Vision: The Eye

Question 1

Anatomy of the human eye

Answer 1

• fluid filled sphere
• 3 layers of tissue
• only the retina contains neurons
• ciliary body encircles the lens: ciliary muscle adjusts the lens and ciliary processes supply eye with fluid
• iris: coloured region that contains two sets of muscles to adjust pupil size
• pupil: the opening in the centre
• sclera: outer most tissue layer
• cornea: transparent tissue that permits light rays to enter the eye

Question 2

accommodation in the human eye

Answer 2

retina contains all the neurons

being able to adjust that lens gives us the ability to make sure the image falls perfectly on the retina

Question 3

accommodation and disorders

Answer 3

• emmetropia (normal): relaxed ciliary muscles results in focused image in the distance
• myopia (nearsighted): light rays are focused in front of the retina
• hyperopia (farsighted): light rays are focused beyond the retina

Question 4

macular degeneration =

Answer 4

the creation of blind spots, reduced acuity, etc

Question 5

photoreceptors for colour are only found in

Answer 5

the small foveal region

Question 6

patients with glaucoma

Answer 6

the mind fills in the blindspot - but if you have a glaucoma in both eyes, it is more difficult for the brain to fill this in

Question 7

structure of the retina 1

Answer 7

FIVE BASIC CLASSES OF NEURONS

• Photoreceptors: rods and cones (they synapse onto bipolar cells which synapse onto ganglion cells which head out the optic nerve to the brain)
• Bipolar cells
• Ganglion cells
• Horizontal cells: modulate interaction between photoreceptors and bipolar cells
• Amacrine cells: modulate interactions between photoreceptors and ganglion cells

2 TYPES OF PHOTORECEPTORS

• rods
• cones

Question 8

Order of the retinal process

Answer 8

Photoreceptor
(horizontal cell modulation - allows sensitivity to a wide range of illumination)
Bipolar cell
(amacrine modulation - modulates the visual response by allowing the response to be more sophisticated)
Ganglion cell
(ganglion axons form optic nerve)
Brain

Question 9

Structure of rods v cones

Answer 9

Rod: found more in the periphery and are not sensitive to one specific wavelength of light - they support our low light vision

Cones: found primarily in the central foveal region –> 3 types of cones that detect light at different wavelengths, giving us colour vision

Question 10

the physiological scotomoa is

Answer 10

a blind region where the ganglion cell axons leave the retina in the optic nerve

Question 11

Why are the photoreceptors in the deepest layer?

Answer 11

1. Photoreceptor disks ‘wear out’ (occurs over 12 days) and so continuously move up the photoreceptor until they reach the top, are pinched off, and then recycled (phagocytosed)
2. Pigment epithelium regenerates the photopigment after exposure to light.
3. Blood flow in pigment epithelium supplies photoreceptors

Question 12

Phototransduction: absorption of light by the photopigment results in change in membrane potential

Answer 12

• photoreceptors do not exhibit action potentials (graded response)
• light activation causes a graded change in membrane potential
• shining a light on a photoreceptor results in hyperpolarization (meaning less transmitter is released onto the post synaptic neurons) –> as in the dark, the receptor is naturally depolarised

Question 13

What changes the membrane polarisation?

Answer 13

The same thing happens in rods and cones

IN THE DARK:

• large amount of cGMP floating around the outer segment –> resulting in a lot of open channels that depend on cGMP (cations)
• cations (particularly Na) enter into the cell (depolarising influence)
• K flows out consistently (hyperpolarising) but the overall cell is DEPOLARISED

LIGHT HITS:

• in the outer segment, photons are absorbed –> decreases cGMP
• many cGMP gated channels close
• cations can’t enter cell
• reduced inward current (less depolarising)
• the inner segment isn’t affected by cGMP therefore K keeps flowing out of the cell
• therefore HYPERPOLARISED cell

Question 14

What happens when a photon is absorbed?

