8.4. Physiology of vision.

Question 1

I. Structure of the eye
3A. Where can you find aqueous humor?

Answer 1

The anterior and posterior chambers of the eye maintain a circulation of aqueous humor fluid that is produced by the ciliary body and drained
through Schlemm’s canal at the corneo-scleral junction

Question 2

I. Structure of the eye
3B. What is the role of aqueous humor?

Answer 2

Aqueous humor is necessary to give nutrients to the lens and cornea, as well as to create refractive power for the eye

Question 3

I. Structure of the eye
3C. What happen if there is a problem in the draining of the aqueous humor

Answer 3

Any problem that occur in the draining of the aqueous humor can result in glaucoma
-> in which the intraocular pressure increases and eventually presses on the optic nerve, causing blindness

Question 4

I. Structure of the eye
4A. What does the middle vascular layer include?

Answer 4

The vascular coat of the eye includes the ciliary body, choroid and iris

Question 5

I. Structure of the eye
4B. What are the features of the choroid (Middle vascular layer)?

Answer 5

The choroid is the most vascular part of the body, and proportionally has a higher
blood + oxygen supply than even the kidneys

Question 6

I. Structure of the eye
4C. Describe the oxygen saturation of the vascular coat?

Answer 6

The oxygen saturation of the vascular coat is 90%, while the retinal artery is only 45%
-> but even then, the eye is always on the verge of partial hypoxia (as indicated by the amount of lactate found in the aqueous humor)

Question 7

II. Image formation of the eyes
1. What is the image formation based on?

Answer 7

The image formation is based on the combined refractive power of the cornea, aqueous humor, lens and vitreous body (refractive power – D = 1/focal length)

Question 8

II. Image formation of the eyes
2. Where does refractive power come from?

Answer 8

The majority of the refractive power comes from the cornea, but the lens provides a refractive power that can be adjusted

Question 9

II. Image formation of the eyes
3. Explain the properties of convex lens

Answer 9

The refractive power of all these layers combined acts as a convex lens that produces an inverted image on the retina for visualization

Question 10

II. Image formation of the eyes
4. What is the role of lens and ciliary muscles in image formation?

Answer 10

• Since the majority of the refractive power of the eye is given by the cornea, the lens is meant to tune the eye so that ideally, the inverted image is projected perfectly on the retina.
• The ciliary muscles contract or relax to adjust the lens according to what the eye is looking at and how far away it is => accommodation (compensation for the distance of the object).

Question 11

II. Image formation of the eyes - Accommodation
5A. Describe the accommodation when viewing at a distant object

Answer 11

When viewing at a distant object => lens must be flat (↑ focal length)
- The ciliary muscles are relaxed
- Zonular fibers are tightened
- Lens is flattened

Question 12

II. Image formation of the eyes
5B. Describe the accommodation when viewing at a close object

Answer 12

When viewing at a close object -> lens must be more round (↓ focal length = ↑D)
- Ciliary muscles are contracted
- Zonular fibers are relaxed
- Lens is contracted -> reduced focal length -> clear view of close object

Question 13

III. The retina
1. What is retina?

Answer 13

The retina is a multi-cellular layered part of the eye that detects light and sends this message through the optic nerve to the CNS to process these signals into the actual image that you are looking at.

Question 14

III. The retina
2. What is retina made up?

Answer 14

The retina is made up of multiple neurons in a circuit as well as glial cells and a pigmented epithelium.

Question 15

III. The retina
3A. What are Müller cells?

Answer 15

Long glial cells that stretch almost the entire length of the retina

Question 16

III. The retina
3B. What are the 6 functions of Müller cells?

Answer 16

1. Function to maintain the EC environment by regulating K+-levels
2. Uptake of neurotransmitters
3. Remove debris
4. Store glycogen for energy
5. Form an electrical insulation
6. Provide protection of the neurons

Question 17

III. The retina
4A. What are the 5 functions of pigmented epithelium?

Answer 17

1. Absorbs excess light
2. Allows transport of vitamin A, chloride and water
3. Helps the retinal visual cycle by converting all trans-retinal to 11-cis-retinal for delivery to the photoreceptors
4. Phagocytoses the apical membrane disks of the rod cells
5. Acts similarly to glial cells in the absorption and secretion of K+

