Retina and Vision

Question 1

4 factors that are necessary for an object to be seen?

Answer 1

1. Pattern of the object must fall on the vision receptors (rods and cones in the retina) - allows accommodation
2. Amount of light entering the eye must be regulated (too much light will “bleach out” the signals)
3. Energy from the waves of photons must be transduced into electrical signals
4. Brain must receive and interpret the signals

Question 2

Structure of the retina?

Answer 2

Inside-out laminar structure

Question 3

Direct (vertical) pathway for signal transmission via the retina?

Answer 3

Photoreceptors to bipolar cells to ganglion cells

IMAGE

Question 4

Importance of the lateral connection to the direct pathway?

Answer 4

Horizontal cells - receive input from photoreceptors and project to other photoreceptors and bipolar cells

Amacrine cells - receive input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells

Question 5

Function of the photoreceptors?

Answer 5

Converts electromagnetic radiation to neural signals (transduction)

Question 6

4 main regions of the photoreceptors?

Answer 6

• Outer segment
• Inner segment
• Cell body
• Synaptic terminal

Question 7

2 types of photoreceptors?

Answer 7

Rods

Cones

Question 8

Resting membrane potential of a photoreceptor?

Answer 8

Have a DEPOLARISED rmp of -20 mV; compared to other neurons, the resting Vm is more positive

Question 9

What happens to the membrane potential when there is light exposure?

Answer 9

Vm hyperpolarises

Question 10

Why is the Vm +ve in a photoreceptor?

Answer 10

Due to the ‘dark current’ - a cGMP gated Na+ channel that is:
• OPEN in the DARK
• CLOSED in the LIGHT

Question 11

Why is the ‘dark current’ important for vision?

Answer 11

Change in Na+ with light is the signal that enables the brain to perceive objects in the visual field

Question 12

Action of the dark current in the dark?

Answer 12

Permeability of Na+ and K+ are equal and thus the Vm is between ENa and EK

Question 13

Action of the dark current in response to light?

Answer 13

Na+ channels close so permeability decreases to below that of K+

Thus, hyperpolarisation occurs (driving the Vm towards the equlibrium potential of K+); this is local and graded (the more light there is, the more hyperpolarisation occurs?)

Question 14

What are the visual pigment molecules?

Answer 14

Rhodopsin (for rods):
• 11-cis-Retinal (vitamin A derivate) + Opsin (GPCR)

These are present in the membrane folds

Question 15

Action of light on rhodopsin?

Answer 15

Converts 11-cis-retinal to all-trans-retinal (activated form)

Question 16

Mechanism of action of all-trans-retinal?

Answer 16

Activates transducin, causing a molecular cascade

cGMP decreases leading to closure of cGMP-gated Na+ channels

Lowered Na+ entry causes hyperpolarisation

Question 17

Explain what is meant by high gain mechanism in phototransduction?

Answer 17

Amplification means that even minute changes in light can change the membrane potential of photoreceptors, i.e: 1 opsin leads to 1000 transducin OR 1 PDE leads to 1000 cGMP

Question 18

What is the basis of phototransduction?

Answer 18

Dark current channel which is a nucleotide-gated channel (opened by cGMP) and is permeable to Na+

It opens in the dark and closes in response to light

It keeps the photoreceptor Vm more positive than other neurons and there is a steady release of neurotransmitter (more glutamate in the dark and less in the light)

Question 19

Define visual acuity? How is it determined?

Answer 19

Ability to distinguish two nearby points; determined largely by photoreceptor spacing and refractive power

Question 20

What do rods and cones allows?

Answer 20

Rods - seeing in dim light

Cones - seeing in normal daylight

Question 21

Which photoreceptors have less acuity?

Answer 21

Rods - they have more convergence, allowing increased sensitivity but acuity is decreased; this is what allows sight in dim light

Question 22

What is meant by high convergence in peripheral rods?