Answer 14

Biochemical process when light hits

• photon –> absorbed by retinal –> changes the configuration –> causes changes to opsin which:
• activates transducin
• activates PDE
• hydrolyses cGMP therefore less cGMP

Question 15

Signal Amplification

Answer 15

1 photon of light results in 1 activated photopigment molecule that activates up to 800 transducing molecules which each activates a single PDE and breaks down 6 cGMP, which closes up to at least 200 ion channels

Question 16

the retinoid cycle

Answer 16

restores the retinal to the form that it can capture a photon

activated photopigment is phosphorylated by a kinase:

• allows arrestin to bind
• prevents the activation of transducin
• stops the process

Question 17

Rods and cones differ in:

Answer 17

1. shape
2. photopigment
3. pattern of synaptic connections - which determines spatial acuity
4. distribution across retina (rods in the periphery, cones in the fovea)

Question 18

2: Photopigment (and response characteristics)

Answer 18

rods = rhodopsin –> illumination

cones –> colour, acuity

Question 19

3. pattern of synaptic connections (spatial acuity)

Answer 19

Rods:
- convergence from rods to rod bipolar cells: pools the signal –> allows for low light vision
- rod bipolar cells synapse on amacrine cells, which synapse on bipolar cells and ganglion cells
whereas
Cones:
- one cone to one cone bipolar cell - and then straight to ganglion cells (less sensitive, but high spatial acuity)

Question 20

4. distribution of photoreceptors in the human retina

Answer 20

Cones: throughout at low density, packed in fovea, high spatial acuity

Rods: throughout periphery, none in centre, low spatial acuity

Question 21

Photopic vision is

Answer 21

vision supported by photoreceptors

Question 22

The fovea is

Answer 22

a region of the retina packed densely with cones

Question 23

Cones and colour vision

Answer 23

• rods contain a single photopigment (rhodopsin)
• ocnes can contain 3 different types of photopigment (3 opsins)
• normal human colour vision is trichromatic

Question 24

Photoreceptors and bipolar cells are unique because

Answer 24

they don’t fire action potentials but instead a graded response (where depolarisation results in more release of transmitter and hyperpolarisation with less release)

Question 25

Responses of retinal ganglion cells

Answer 25

Each ganglion cell responds to stimulation in a small patch of retina, which reflects a place in the visual field.

Ganglion cells respond to changes in illumination in specific locations of absolute space

ON-centre neurons: increase firing rate with increases of luminance in the receptive field

OFF-centre neurons: increase firing rate with decreases in luminance in the receptive field - i.e. if it is darker in the centre, action potentials occur, but will hyperpolarise when light is in the centre

Question 26

Therefore photoreceptors and off centre neurons

Answer 26

react in the same way (hyperpolarise with light)

Question 27

What leads to the centre-surround receptive fields of retinal ganglion cells?

Answer 27

Photoreceptors reduce release of transmitter (glutamate) when photons are absorbed.

The bipolar cells are going to also change the amount of glutamate they release in a graded way, then the ganglion cells decides whether or not that is worth an action potential.

Question 28

Tree for the centre-surround receptive fields of retinal ganglion cells

Answer 28

Cone gets input from the centre of a ganglion cell receptive field

Photoreceptor absorbs light (hyperpolarises) = less transmitter released

2 OPTIONS

a) (sign inverting) Depolarises ON-centre bipolar, which activates on-centre ganglion
b) (sign conserving) Hyperpolarises OFF-centre bipolar, deactivates off-centre ganglion

Question 29

How our ganglion cells adapt

Answer 29

• increase in background illuminance means an adaptive shift in the ganglion cells so that you need more photons to result in the same number of action potentials heading to the brain
• ganglion cell firing therefore reflects general illuminance and the specific field
• ganglion cells respond more vigorously to a small spot of light

Question 30

Ganglion cells signal luminance contrast

Answer 30

t1: cone with an on-centre ganglion cell: light in centre hyperpolarises cell
- -> less transmitter released on bipolar cell
- -> on centre depolarises
- -> releases more transmitter
- -> ganglion cell fires action potential

t2: cone in surroudn also absorb light - hyperpolarise cells
- -> less transmitter released
- -> hyperpolarises horizontal cells
- -.> reduces activity on synapses with central photoreceptor
- -> depolarises centre cone
- -> increases transmitter relase
- -> on centre bipolar cell hyperpolarises
- -> ganglion cell fires less action potentials