Question 18

III. The retina
4B. What does it mean when Pigmented epithelium absorb excess light?

Answer 18

A rod can absorb a single photon of light, so it is important to absorb any excess light to avoid overstimulation
- People with albinism lack this layer and thus light refraction within the eye leads to difficulty in vision

Question 19

III. The retina
5. What are the 8 layers of the retinal neurons?

Answer 19

1. Pigmented cell layer
2. Rods + cones
3. Outer nuclear layer – cell bodies of rods + cones
4. Outer plexiform layer – axons of rods + cones, horizontal + bipolar cells
5. Inner nuclear layer – cell bodies of horizontal cells + bipolar cells
6. Inner plexiform layer – axons of bipolar, amacrine and ganglion cell dendrites
7. Ganglionic cell layer – cell bodies of ganglion cells
8. Optic nerve fibers – long axons of ganglion cells which all verge to form the optic N
   => Light goes from layer 8 and upwards (opposite in picture)

Question 20

III. The retina
6. What happen when light hit retina?

Answer 20

When light hits the retina, it passes all layers until it reaches the rods + cones (photoreceptors).
=> Rods + cones both function differently and are located in different parts of the retina.

Question 21

III. The retina - RODS
7. What are rods?

Answer 21

Rods are photoreceptor cells that function to detect light

Question 22

III. The retina - RODS
8. What are features of rods?

Answer 22

• They are extremely sensitive to light due to the high amount of rhodopsin in the cell, and thus are useful for vision in darkness
• Rods have low acuity, because many rods synapse on a single bipolar cell -> light hitting any part of the retina will activate the rods but the visual acuity (clarity of vision) will not be very good

Question 23

III. The retina - RODS
8. What is the location of rods?

Answer 23

Rods are primarily in the peripheral part of the retina, while the macula contains mostly cones

Question 24

III. The retina - RODS
9. Describe the physical shape of rods

Answer 24

• Rods are long with the outer segment being rod-like in shape and stacks of free-floating membrane discs that
  contain large amounts of rhodopsin (light-sensitive pigment) in the membrane
• Outer segment is connected to inner segment (lots of mitochondria here) by a single cilium
• Cell body follows with the axon and synaptic terminal

Question 25

III. The retina - CONES 10. What are cones?

Answer 25

Cones are photoreceptor cells responsible for color vision

Question 26

III. The retina - CONES 11. What are the characteristics of CONES?

Answer 26

- They are not as sensitive to light as rods, but have much higher visual acuity since only a few cones synapse on a bipolar cell - Cones are located in the macula, with the fovea having the highest density of cones and thus the highest visual acuity

Question 27

III. The retina - cones 12. Describe the physical shape of cones

Answer 27

- The outer segment is cone-shaped, with an infolded membrane containing rhodopsin - Structure of the cone is similar to rods when it comes to the inner segment and cell body

Question 28

III. The retina - cones 13. Why are cones less sensitive to light?

Answer 28

Since there is less rhodopsin here, it is not very sensitive to light -> several hundred photons are needed to activate one cone

Question 29

III. The retina - cones 14. List different kinds of cones?

Answer 29

There are different kinds of cones: red, green or blue cones -> absorb colored light as described

Question 30

III. The retina - cones 15. What is Rhodopsin (light-sensitive pigment)?

Answer 30

Rhodopsin (light-sensitive pigment) is a molecule consisting of opsin + retinal

Question 31

IV. Photoreception 1. What is the general mechanism of photoreception?

Answer 31

- This process takes light and turns it into electrical energy in rods + cones – using rhodopsin (visual pigment - GPCR) composed of opsin (7TM protein) + retinal - When light hits the rhodopsin molecule  retinal chemically transformed to begin photoreception process - When light hits a photoreceptor cell, it hyperpolarizes the cell due to the closure of Na+-channels - This hyperpolarization will lead to activation + deactivation of bipolar and horizontal cells to create a signal - In the dark, there is a depolarization of the photoreceptor cells and a strong inward Na+-current = dark current

