Lecture 26 - Retinal Dystophies

Question 1

Where is the retina situated?

Answer 1

In the back of the eye and is part of the CNS (an outpost of the brain).

Question 2

What is the vertebrate retina?

Answer 2

Complex, layered structure that performs all of the initial steps of visual processing.
Seven cell types, three nuclear layers and two synaptic layers.

Question 3

Describe retinal organisation.

Answer 3

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.

Question 4

What does Bruch’s membrane do?

Answer 4

Divides the RPE from choroid (network of blood vessels).

Question 5

Describe the fovea.

Answer 5

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.

Question 6

Describe the optic disc.

Answer 6

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.

Question 7

How many types of cone cells are there?

Answer 7

3

Question 8

Describe the different kinds of cone cells.

Answer 8

Short cones - absorb blue light.
Medium cones - absorb green light.
Long cones - absorb orange/red light.

Question 9

What kind of light do rod cells absorb?

Answer 9

Green

Question 10

What are the only spiking neurons in the eye?

Answer 10

RGCs.
They convey information about the visual scene in the form of spike trains - retinal code.

Question 11

What neurotransmitter is used in vertical connections?

Answer 11

Glutamate.

Question 12

What neurotransmitters are used in horizontal connections?

Answer 12

GABA (horizontal and amacrine)
Glycine (amacrine)
Acetylcholine (amacrine)
Dopamine (amacrine)

Question 13

What is the function of the retina?

Answer 13

To convert light into electrical signals - phototransduction performed by rods and cones.

Question 14

What does processing in retinal neuronal networks convey?

Answer 14

Information on luminance, contrast, colour and on more complex image features such as orientation and motion.

Question 15

Describe peripheral vision (rods).

Answer 15

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.

Question 16

Describe central vision (cones).

Answer 16

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.

Question 17

What happens when cones degenerate?

Answer 17

Loss of colours, details and a central blind area.

Question 18

What happens when rods degenerate?

Answer 18

Tunnel vision, narrow angle of coverage.

Question 19

What visual pigment is in rods?

Answer 19

Rhodopsin.

Question 20

What visual pigment is in cones?

Answer 20

Conospin.

Question 21

Describe how opsin works.

Answer 21

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.

Question 22

What are horizontal and amacrine cells important for?

Answer 22

Horizontal - delimit the receptive field centre-surround (contrast sensitivity).
Amacrine - temporal aspects of light responses (transient vs sustained response).

Question 23

What are the 2 big families of retinal disorders?

Answer 23

Optic neuropathies where photoreceptors are not dead but communication to the brain is lost.
Photoreceptor where photoreceptors degenerate/die.

Question 24

What are some examples of optic neuropathies?

Answer 24

Diabetic retinopathy
Gluacoma
Mitochondrial disorders
MS

Question 25

What are some examples of photoreceptor dystrophies?

Answer 25

Retinitis pigments
Age related macular degeneration (AMD)

Question 26

Why are optic neuropathies hard to treat?

Answer 26

Because RGCs and the optic nerve have degenerated so visual centres of the brain need to be stimulated directly.

Question 27

What are some treatments for retinal dystrophies?

Answer 27

Easier to restore vision at the retinal level.
Retinal prosthetics create light perception in a blind person by stimulating a small area of RGCs.
Optic nerve can be stimulated via a cuff electrode.
Direct stimulation of the visual cortex.
Subretinal implants and epiretinal implants.
Optogenetics.