Retina (EF) Flashcards
Which neurons are located in which areas?
(vitreous cavity-inside)
• Nerve fibre layer- Ganglion cells
• Ganglion cell layer- Ganglion cell bodies
- Inner plexiform layer- Bipolar, ganglion and amacrine synapses
- Inner nuclear layer- Bipolar, horizontal and amacrine cell bodies
- Outer plexiform layer- Bipolar, horizontal and photoreceptor synapses
- Outer nuclear layer- Photoreceptor cell bodies
• Photoreceptor layer
• Pigment epithelium
(sclera- closest to eyeball)
Photoreceptors
How do photoreceptors respond to light?
• PHOTORECEPTORS HYPERPOLARISE TO LIGHT
- resting membrane potential becomes more negative
- when light falls on a photoreceptor
- retinal confirmation changes from 11-cis (kinked) to all-trans straight form
- initiates a cascade of events that activates a G-protein, transducin
- phosphodiesterase enzyme breaks down cGMP
- sodium channels close
- hyperpolarisation of the photoreceptor membrane
Photoreceptors
Photoreceptor outer segment in the dark
- cGMP gated sodium channels found in the photoreceptor outer segment are open
- photoreceptor membrane potential depolarise- release neurotransmitter Glutamate
Photoreceptors
What is phototransduction?
Phototransduction- when light is converted to a neural signal within photoreceptors
Parallel pathways
Describe the cone pathway
Parallel processing
- photoreceptor signal is sent both to an ON bipolar cell and an OFF bipolar cell
• ON pathway (depolarising):
- signal is being sent to an ON bipolar cell, which goes to an ON ganglion cell and off to the brain
• OFF pathway (hyperpolarising):
- at the same time, same signal is being sent to an OFF bipolar cell, which goes to an OFF ganglion cell and off to the brain
Parallel pathways
Describe the rod pathway
- rod bipolar cells (similar to ON) piggyback on the ON and OFF pathways in the dark
Parallel pathways
Centre-surround receptive field
Info coming out from the ganglion cells:
- ganglion cells respond depending on where the light falls on its receptive field
- if we shine a light in the centre of the receptive field of an ON ganglion cell, this cell will depolarise and fire action potentials
- moving the light to the peripheral part of the ganglion cell turns the ganglion cell off and they will hyperpolarise
Parallel pathways
What creates the centre-surround?
- The way the retina is wired up is important
Central response: - determined by the “Through” pathway (Ph-BC-GCs)
Surround response: - determined by inputs from horizontal cells
- Whatever the centre does, the surround does the opposite
- cannot see an edge if the target is so big that it covers the entire receptive field
Parallel pathway
Horizontal cells
Horizontal cells receive input from surrounding receptors and send info to the other photoreceptors that happen to be in the central part of the ganglion cell’s receptive field
- Horizontal cells express the same types of glutamate receptors as OFF bipolar cells
- express the ionotropic class: AMPA, kinate receptors
- when horizontal cells hyperpolarise, they release less GABA as their neurotransmitter, membrane potential goes up (increases), less inhibition
Parallel pathway
Glutamate
- Glutamate is the signal that comes out in the dark
- it will cause hyperpolarisation of ON bipolar cells and depolarisation of OFF bipolar cells
- Glutamate causes a hyperpolarisation on metabotropic glutamate receptors- mGluR6 (opposite to ionotropic glutamate receptors- iGluR)
- ON BIPOLARS- only cell type that expresses an mGluR6 in the retina
- whatever the ON/ OFF bipolars do, the ON/ OFF ganglion does as well
Parallel pathway
What happens to horizontal cells when light falls on their receptive field
- HORIZONTAL CELLS HYPERPOLARISE TO LIGHT
- horizontal cells mediate their surround, get into from photoreceptors
- Photoreceptors hyperpolarise when light falls on it, horizontal cells does the same
Amacrine cells (AII) What do amacrine cells do?
- AII (A2) amacrine cell is the interneuron in the rod pathway
- The rod bipolar cells do not communicate with ganglion cells directly, the rod pathway piggybacks
- Rod bipolar cell (only one type) synapses with the AII (A2) amacrine cell type
- AII allows the rod signal to piggyback onto the cone pathway (ON and OFF bipolar cells)
- Synapse from AII to OFF bipolar cells
- A gap junction between the AII and ON bipolar
Starburst amacrine cells
Starburst amacrine cells- inhibitory
- Motion detection
- When light moves from soma to dendrite, GABA is released
- When light moves in the opposite direction, nothing happens
Amacrine cells (AII) When light falls on rods, what happens to every cell in the circuit?
- light falls on rods, causes hyperpolarisation
- ROD BIPOLAR IS AN ON BIPOLAR CELL TYPE, it will depolarise to light and release glutamate
Role of ganglion cells
What are the type of ganglion cells?
• Midget ganglion cells
- colour processing
• Parasol ganglion cells
-encode movement
• Melanopsin ganglion cells
