retinal function

Question 1

what is difference between rod and cone system when contacting horizontal cells?

Answer 1

• in rod system there was a lot of convergence meaning lots of rods contact a single horizontal cell
• in cone system less convergence so less cones contact a single horizontal cell

Question 2

what cells synapse with photoreceptors?

Answer 2

• horizontal cells synapse with photoreceptors and bipolar cells in the OPL

Question 3

what are bipolar cells ?

Answer 3

• dendrites of bipolar cells synapse with photoreceptors and horizontal cells in the OPL
• have their cell body in the inner nuclear layer and send axons to the inner plexiform layer where bipolar cells synapse with amacrine cells, ganglion cells and interplexiform cells

Question 4

where do bipolar cells have their dendrites?

Answer 4

OPL- where they synapse with photoreceptors and horizontal cells

Question 5

where do bipolar cells have their synaptic processes ?

Answer 5

IPL- where they synapse with ganglion cells and amacrine cells

Question 6

what are the two types of photoreceptors ?

Answer 6

rods and cones

Question 7

how many types of horizontal cells do we have ?

Answer 7

3 types
DH1
DH2
DH3

Question 8

how many types of bipolar cells ?

Answer 8

9 different bipolar cells in the human retina

Question 9

what is rod bipolar cells ?

Answer 9

• type of bipolar cell that only contacts rods
• it receives input from 15-20 rods centrally and 40-50 rods in the periphery
• lots of convergence- many rods give information to a single bipolar cell

Question 10

what are the other types of bipolar cells?

Answer 10

• all other bipolar cells are cone

- they synapse with a cone pedicle

Question 11

what are the two general types of cone bipolar cells ?

Answer 11

1. invaginating cone bipolar
   - which end within triads
   - form central element within triad
2. flat cone bipolar
   - don’t go into the invaginations of the cone pedicle
   - they make superficial contact with pedicles

Question 12

why some types of bipolar cells called midget bipolar cell?

Answer 12

• small dendritic field meaning only contact single cones

Question 13

what are the two types of midget bipolar cell?

Answer 13

1. invaginating midget bipolar

• all of its dendrites form the central elements of the triads of just a single cone pedicle
• one to one relationship between cone pedicle and invaginating midget cell

2. flat midget bipolar
   - bipolar cell will contact a single cone in the central retina, forming superficial non-invaginating contacts

• in the cone system there is no convergence in central retina

Question 14

summary of bipolar cell types ?

Answer 14

• there are 9 in the human retina
• 1 type of rod bipolar contacting many rods ( high convergence )
• 8 types of cone bipolar - which can be either flat or invaginating
• midget cone bipolar cells contact individual cones within the central retina ( no convergence )

Question 15

what are amacrine cells?

Answer 15

• have their cell bodies in the lower half of the inner nuclear layer
• send dendrites into the inner plexiform layer where they synapse with bipolar cells and ganglion cells
• suited for mediating lateral interactions in the inner retina
• amacrine cells have no axon

Question 16

where do most amacrine cells have their cell bodies?

Answer 16

most amacrine cells have their cell bodies within the INL and synapse in the IPL

Question 17

how many types of amacrine cells ?

Answer 17

there are at least 25 different types in human

Question 18

what is the most spectacular class of amacrine cell?

Answer 18

• starburst cell
• no axon
• cell body with a lot of dendrites coming out

Question 19

what are ganglion cells?

Answer 19

• have their cell body in the ganglion cell layer
• send dendrites outwards into the innerplexiform cells where they synapse with amacrine and bipolar cells
• in the other direction from cell body - send axons to the the nerve fibre layer and axons run towards optic nerve head and leave retina as optic nerve

Question 20

what is structure of ganglion cells ?

Answer 20

• ganglion cells have cell bodies in the ganglion cell layer, and their axons makeup the nerve fibre layer, eventually forming the optic nerve

Question 21

how many types of ganglion cells are there ?

Answer 21

18 different types

Question 22

what is an example of ganglion cell?

Answer 22

• midget ganglion cell
• small dendritic field and it contacts only single midget bipolar cells in the central
  retina
• each central cone has its own ganglion cells

Question 23

what happens in rod system?

Answer 23

• rod system - many rods feed onto a few bipolar cells

lot of rods feed into a single ganglion cells

Question 24

what happens in cone system ?

Answer 24

• cone system - each cone contacts its individual midget bipolar cells which contact its individual midget ganglion cell
• 1:1 relationship between photoreceptor and ganglion cells

Question 25

what is another retinal cells type ?

