerve (slow onset).

Question 59

What is macular degeneration affecting?

Answer 59

Photoreceptors in macula and and fovea. In the case of wet AMD, leaky blood vessels (in macula) disturb connection between retina and retinal pigment epithelium, obstructing photoreceptors.

Question 60

What is the cause of retinitis pigmentosa and what are its consequences?

Answer 60

Caused by an heterogenous group of hereditary diseases. Causes photoreceptor degeneration in the peripheral retina -> creates tunnel vision and night-blindness.

Question 61

In what does result diabetic retinopathy?

Answer 61

Macular oedema (blood vessels leaking their content on retina).

Question 62

What is the cause of blindsight?

Answer 62

Damage to primary visual cortex = conscious blindness. Other forms of subconscious vision can still persist.

Question 63

True or false: hemispatial neglect arises from the damage to the left eye.

Answer 63

False: most often arises from strokes and brain injury occuring unilaterally in the right cerebral hemisphere in the parietal lobe.

Question 64

What is the link between cerebral akinetopsia and middle temporal area (MT)?

Answer 64

Cerebral akinetopsia, known as motion blindness, arises from damage to MT.

Question 65

What does cause the damage or the improper development of face selective visual regions in the inferotemporal cortex?

Answer 65

Prosopagnosia.

Question 66

What are 4 known vision restoration techniques?

Answer 66

Cell therapy, gene therapy, retinal prosthetic implants and optogenetic therapy.

Question 67

How is transducin inactivated?

Answer 67

Arrestin.