The Retina

Question 1

What are the factors that determine visual acuity?

Answer 1

Neural factors and optical factors

Question 2

What is visual acuity?

Answer 2

Ability to resolve fine detail

Question 3

What does the denominator of visual acuity mean eg 6/12?

Answer 3

What a normal person sees at 12m, this person sees at 6m (this is the legal driving limit)

Question 4

What are the optical factors that affect visual acuity?

Answer 4

pupil size; clarity of optical media: cataracts, corneal opacities; refractive errors (cause blurriness): usually caused by length of eyeball eg myopia, hypermetropia, astigmatism, presbyopia

Question 5

Visual acuity is defined by

Answer 5

density of cones; tf fovea has highest visual acuity

Question 6

Phototopic light levels

Answer 6

daylight

Question 7

Scotopic light levels

Answer 7

dark light levels

Question 8

At phototopic light levels, the best VA is

Answer 8

6/6 or better

Question 9

At scotopic light levels, the best VA is

Answer 9

6/60

Question 10

Visual acuity decreases as you move away from the fovea proportional to

Answer 10

decrease in cones

Question 11

T/F The best visual acuity under scotopic conditions is at the fovea

Answer 11

False; it’s about 8 degrees from the fovea where the concentration of rods is increasing

Question 12

What are the 6 types of neurons in the retina?

Answer 12

rods, cones, horizontal cells (HCs), bipolar cells (BCs), amacrine cells (ACs), and ganglion cells (GCs)

Question 13

The output neurons of the retina are

Answer 13

ganglion cells (GCs)

Question 14

Where in the retina do the neurons synapse?

Answer 14

Outer and inner plexiform layers

Question 15

T/F Photoreceptors are the first cells light hits

Answer 15

False; they are the LAST cells, at the back of the retinal layers

Question 16

95% of photoreceptors are

Answer 16

rods

Question 17

T/F Rods are more sensitive to light than cones

Answer 17

True

Question 18

What are the 3 types of cones?

Answer 18

Red, green, and blue

Question 19

What is the order of layers in the retina from where light contacts it?

Answer 19

nerve fibre layer
ganglion cell layer
inner plexiform layer (bipolar-amacrine-ganglion synapses)
inner nuclear layer (nuclei of horizontal, bipolar, and amacrine cells)
outer plexiform layer (bipolar-horizontal-photoreceptor synapses)
outer nuclear layer (nuclei of photoreceptors)
photoreceptor layer
pigment epithelium
choroid

Question 20

Bipolar cells are in which layer of the retina?

Answer 20

Inner nuclear layer

Question 21

What are the 10 different types of bipolar cells?

Answer 21

1x for rods; 9x for cones

Question 22

Bipolar cells are important for

Answer 22

spatial vision and colour vision

Question 23

OFF BCs

Answer 23

always HYPERpolarize when light shines on the retina

Question 24

ON BCs

Answer 24

always DEpolarize when light falls on the retina

Question 25

Bipolar cells (BCs) receive inout from

Answer 25

photoreceptors

Question 26

What is the function of horizontal cells?

Answer 26

modulation in outer retina: receive input from photoreceptors and provide output to other photoreceptors - mainly INHIBITORY

Question 27

Horizontal cells are mainly __________ to photoreceptors

Answer 27

Inhibitory, via GABA

Question 28

Horizontal cells use which NT?

Answer 28

GABA tf inhibitory

Question 29

Horizontal cells respond to light by

Answer 29

HYPERpolarizing

Question 30

ON ganglion cells

Answer 30

DEpolarize when light falls on the retina

Question 31

OFF ganglion cells

Answer 31

HYPERpolarize when light falls on the retina

Question 32

What is unique about ganglion cells in the retina?

Answer 32

They are the only cells that fire APs; the rest of the retina fires only graded potentials

Question 33

Ganglion cells release

Answer 33

Glutamate (excitatory)

Question 34

What is the role of ganglion cells?

Answer 34

Main output neuron of the retina

Question 35

T/F The visual system is a detection system

Answer 35

False; it is a comparator - how we interpret what we see (greyness, colour, motion) relates to the context in which it is presented

Question 36

What is meant by receptive field of a ganglion cell or bipolar cell?

Answer 36

area of retina that when stimulated with light changes the cell's membrane potential

Question 37

T/F Ganglion cells respond to light by firing action potentials

Answer 37

False; GCs respond to light by increasing or decreasing their AP firing rate - depending on where in the receptive field the light is shone

Question 38

What are photopigments?

Answer 38

Proteins in photoreceptors: rods contain rhodopsin, cones contain cone-opsins (1 of 3) - all bind to vitamin A (retinal); they convert light into neural signals

Question 39

How do photoreceptors turn light into neural signals?

Answer 39

Photopigments (rhodopsin, coneopsins) are conformationally changed when light interacts with their bound retinal

Question 40

Photoreceptors release NT relative to

Answer 40

graded membrane potential; lower (hyperpolarized/more negatie) means less firing and less release of NT

Question 41

Photoreceptors

Answer 41

HYPERpolarize in response to light

Question 42

Photoreceptors use ________ as their NT

Answer 42

glutamate (excitatory)

Question 43

In the dark, photoreceptors are ___________ due to _________

Answer 43

depolarized; Na+ cycling in and out of the photoreceptors via cGMP-gated channels

Question 44

In the light, photoreceptors are _________ due to _________

Answer 44

hyperpolarized; phototransduction leads to formation of PDE which breaks down cGMP, closing the Na+ channel thus the cell hyperpolarizes

Question 45

What is phototransduction?

Answer 45

Light activates rhodopsin via vitamin A; this initiates the production of transducin, then PDE; PDE breaks down cGMP that, in the dark, keeps cGMP Na+ channels open (depolarizes cell); the cGMP Na+ channels close, leading to hyperpolarization of the cell

Question 46

The photoreceptors that form the central part of a ganglion cell receptive field are

Answer 46

those that directly contact ganglion cells via bipolar cells ie Ph-BC-GCs

Question 47

The central response is generated by

Answer 47

light contacting photoreceptors in the central part of the ganglion cell receptive field

Question 48

The central response of ganglion cells to light is

Answer 48

depolarization

Question 49

The receptive field of a retinal ganglion cell is

Answer 49

the area of retina that the ganglion cell will respond to

Question 50

Light shone on the periphery of a receptive field generates the _______ response

Answer 50

surround response

Question 51

The surround response is generated by

Answer 51

shining light on the periphery of a ganglion cell receptive field

Question 52

Photoreceptors in the surround send information to

Answer 52

horizontal cells

Question 53

Which cell type creates the 'centre-surround'?

Answer 53

Horizontal cells