Retinal Receptive Fields - Week 5 Flashcards
What is the advantage of a centre-surround detection system and how does it differ from simple detection?
Centre-surround receptive fields give you information on:
- size of stimulus
- location of stimulus
This is because one region will respond to stimulus and the other responds to stimulus inhibition
(simple detection is just photorceptors connected to bipolar cells connected to ganglion cells. Need lateral pathway)
How do the rods, horizontal cells and bipolar cells react to an annulus compared to a small spot stimulus?
Rod response is reduced, Horizontal cell response is similar, and bipolar cell response is changed
The horizontal cell is connected to all the receptors which is why it is unchanged (actually a bit bigger even because more receptors are stimulated with the annulus)
How does size of an annulus effect horizontal cell response?
Larger annulus increases the response (because more receptors are stimulated)
Why is there a small rod response in an annulus stimulus for a cell’s receptive field?
Some of the light leaks across
During patch clamp experiments, what happens when the light stimulus is on an excitable area?
Number of spikes from the patch clamp increases
What types of centre-surround receptive fields for ganglion cells are there?
On-centre Off-surround ganglion cell
Off-centre On-surround ganglion cell
Describe the link between the anatomy and physiology for centre-surround responses.
Cells can be excitatory or inhibitory for the centre/surround. So there are 2 different channels.
Anatomically: bipolar and ganglion cells synapse in 2 different layers.
(So there’s an anatomical segregation of this physiological outcome)
What Neurotransmitters are used by the:
- Through pathway
- Lateral pathway?
What type of signals do they carry?
Through: uses Glutamate, carries excitatory signal
Lateral: uses GABA, carries inhibitory signal
What 2 cells are important for the lateral pathway?
- Horizontal cells
- Amacrine cells
Where do horizontal cells synapse?
Outer Plexiform Layer
Where are the horizontal cell bodies?
Inner Nuclear Layer
Where do amacrine cells synapse?
Inner Plexiform Layer
Where are amacrine cell bodies?
Inner Nuclear Layer
How many types of horizontal cells are there in primates (and cats)?
2
- H1 and H2 in primates
- A and B in cat
How many types of amacrine cells are there in primates?
Over 30 types in primates
How can info be patched from receptors to create the surround characteristics of a bipolar cell?
Horizontal cells are patching info from a lot of cones + some rods into nearby bipolar cells to create the surround characteristics of that bipolar cell
What do the different horizontal cells communicate with?
H1: speaks to all the cones and some of the rods
H2: only speaks to cones
also there’s a H3, which speaks to a certain subclass
How does the Bipolar centre-surround compare to the Ganglion cell centre-surround?
Bipolar cell surround is not as inhibitory (still is somewhat inhibitory though)
What anatomical evidence suggests that bipolar cells and horizontal cells communicate with each other?
They both are at the same synapse
Where do you find lots of GABA
Found in both the outer and inner retina
List the 2 classes of Glutamate receptors and explain which one is preferentially used for the ON pathway and OFF pathway.
1) Metabotropic receptors - goes through a series of intermediary cascades; used for ON pathway
2) Ionotropic receptors - is a direct gate of an ion channel; used for OFF pathway
What is the resting membrane potential (mV) for Bipolar cells?
On and Off bipolar cells rest at slightly either side of -70
What is the equilibrium potential (mV) for Sodium?
+20
What is the equilibrium potential (mV) for Potassium?
-75
What is the equilibrium potential (mV) for Chloride?
-70
How are chloride channels and bipolar cells related to GABA?
Bound GABA opens chloride channels on bipolar cells
- because the chloride equilibrium potential is close to the membrane potential, it can potentially have either depolarising or hyperpolarizing effects
Where in bipolar cells is chloride the highest?
Highest at the bipolar cell dendrites
How have we measured the amount of chloride in bipolar cells?
Use a fluorescent chlorine indicator in mice and use fluorescence microscopy
Describe the motivations of chloride comparing ON and OFF bipolar cells?
On Bipolar cell: (Cl Efflux) has membrane potential slightly more than -70 therefore chloride wants to leave
Off Bipolar cell: (Cl Influx). membrane potential slightly less than -70; chloride wants to enter
How do cones act in the dark?
- Glutamate released from cones depolarises horizontal cells
- Glut activates horizontal cell to release GABA
- and then GABA keeps the chloride channels open
Where is spatial sensitivity best? And why?
Best at the fovea
- As we go from the centre, the dendritic field of the bipolar cell gets bigger and the dendritic field of the ganglion cell gets bigger. Meaning that the number of cones feeding the centre and number of cones feeding the surround increases.
What happens with bipolar cells when light stimulates centre and surround?
- Glutamate and therefore GABA release STOPS
- Less GABA closes chloride channels
- now chloride is stuck in cell
Result in: - Less Cl efflux from ON-BCs: which become more negative; less depolarisation response to light
- Less Cl influx from OFF-BCs: becomes less negative; less hyperpolarisation response to light
What happens with bipolar cells when light stimulates the centre only?
Glutamate release stops in centre but continues in surround, allowing for GABA release and therefore Chloride channels stay open.
ON-BC: depolarise
OFF-BC: hyperpolarise
How do we express how strong a centre vs surround is?
By comparing difference of gaussian functions
What is the highest/’cutoff’ spatial frequency dependent on?
Centre size
What do low spatial frequencies affect? Centre, Surround or both?
Both
What size of receptive fields help you move around your environment in day to day life?
Small receptive fields
What level of contrast is needed to detect large receptive fields?
Need High contrast
How many types of amacrine cells are there?
30 different types
What is the role of A2 amacrine cell?
patch rod system onto cone system
Where are Starburst amacrine cells found?
In lower mammals only
Are starburst amacrine cells polarised or unpolarised? Explain.
They are polarised, meaning they have GABA receptors on the outside and excitatory receptors on the inside
Where are the directionally-sensitive characteristics of the centre-surround system constructed in higher order mammals compared to lower order mammals?
Made further up in our visual system
What is the special role of starburst amacrine cells?
The prefer certain centre-surrounds but also prefer DIRECTION of movement
Where does the majority of visual processing happen in lower order mammals?
At the retinal level
How can we investigate the relationship between horizontal cells and inhibitory surround?
Inject hyperpolarising current into the surround region to get same response as if a light were shined there
How would applying picrotoxin influence the ganglion cell centre-surround response?
Picrotoxin is an antagonist of GABA receptors.
Normally, increasing spot of light size would eventually decrease the response due to the inhibitory surround. With Picrotoxin, the response stays at it’s maximal and now no longer decreases.