Answer 25

- interplexiform cells - connects IPL to OPL - take information from the IPL to the OPL - involved in dark and light adaptation of retina - have cell body in the inner nuclear layer or the ganglion layer

Question 26

what is meant by efferent innervation/centrifugal information in humans?

Answer 26

- brain has some control over the functioning of retina - the human retina receives modulating input from the brain via fibres within the optic nerve. - although there are only small number of retionpetal/centrifugal axons they branch extensively before terminating in the IPL

Question 27

what is the difference in function between rods and cons?

Answer 27

rods - vision in low light intensity, which means they are designed to maximise sensitivity - scotopic vision cons - high intensity , high acuity colour vision - photopic vision

Question 28

why are rods good for mediating high sensitivity ?

Answer 28

1. maximise photon capture - this is done by rods being more numerous than cones and having larger outer segments 2. transduction cascade in rods has a higher gain - single photon closes more cation channels in rods than in cones 3. the main reason the rod system is more sensitive than the cone system is its connectivity

Question 29

how to activate ganglion cell to stimulate light ?

Answer 29

1. in darkness the bent 11-cis retinal is bound to opsin 2. when a photo comes along, 11-cis retinal become straight into all trans retinal which no longer fits into binding pocket of opsin 3. chromophore separates from the opsin 4. triggers the photo transduction cascade- the activated Rh activates T which activates PDE which hydrolyses cGMP which was holding open cation channel 5. cGMP is hydrolysed to GMP ion channel shut 6. photoreceptor hyperpolarisation 7. glutamate release from photoreceptor is decreased 8. this activates ganglion cell thus stimulating light

Question 30

what happens when photon is absorbed by cone ?

Answer 30

- when photon is absorbed by cone it will activate the bipolar cell which stimulates ganglion cell

Question 31

what is thermal isomerisation ?

Answer 31

visual pigment molecules in the outer segment are inherently unstable which means they will occasionally isomerise even in the absence of light

Question 32

what do ganglion cells require to fire ?

Answer 32

-- in order to activate ganglion cell it has to be hit by information that 5 photons have been absorbed by cones which is due to thermal isomerisation - therefore ganglion cells require 5 isomerisations in order to fire

Question 33

why is cone system not activated in low light levels?

Answer 33

- cone system is connected to its own ganglion cell, this means they will need to absorb 5 photons in order to activate ganglion cell in low light level - not enough photons absorbed by single cone in order to activate ganglion cells

Question 34

how is rods system able to function in low light levels?

Answer 34

- in rod system there is a lot of convergence, meaning rods connect to few biploar cells which connect to fewer ganglion cells - ganglion cells connected to rods is likely to receive information of 5 isomerisation even when photons are short in supply - thus mediating high sensitivity ( scotopic vision )

Question 35

what is the issue with convergence in rods ?

Answer 35

- good for sensitivity | - poor for high acuity

Question 36

what is spatial acuity ?

Answer 36

- ability to resolve detail which tested with a test chart - acuity is our ability to resolve 2 points - the closer the points are when we can still resolve them, the higher the acuity

Question 37

what do you need to resolve two points ?

Answer 37

- must stimulate separate cones - must have one unstimulated cone in between the stimulated cones - the point must be imaged on separate photoreceptors

Question 38

why is rod system not good for mediating high acuity vison ?

Answer 38

- the convergence of the rod system means both the single large stimulus and the 2 smaller spots result in the same ganglion cell activity - brain can't distinguish one spot from two spots due to the convergence

Question 39

why is cone system good for mediating high acuity vision ?

Answer 39

- in central retina each cone is wired to its own midget bipolar cells which is wired to its own midget ganglion cell - brain has ability to distinguish one spot from two spots

Question 40

reasons why cone system is good for high acuity ?

Answer 40

- lack of convergence - cones are numerous in the foveola, they were thinner and longer to ensure a high packing density, results in higher sampling frequency of image, thus increasing acuity - fovea is also avascular, inner 5 layer of retina are pushed, light scatter is reduced which results in high acuity - macular pigment absorbs short wavelength, this part of spectrum which is most prone to scatter and aberration, their removal thus increases acuity

Question 41

where do we move our eyes in high light levels?

Answer 41

we move our eyes to put image on fovea, where acuity is highest

Question 42

where is sensitivity most low?

Answer 42

in fovea , as there are no rods

Question 43

what to do when you want to see a dim object ?

Answer 43

image the object on peripheral retina where sensitivity is the highest