Lecture 10: Vision 1

Question 1

What fundamentally limits visual acuity?

Answer 1

neural and optical factors

Question 2

What are the optical factors which affect visual acuity?

Answer 2

pupil size
clarity of optical medial (cataracts, corneal opacities)
refractive errors -> blur (myopia, hypermetropia, astigmatism, presbyopia)

Question 3

How does light trigger a neural response in the eye?

Answer 3

light travels through to the back of the eye where photoreceptors are located
light activates the photoreceptors which kicks off the process of seeing

Question 4

What are the characteristics of rods?

Answer 4

useful for night vision
very sensitive
one type only
no colour vision
absent from fovea

Question 5

What are the characteristics of cones?

Answer 5

useful for day vision
less sensitive
three types
allow colour vision
densest in fovea

Question 6

What is the ratio of rods to cones?

Answer 6

20:1

Question 7

What are the photopigments in rods and cones and what is their role?

Answer 7

rods = rhodopsin
cones = cone-opsin
their role is to be activated by light which results in the conformational change of a protein

Question 8

What do opsins bind to?

Answer 8

vitamin A

Question 9

What happens when light hits 11-cis retinal?

Answer 9

all-trans retinal is formed

this molecular change in shape kicks off the process which allows us to see

Question 10

How do photoreceptors respond to light?

Answer 10

photoreceptors are hyperpolarised by light and respond with graded changes in membrane potential (not action potentials)

Question 11

What do photoreceptors use as their neurotransmitter?

Answer 11

glutamate

Question 12

What happens to photoreceptors in the dark?

Answer 12

cGMP gates a sodium channel causing continuous influx of sodium ions -> depolarisation of the cell

Question 13

Explain the sequence of events that occurs when photoreceptors are exposed to light.

Answer 13

in the light cGMP breaks down to GMP -> cGMP no longer gates the sodium channels -> flow of Na ions ceases -> cell is hyperpolarised

Question 14

What happens when light activates rhodopsin?

Answer 14

initiates a cascade of events that ultimately leads to the closure of cGMP gated sodium channels which results in hyperpolarisation
Rh -> transducin -> PDE -> breaks down cGMP

Question 15

How is information about light passed through the retina?

Answer 15

there is a “through” pathway (Ph - BC - GCs) and there are lateral interactions (horizontal cells / amacrine cells)

Question 16

What are the second order neurons (bipolar cells)?

Answer 16

important neurons in “through” pathway

important for spatial vision and colour vision

Question 17

What do bipolar cells receive input from and what do they form output synapses with?

Answer 17

receive input
from photoreceptors and
form output synapses with
retinal ganglion cells (and amacrine cells)

Question 18

What are the 10 different types of second order neurons?

Answer 18

1x rod bipolar cell

9x cone-bipolar cells

Question 19

What are the many different types of ganglion cells? What is their role?

Answer 19

ON, OFF, M and P

release glutamate and fire action potentials

Question 20

How do ganglion cells respond to light?

Answer 20

by either increasing or decreasing their action potential firing rate

Question 21

What is the receptive field of a ganglion cell?

Answer 21

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

Question 22

What are the responses of ganglion cells?

Answer 22

increase or decrease in firing
transient or sustained response
visual information is passed to higher cortical centres in parallel

Question 23

What is the role of horizontal cells?

Answer 23

provide output onto photoreceptors and use inhibitory neurotransmitter GABA to induce lateral inhibition
respond to light by hyperpolarising

Question 24

What is the role of amacrine cells?

Answer 24

important for lateral inhibition as well

for the most part are considered inhibitory cells (release glycine and GABA)