Visual System Flashcards
Lecture 1
Dark and Light Responses
What is the fovea?
Specific region of the retina upon which light focuses.
What type of photoreceptive cells are present in the eye?
Rod and Cone cells.
Which layer are rod and cone cells in?
Outer nuclear layer.
How are rod and cone cells connected to each other?
By horizontal cells.
Where are horizontal cells?
Outer plexiform layer.
What do rod and cone cells synapse onto to convey information?
Bipolar cells.
Where are bipolar cells?
Inner nuclear layer.
How are bipolar cells connected to each other?
By Amacrine cells.
Where are amacrine cells?
Inner plexiform layer.
What do bipolar cells synapse onto to convey information?
Ganglion cells.
Where are ganglion cells?
Ganglion cell layer.
Where is visual information carried from ganglion cells?
To the optic nerve, then lateral geniculate nucleus, then visual cortex.
What is the blind spot?
Optic disc made up of ganglion cells.
How is the light prevented from reflecting from the retina?
Pigmented epithelium posterior to cone and rod cells absorbs the light that hasn’t activated photosensitive cells.
What is the function of the interconnecting cillium?
Part of the centriole.
Describe the effect of light on rod cells.
Light hits rhosopsin on the rod cell’s disc. Rhodopsin activates Transducin G-protein which converts GTP to GDP, powering cGMP phosphodiesterase. Phosphodiesterase converts cGMP (from Guanylyl Cyclase) to 5’-GMP. Rod cell’s sodium channels require high cGMP concentration to open and cause sodium influx (3 molecules at once).
Under what conditions are rod cells more likely to be depolarised?
During the dark.
How is the light circuit signal amplified?
1 rhodopsin activates 100s of transducins. Phosphodiesterase converts 1000s cGMPs/sec.
Describe the effect of light on rhodopsin.
Light induces a conformational change of all-trans retinal to 11-cis retinal by swiveling of the chain around the 11C=C12 bond.
Describe the dark current in a rod cell.
Sodium enters outer segment via cGMP gated channels. Sodium pumped out in exchange for potassium in the inner segment by the Na/K pump. Potassium efflux via potassium selective channels in the inner segment.
What sodium channel type is used in rod cells?
CNGB1A
What sodium channel type is used in cone cells?
CNGA3
Where in the retina is active conduction observed?
Ganglion cells and one subtype of Amacrine cells.
How is the light response terminated?
By phosphorylation of rhodopsin by opsin kinase.
Describe the light adaptation response.
Dark conditions increase cGMP concentration. Sodium channels open causing influx. Depolarisation causes influx of calcium at the synaptic terminal. Calcium negatively regulated Guanylyl cyclase. GC no longer synthesises cGMP. No more sodium influx. Decrease in intracellular calcium. Negative effect on GC removed. cGMP concentration rises.
What are the common properties of rod cells?
Highly sensitive. Night vision. More photopigment means more light can be captured. High amplification allows detection of a single photon. Low resolution. Slow response. More sensitive to scattered light (peripheral vision).
What are the key features of the rod system?
Highly convergent. Not present in fovea. Achromatic (B&W).
What are the common properties of cone cells?
Low sensitivity. Day vision. Less photopigment. Less amplification. High resolution. Fast response. More sensitive to direct (axial) light.
What are the key features of the cone system?
Less convergent. Concentrated in fovea. Chromatic (3 pigments).
Lecture 2
Processing of visual information
What are the main types of ganglion cells?
P-type. M-type.
Describe M-type ganglion cells.
Large receptive fields. Low resolution (gross features). On/off centre. Not wavelength selective.
Describe P-type ganglion cells.
Small receptive fields. High resolution (fine features). On/off centre. Wavelength selective.
What are the general properties of other ganglion cells?
Respond to diffuse light. Detect brightness. Initiate pupilliary reflexes.
Which type of ganglion cell is more abundant in the fovea?
P type.
Describe the receptive field.
Circular centre and antagonistic surround. Allows for edge detection. 2 parallel pathways of information processing.
Why is the ganglion cell system good for edge detection.
Light localised to centre or surround only will have a strong excitatory or inhibitory effect. Diffuse light will give an effect slightly greater than no light at all.
Describe the effects of on-centre light on the on-centre cells.
Decreased glutamate release from cones. Muscarinic glutamate receptors of bipolar cells are activated. Unknown 2nd messenger causes opening of TRPM1 channels leading to depolarisation hence increased firing of ganglion cells.
What is the TRPM1 channel?
Transient Receptor Potential channel of Melistatin subfamily.
Describe the effects of on-centre light on the off-centre/surround cells.
Increased glutamate release from cones. Activation of AMPA causing sodium influx and depolarisation of off-centre cells. Increased activity of ganglion cells.
Describe the effects of surround light on on-centre cells.
Increased glutamate release. Glutamate binds to muscarinic glutamate receptors. Unknown 2nd messenger negatively controls TRPM1 channels (closed). Cell hyperpolarises. No firing in ganglion cell.
Describe the effects of surround light on surround cells.
Decreased glutamate release. AMPA not activated. Cell hyperpolarises. No firing in ganglion cell.
Describe centre surround inhibition pathway.
Light on surround causes surround hyperpolarisation. Surround cones hyperpolarise horizontal cells. Horizontal cells negatively control centre cones (depolarise). Centre cones negatively control bipolar cells (hyperpolarise). Hyperpolarised bipolar cells mean no firing in ganglion cells of centre.
What is the function of centre surround inhibition?
Edge detection.
How do rod and cone cells cooperate?
When light levels are low, rods and cones communicate via horizontal cells between groups and via electrical synapses within groups. Rod cells act on their own when in near-dark conditions.
How is red-green colour blindness inherited?
X-linked recessive.
What are the 3 types of colour blindness?
Protanomaly - abnormal red cone (RG). Deuteranomaly - abnormal green cone (RG). Tritanomaly - abnormal blue cone (BY).
What is dichromatopsia? Give examples.
2 out of 3 cones present. Protanopia. Deuteranopia. Tritanopia.
Which chromosome is the red pigment gene located on?
X
Which chromosome is the green pigment gene located on?
X
Which chromosome is the blue pigment gene located on?
7
Which chromosome is the rhod pigment gene located on?
3
What is typical monochromatopsia?
No cone cells.
What is atypical monochromatopsia?
1 type of cone cells
