Visual Pathway: Retina to Cortex

Question 1

What are the steps in the process of seeing?

Answer 1

Refraction of light by the optics of the
Transduction of light energy into electrical signals by photoreceptors
Refinement of these signals by synaptic interactions with the neural circuits of the retina
130 million photoreceptors
1 million ganglion cells
The process of information compression and sorting into parallel pathways begins in the retina

Question 2

what is the basic circuitry of the retina?

Answer 2

Photoreceptors at the back
Rods and cones
Bipolar cells
Ganglion cells with long axons that extend up into the LGN - only cells in the retina that use action potentials (others use graded potentials)
Light must pass through these cells to get to photoreceptors
Photoreceptors → bipolar cells → retinal ganglion cells → LGN (3 neuron relay)
Horizontal cells and amacrine cells are involved in setting up the centre-surround RFs

Question 3

What is the circuitry for cones?

Answer 3

Fovea - central retina
Each cone is connected to one bipolar cell connected to one ganglion cell
One to one relationship between cones and ganglion cells
Small RFs and highly densely packed - fine detail
Loses sensitivity in favour of acuity
Peripheral retina
Cones pool information onto a smaller number of bipolar cells which in turn pool information onto an even smaller number of ganglion cells
Convergence - less able to say exactly where in the visual field something came from but has more sensitivity as its pooling information across a broader area.
More important to see something in peripheral vision that may cause concern to then turn head and look at it using central vision to get detail
Combination is very powerful

Question 4

What is the circuitry for rods?

Answer 4

Involved in very dim light as they bleach easily - don’t get a lot of detail
Pool lots of information into a single bipolar cell and then pooling lots of information from bipolar cells into ganglion cells.
Acuity is very low as a result of the convergence of information but sensitivity to light is very high
Very sensitive to small amounts of light in darkness - allow us to see differences/ contrast
Combination of rods and cones and their respective spacing creates a powerful system

Question 5

what are the three different conditions we see in?

Answer 5

Rod vision - scotopic (very sensitive but not very accurate- low resolution)
Cone vision - photopic (not very sensitive but very accurate - high resolution)
Both - mesopic (twilight - halfway house

Question 6

What happens to the rod circuitry during mesopic vision?

Answer 6

All amacrines have a seminal role in the rod pathways - they link the rod and cone pathways via gap junctions so that the rod signals can also use the cone bipolar pathways to ganglion cells.
Rod → rod bipolar → A2 amacrine cell → cone bipolar → ganglion cell
Mix of the two so reduced acuity but improved sensitivity
Convergence at all synapses

Question 7

How is phototransduction mediated in rods and cones?

Answer 7

Photopigment contains an oxine: G-protein-coupled receptor together with the ligand retinal
Retinal (inside rhodopsin) is what’s activated by photons - they induce conformational change from cis retinal to trans retinal thus activating the oxine
The activation of oxine activates the G protein
Transducer with alpha, beta, gamma subunits
Alpha GTP subunit of the G protein then dissociates and binds to phosphodiesterase
This activates the phosphodiesterase to hydrolyse cGMP
cGMP was the second messenger molecule that opened the Na+ channel on the photoreceptor - what was causing the dark current in the initial stages in the dark
Removal of cGMP closes the Na+ channel and the photoreceptor then hyperpolarises in a graded manner
More light activates more transducin which activates more phosphodiesterase which activates more hydrolysis of cGMP and more channels close
Get to a point of maximum hyperpolarisation - normal bleaching is approx -65mV from a resting membrane potential of approx -35mV. Photoreceptor no longer able to respond to light.

Question 8

how does the sensitivity of their different photopigments affect activity of photoreceptors?

Answer 8

Resting membrane potential -35mV
Light produces hyperpolarisation
Graded hyperpolarisation observed of photoreceptor until at most intense flash it caps at -65mV and photoreceptor is fully bleaches
Rods are much more sensitive to light and saturate at ambient light levels
Why they don’t function in daylight
Cones are fully active even in bright sunshine
As they contain different photopigments, its the sensitivity of these that allows them to be more or less sensitive to the amounts of light that are coming in.
Light activation causes a graded change in membrane potential and a corresponding graded reduction in the rate of neurotransmitter release on bipolar cells.

Question 9

how is the phototransduction cascade terminated?

Answer 9

Must be able to regenerate photoreceptor pigment in order to encode new information
Several mechanisms:
Inactivation of rhodopsin
Inactivation of transducin g protein
Inactivation of phosphodiesterase (PDE)
Activation of guanylate cyclase (produced cGMP)

Question 10

what are bipolar cells?

Answer 10

Second receptors
RFs: centre surround
When light falls in the centre this causes a stimulatory response
The surround causes an inhibitory response
Ideal stimulus is a dot of light against dark background
And the inverse
On-centre and off-centre
Lateral inhibition
Respond to local contrast
Have opposing input from local receptors - photoreceptors - and this generates the RF. Hence respond to local contrast rather than overall light.
Interested in seeing edges rather than uniform illumination or darkness

Question 11

what is the model of lateral inhibition?

Answer 11

Theory of how the centre-surround RFs are generated
RFs are set up as a centre with antagonistic surround (annulus)
Centre and surround feed into bipolar cell
Horizontal cells are GABAergic and collect information from the antagonistic surround and make connections with the bipolar cell that cause IPSPs opposing the light appearing in the centre.
Cell type is not responsive to light in both places as one will cancel out the other
Many molecules and connections thought to be involved in setting up lateral inhibition
The horizontal cells are also thought to feedback into the photoreceptors
Regardless of actual mechanism is true that the stimulus for the centre is opposite for that of the surround so that only contrast will causes a response in the bipolar cell
Two cells one sensitive to dark dots and one to light dots.