Answer 22

They have larger spacing, i.e: there is a lower density of rods, allowing more convergence on a large ganglion cell

Question 23

What is meant by low convergence in foveal cones?

Answer 23

They have smaller spacing, i.e: there is a high density of cones; there is less convergence on the small ganglion cell but acuity is increased

Question 24

Basis of colour vision?

Answer 24

Light is comprised of discrete wavelengths and photoreceptor are only activated by a small portion (not infrared or UV)

Question 25

What is basis of different colours being seen?

Answer 25

Different opsins perceive different wavelength, e.g: there are 4 types of photoreceptors in the human retina:
• Short-wave cone (blue)
• Middle-wave cone (green)
• Long-wave cone (red)
• Rod

Different shades of colour be seen

Question 26

5 differences between rods and cones?

Answer 26

Rods:
• Achromatic
• Peripheral retina
• High convergence
• High light sensitivity
• Low visual acuity

Cones:
• Chromatic
• Central retina (fovea)
• Low convergence 
• Low light sensitivity
• High visual acuity

Question 27

In essence, what does out visual system actually perceive?

Answer 27

Local differences in light intensity, not the absolute amounts of light

Question 28

What does each eye see?

Answer 28

A part of the visual space, i.e: monocular visual field (+/- 45 degrees)
These overlap extensively to create a binocular visual field (+/- 45 degrees)

Question 29

How is the retina divided?

Answer 29

In half, relative to the fovea, into:
• Nasal hemiretina
• Temporal hemiretina

Question 30

Pathway of the nerve fibres and optic tract?

Answer 30

Nerve fibers from the nasal half of each retina cross over at the optic chiasm

Resulting 2 optic tracts allow right and left visual fields to reach the left and right hemispheres separately:
• 60% (nasal retina) cross
• 40% (temporal) do not

Question 31

What is meant by retinotopy?

Answer 31

Image is picked up at the photoreceptor and is NOT MIXED; it goes all the way to the striate cortex

Question 32

Where is the visual field mapped?

Answer 32

In the retina, lateral geniculate nucleus (LGN) and cortex

Question 33

Where are eye specific inputs segregated in the brain?

Answer 33

In the primary visual cortex, more specifically in layer 4

Both eyes project to each visual cortex but, at the primary visual area (17), they remain largely segregated into ocular dominance columns, i.e: each column is dominated by input from 1 of the 2 eyes

Cells outside of layer 4 receive input from both eyes

Question 34

How is visual perception shaped?

Answer 34

Based on the brain’s interpretation of distributed patterns of activity

Visual perception is shaped by early activity

Question 35

Consequences of congenital cataracts and treatment?

Answer 35

Opaque covering of lens results in impaired vision from birth; they are typically removed between 10-20 years of age

They have difficulty perceiving shape and form

Question 36

What is amblyopia?

Answer 36

AKA cortical blindness - variety of visual disorders where there is not issue with the eye but 1 eye has better vision than the other; causes include:
• Strabismus (wandering eye) if not corrected in infancy

Question 37

Treatment of wandering eye?

Answer 37

Often surgically corrected but delayed until the child is old enough

Until then, cover the infant’s better eye for a few hours each day, making the brain rely on signals from the affected eye; allows the brain to develop properly to process signals from both eyes

Question 38

What is Hebb’s postulate?

Answer 38

Axon of cell A is near enough to excite cell B, and repeatedly/persistently takes part in firing it, some growth process or metabolic change occurs in one/both cells

Results in cell A’s efficiency, as once of the cells firing cell B, increasing

Correlated activity between pre-synaptic and post-synaptic cells strengthens synaptic connections between them;
cells that fire together, wire together

Question 39

Describe long-term potentiation (LTP)

Answer 39

Hebb’s postulate is also important for learning and memory

Question 40

How are the terminal arborisation (branches) of LGN axons affected in monocular deprivation?

Answer 40

Lack of visual activity leads to less branching, i.e: there are less branches in the deprived eye