Lecture 26 - Retinal Dystophies Flashcards
Where is the retina situated?
In the back of the eye and is part of the CNS (an outpost of the brain).
What is the vertebrate retina?
Complex, layered structure that performs all of the initial steps of visual processing.
Seven cell types, three nuclear layers and two synaptic layers.
Describe retinal organisation.
Outer nuclear layer contains photoreceptors.
Inner nuclear layer contain horizontal, bipolar and amacrine cells.
Ganglion cell layer contains ganglion cells.
Outer plexiform layer contains glial cells and makes synaptic connections between photoreceptor terminals, horizontal and bipolar cells.
Inner plexiform layer makes thicker connections between bipolar and amacrine cells.
Retinal pigment epithelium is attached to the back of the eye and is important for survival and health of photoreceptors.
What does Bruch’s membrane do?
Divides the RPE from choroid (network of blood vessels).
Describe the fovea.
Also known as the macula.
Dark central area devoid of blood vessels.
2° of visual angle.
All inner layers pushed aside, light falls directly onto photoreceptors.
Foveal pit has no blood vessels, RGCs, INL cells and only contains cones.
Describe the optic disc.
6° of visual angle.
Located 15° nasal to fovea.
RGC axons converge and leave eye together with blood vessels.
No photoreceptors - corresponds to blind spot.
How many types of cone cells are there?
3
Describe the different kinds of cone cells.
Short cones - absorb blue light.
Medium cones - absorb green light.
Long cones - absorb orange/red light.
What kind of light do rod cells absorb?
Green
What are the only spiking neurons in the eye?
RGCs.
They convey information about the visual scene in the form of spike trains - retinal code.
What neurotransmitter is used in vertical connections?
Glutamate.
What neurotransmitters are used in horizontal connections?
GABA (horizontal and amacrine)
Glycine (amacrine)
Acetylcholine (amacrine)
Dopamine (amacrine)
What is the function of the retina?
To convert light into electrical signals - phototransduction performed by rods and cones.
What does processing in retinal neuronal networks convey?
Information on luminance, contrast, colour and on more complex image features such as orientation and motion.
Describe peripheral vision (rods).
Rods distributed throughout the retina, responsible for being able to see in dim light.
High sensitivity.
Operate in low light only with a narrow spectrum of intensity.
Monochromatic.
Low-resolution images.
Slow temporal responses to changes in illumination.
Describe central vision (cones).
Cones concentrated around the fovea.
Works only in daylight.
Low sensitivity operating over a broad spectrum of intensity.
Chromatic.
High spatial acuity.
Narrow angle of coverage.
Fast responses to changes in illumination.
What happens when cones degenerate?
Loss of colours, details and a central blind area.
What happens when rods degenerate?
Tunnel vision, narrow angle of coverage.
What visual pigment is in rods?
Rhodopsin.
What visual pigment is in cones?
Conospin.
Describe how opsin works.
Molecule changes shape when they hit a certain wavelength of light.
Light activates opsin > closure of cgMP-gated sodium channels in outer segment membrane.
Channels open in the dark allowing sodium influx = dark current.
Channels close in light, stopping sodium influx = membrane hyper polarisation.
What are horizontal and amacrine cells important for?
Horizontal - delimit the receptive field centre-surround (contrast sensitivity).
Amacrine - temporal aspects of light responses (transient vs sustained response).
What are the 2 big families of retinal disorders?
Optic neuropathies where photoreceptors are not dead but communication to the brain is lost.
Photoreceptor where photoreceptors degenerate/die.
What are some examples of optic neuropathies?
Diabetic retinopathy
Gluacoma
Mitochondrial disorders
MS