Question 12

What give bipolar cells on or off centre characteristics?

Answer 12

Expression of different glutamate receptors in their dendrites which have different effects
Off bipolar expresses AMPA - when Glu binds it opens a cation channel, Na+ enters, depolarisation and neurotransmitter release.
On bipolar expresses mGluR6 - when Glu binds it closes a cation channel, as its part of a metabotropic receptors that’s involved in the activation of different signalling pathways, and that causes hyperpolarisation.
Off bipolar cell is depolarised in the dark when photoreceptors release Glu and vice versa in light
On bipolar cell is hyperpolarised in the dark when photoreceptors release Glu and vice versa in the light
Glu is released by the photoreceptors onto the bipolar cells in the dark

Question 13

what does ABP do

Answer 13

a receptor blocker for on centre cells
deficit in ability to detect stimuli brighter than the background but could still see stimuli darker than the background
therefore two separate parallel pathways

Question 14

what are ganglion cells?

Answer 14

invaginated cone bipolars are connected to on ganglion cells
Flat cone bipolars are connected to off ganglion cells
The dendrites of off ganglion cells ramify deeper in the inner plexiform layer than those of on ganglion cells and they receive input from different bipolar cells
On bipolar cells connecting to on ganglion cells and off bipolar cells connecting to off ganglion cells
Know where they came from as they ramify within different layers of the retina

Question 15

What are the different retinal ganglion cell responses?

Answer 15

On-center
Complete darkness - baseline number of action potentials
Spot of light in centre - preferred stimulus highest number of action potentials
Light covering both centre and surround - baseline
Darkness in centre and light surround - completely turns off
Responds to spots of light and is silenced by spots of darkness
Interested in contrast rather than overall darkness or light like in bipolar
Off-centre
Spot of light: turned off
Spot of darkness: fires at highest level of action potentials
All over light/dark - baseline
Two different types of cells sending information up in parallel pathways into the LGN

Question 16

how will an off-centre on-surround ganglion cell respond to a single dark edge cross the cell RF?

Answer 16

Dark centre - depolarisation
Dark in surround - hyperpolarisation
Dark edge in annulus turns of response
Move to cover centre turn on high action potential
Information about increases or decreases in luminance is carried to the brain by the axons of 2 separate types of retinal ganglion cell in parallel pathways
Respond to edges - contrast

Question 17

What are P type cells?

Answer 17

90%
Parvocellular pathway
Very small dendritic tree
smaller RFs than M cells
sustained responses to stimuli, especially colour
require high contrat stimuli
function is to detect visual signals that relate to fine details and high contrast and to different colours but are relatively insensitive to low contrast stimuli and rapid movement visual signals
red-green colour opponent channels

Question 18

What are M type cells?

Answer 18

5%
Magnocellular pathway
Extensive dendritic tree
Lot of convergence within pathway
Most have an on or off centre
larger RFs than P cells
more transient responses to stimuli
not sensitive to colour
more sensitive to low-contrast black and white stimuli and rapid movement visual signals
feed into motion pathway sat higher levels in the visual cortex (no motion sensitive retinal ganglion cells)

Question 19

What are non M/non P cells?

Answer 19

All cells not in the main pathways
Koniocellular pathway/ bistratified cells
Less well categorised with many subtypes
Most have an on or off centre

Question 20

What are photosensitive ganglion cells?

Answer 20

Not involved in perceptual vision
Contain the photopigment melanopsin and project to the SCN (suprachiasmatic nucleus) via the retinohypothalamic tract
involved in sett and maintaining circadian rhythms
Others project to the LGN and are involved in the pupillary light reflex
Others go to the superior colliculus for saccadic eye movement

Question 21

What are the koniocellular RFs?

Answer 21

Blue-yellow colour opponency
Yellow is provided by a combination of input from the red and green cones
On responses for blue off responses for yellow light - appears to take up whole receptive field rather that centre surround

Question 22

What is the need for colour-opponency?

Answer 22

Principle of univariance
Blue, green and yellow cone all have optimal stimuli but they also overlap
The wavelengths of light coming in are detected because of this overlap and the colours are likely to activate photoreceptors simultaneously
The only way to discriminate therefore is by comparing the firing of red green blue cones

Question 23

how would a P ganglion cell with a red on-centre and green off-surround respond to colour?

Answer 23

Centre of RF fed by red cones → depolarisation → increase in activity
Overall red light → effective stimulus but response is reduced because some tof the red will stimulate the green cones that feed into the inhibitory surround but not enough to cause overall cancellation
Depolarising response to red in the centre is only cancelled by the hyperpolarising response to green light in the surround

Question 24

what causes colour blindess?

Answer 24

Absence of red or green pigment give similar set of losses
Absence of blue pigment lose comparison of blue yellow
Used colour blind people to look at camouflage as they were better at picking up differences between greens, browns and yellows then those who can see normal set of colours.

Question 25

What type of cone cells are not in the fovea?

Answer 25

No blue cones
Must be using red green cones for the achromatic contrast used for reading text etc depending on where light falls within RF

Question 26

what are amacrine cells?

Answer 26

Piggybacks rod bipolar onto cone bipolar circuitry
Contribute to inhibitory surrounds
Respond transiently to illumination giving on-off responses to
Large dendritic trees
No axons
Connect to each other laterally across the retina