Question 32

IV. Photoreception 2A. What are the 8-step process of photoreception?

Answer 32

1) Light hits rhodopsin, causing conformational change: 11-cis retinal -> all-trans retinal -> meta-rhodopsin II 2) Meta-rhodopsin II activates transducin (Gt = heterotrimeric G- protein) 3) Transducin activates cGMP-PDE (phosphodiesterase) 4) PDE degrades cGMP into 5’-GMP -> ↓[cGMP] 5) Decrease in cGMP -> closure of CNG (cyclic-nucleotide gated) channels = Na+-influx↓ 6) Closure of Na+-channels = hyperpolarization + less glutamate release 7) Hyperpolarization affect bipolar + horizontal cells 8) This inhibition and activation produces an on- off pattern which is necessary to form visual receptive fields

Question 33

IV. Photoreception 2B. When light hits rhodopsin, causing conformational change. => What is this change?

Answer 33

11-cis retinal -> all-trans retinal -> meta-rhodopsin II

Question 34

IV. Photoreception 2C1. Hyperpolarization affect bipolar + horizontal cells in 2 ways. What are they?

Answer 34

1. Cells with inotropic receptors respond to decreased Glu with a decreased excitatory Glu response 2. Cells with metabotropic receptors respond to decreased Glu release with a decreased inhibitory Glu response.

Question 35

IV. Photoreception 2C2. Cells with inotropic receptors respond to decreased Glu with a decreased excitatory Glu response. How?

Answer 35

Cells with inotropic receptors respond to decreased Glu with a decreased excitatory Glu response. => ionotropic receptors will be inhibited by photoreceptor hyperpolarization => hyperpolarization of bipolar + horizontal cells = inhibition

Question 36

IV. Photoreception 2C3. Cells with metabotropic receptors respond to decreased Glu release with a decreased inhibitory Glu response. How?

Answer 36

Cells with metabotropic receptors respond to decreased Glu release with a decreased inhibitory Glu response. => metabotropic receptor cells will be activated by photoreceptor hyperpolarization => depolarization of bipolar + horizontal cells = excitation

Question 37

V. Visual receptive fields 1. What is lateral inhibition?

Answer 37

1. Lateral inhibition is seen in the visual receptive field via the on-off patterns of bipolar cells in order to increase the visual clarity of the image. This process deals with a ‘’center’’ and a ‘’surrounding’’ region of bipolar cells that is represented by: - Photoreceptor cells that are directly in contact with the bipolar cell = central part - Cells that are indirectly touching bipolar cells via horizontal cell = surrounding part 2. Based on the idea that ionotropic receptors cause inhibition and metabotropic receptors cause activation, this is what decides if something ‘’on’’ or ‘’off’’ - ‘’on’’ = excitation, ‘’off’’ = inhibition - If the center of a cell is ‘’on’’, then the inhibitory horizonal cells will always reverse the surrounding area to be opposite => ‘’off’

Question 38

VI. Color vision 1. What is color determined?

Answer 38

Color is determined by the relative absorption by the three cone photopigments 1. ‘’blue’’ cone S 2. ‘’green’’ cone M 3. ‘’red’’ cone L

Question 39

VI. Color vision 2. State Trichromacy theory

Answer 39

Trichromacy theory: suitable mixture of 3 colors can produce all colors (explained by the 3 different cone photopigments: S, M, L)

Question 40

VII. Explain Visual pathways

Answer 40

- 120 million rods and 6 million cones - 1 million fibers in the optic nerve (convergence) - LGN (lateral geniculate nucleus – thalamus) -> optic radiation -> primary visual cortex (pathway for visual perception) - Colliculus superior: controls eye movements - Pretectal area: pupillary reflexes - Suprachiasmatic nucleus (hypothalamus): circadian rhythm

Question 41

VIII. Optic pathways and lesions 1. Explain optic pathways

Answer 41

- The nasal (purple) and temporal (green) retina are represented contralateral and ipsilateral - The primary visual cortex is at the occipital lobe (Br. 17)

Question 42

VIII. Optic pathways and lesions 2. Describe the lesion on the optic radiation

Answer 42

A lesion on the optic radiation (geniculocalcarine tract on picture) will cause vision on the contralateral side, but the macule will remain functional (number 4) - Number 2: bitemporal hemianopia (heteronymous) - Number 3: right homonymous hemianopia