Eyes and Photoreceptors

Question 1

Rods

Answer 1

130 million
scotopic (v high sensitivity to light)
high convergence
low resolution
peripheral
500nm

Question 2

Cones

Answer 2

6,5mil
photopic(low sensitivity)
low convergence
high resolution
focused in foves
red/green/blue

Question 3

Connections between rods cones and ganglion cells etc?

Answer 3

1 and 2Rods       cones
3 horizontal cells
bipolar cells + muller glial cells
4 Amacrine cells
5 Ganglion cells
6 to optic nerve

(order of cells)
1= outer segment of photoreceptors
2= outer nuclear layer
3= outer plexiform layer
4= inner nuclear layer
5= inner plexiform layer
6= ganglion cell layer

note: cones can be directly connected to ganglion cell

Question 4

Blind spot is where?

Answer 4

where the ganglion cell connects to the optic nerve

Question 5

cone to bipolar neuron ratio?

rod to bipolar neuron ratio?

Answer 5

1 cone to 1 bipolar to 1 ganglion
many rods to 1 bipolar cells (then all bipolar cells converge onto one ganglion)

Rods don’t contact ganglion directly - they are connected via amacrine cells

Question 6

Duplicity theory

Answer 6

= can’t have high sensitivity and high resolution in single recpetor - so, seperate systems for colour and monochrome

Question 7

Pigments in rods and cones?

Answer 7

rods - Rhodopsin

Cones - Photopsin

Question 8

Phototransduction

Answer 8

Rhodopsin activated by photons (reversible via enzyme) 
causes transducin GDP to convert to transducin GTP
ACTIVATES PDE (phosphodhesterase)
this hydrolyses cGMP-->GMP = Na+channels close
This reduces Ca++ down in cytosol
causes recoverin into guanylate cyclate
=converts GMP--> cGMP = Na+ channels open!!

Question 9

In phototransduction what happens in the dark??

Answer 9

‘Dark current’ = runs through cells at all times
-keeps cell depolarised (-40mv) = secretes glutamate
IPSP in bipolar cell = cell is inhibited = no synpatic activity
==’ON bipolar cell’

Question 10

In phototransduction what happens in the light?

Answer 10

channels close = no dark current= hyperpolarises cell
so, no glutamate released = no inhibition
= NETs released

Question 11

2 types of bipolar cell

Answer 11

ON = glutamate inhibits (hyperpolarises) ON bipolar cells via metabolic glutamate recpetors
OFF = glutamate excites (depolarises) OFF bipolar cells via ionotropic glutamate receptors

Question 12

Photorecpetors, bipolar cells and glutamate

Answer 12

Photreceptors only release glutamate
Photoreceptors depolarised and release glutamate in dark
light inhibits glutamate release by photoreceptors

Question 13

Can bipolar cells received mixed input?

Answer 13

yes, some receive inputs from rods and cones and some just from one or the other

Question 14

Retinal ganglion cell types
W
X
Y
Melanopsin RGCs

Answer 14

W- mainly from rods, 40%, <10um diameter bodies, slow conducting, broad field- dendritics spread, directional movement, visions in low light
X- mainly cones, 55%, 10-15 um diameter bodies, medium conduction rate, small fields- dendrites not spread, image formation, colour vision
Y - inputs less defined, 5%, 35um diameter bodies, fast conduction, broad fields, rapid response to changes in visual field
Melanopsin RGCs - large, slow, light reponse, independent of rods and cones, circadian rhythm role

Question 15

Opsin (GPCR) –> Meta-Rhodopsin –> transducin
What does GPCR stand for?
What is the 7-transmembrane receptors function?
3 forms of vitamin A?

Answer 15

G protein coupled receptor
7-transmembrane receptors = acts as inbox for messages in the form of light energy, peptides, lipids, sugars and proteins
3 forms:
1) Retinal-vitamin A aldehyde
2) Retinol
3) retinoic acid

Question 16

Effects of vitamin D on vision:

Answer 16

preventing angiogenesis(new blood vessel formation)
also anti-inflammatory (inflammation plays a key role in development of AMP
may help prevent dry eyes by inducing cathelicidin an anti-microbial protein that can be produced by cells in the eyes and heal eye wounds

Question 17

ON and OFF bipolar cells leads to ‘on-centred’ and ‘off-centred’ receptive fields:

Answer 17

On- bipolar cells = respond progressively more to increasing levels of light
Off- bipolar cells = respond progressively more to decreasing levels of light
= between them they balance the inputs to enable image formation, especially contrast, across a wide range of light intensity