Visual Defects Flashcards
What is the ‘lens’? What structures is it suspended by? What muscles is it connected to?
- A transparent structure
- Suspended by ligaments (zonule fibres) attached to the ciliary muscles which control the shape of the lens.
What is the vitreous humour? Function?
Viscous, jelly-like substance that lies between the lens and the retina. Keeps the eye spherical.
Which part of the eye keeps it spherical?
Vitreous humour
Which part of the eye is light transformed into neural activity?
Retina
Which part of the eye is the point of highest visual acuity? Why?
Fovea - in the line of the visual axis where light can reach the photoreceptors directly
OPHTHALMOSCOPIC VIEW OF RETINA
Macula: central vision, colour vision
Fovea: central/ thinner region of retina
Optic disc: origin of blood vessels, where the optic nerve axons exit the eye (blind spot)
Which part of the eye is responsible for colour vision?
Macula
How is light focused onto the retina?
Refraction by the cornea (and lens) - then passes through the vitreous humour to the retina
What are the 2 layers of the retina?
An outer pigmented layer called the pigment epithelium, which adheres to the choroid, and an inner layer of nerve tissue called the sensory (or neural) retina.
What are the cells in the pigment epithelium of the retina filled with? Purpose of this?
Cells in the pigment epithelium are filled with melanin which absorbs light which isn’t passed on to photoreceptors (stops damage).
What composes the neural layer of the retina?
Photoreceptors, bipolar cells, ganglion cells etc
What is the other function of the pigment of the retina?
Pigment also provides nutrients which are required for photoreceptors to work properly
Light passes through all the other retinal cells to reach the photoreceptors at the back of the retina. Then describe the order which light passes between cells
- Photoreceptors: receive photons
-
Retinal biopolar cells: connect photoreceptors and ganglion cells
- Receive signals from photoreceptors and excite ganglion cells but releasing glutamate onto the dendrites of ganglion cells
- Ganglion cells: receive signal from bipolar cells, axons form the optic nerve
BUT 2 inhibitory neurons are involved:
- Horizontal cells: inhibitory neurons in the outer retina which help integrate and regulate the input from photoreceptors, also help adapt retina to different light levels
- Amacrine cells: inhibitory neurons in the inner retina which
- modulate transmission from bipolar cells to ganglion cells (form synapses onto the synaptic terminal of the bipolar cells as well as onto the dendrites of ganglion cells)
What 2 interneurons (inhibitory neurons) are found in the retina?
- Horizontal cells
- Amacrine cells
Which neurotransmitter does biopolar cells release?
Glutamate (excitatory)
What is phototransduction?
Conversion of light energy into an electrical signal (how information is moved along neurons).
Where does phototransduction occur?
Outer segments of photoreceptors are filled with lots of membranous discs – this is where phototransduction happens.
Cones vs rods;
a) sensitivity
b) light level
c) number
d) photopigment
e) acuity
f) convergence
Rods:
a) Very sensitive to light, can pick up scattered rays
b) low light, nighttime
c) 20x more common
d) High photopigment so can capture more light
e) low acuity
f) High level of convergence
Cones:
a) Not sensitive to light, must have direct ray
b) Day time, bright light
c) uncommon
d) Low photopigment so captures less light
e) high acuity
f) Lower level of convergence: one per ganglion in macula
Where are rods found? Where are cones found?
Rods: All over the retina except the fovea
Cones: Most dense at the fovea and macula
In the dark, are photoreceptors depolarised or repolarised? Describe their glutamate release
In the dark photoreceptors are depolarised (to around -30 mV) and continuously release glutamate
In the light, are photoreceptors depolarised or repolarised? Describe their glutamate release
Light causes these photorecptors to repolarise/hyperpolarise (as depolarising ion channels close) and decrease their glutamate release.
What are membranous discs of photoreceptors packed full of?
Photopigments
What are photopigments?
Each photopigments is a single transmembrane protein (G-protein coupled receptors) –> ‘opsin’ proteins
What are the photopigments called in rods?
Rhodopsin
What is found inside each ‘opsin’ protein?
A central molecule of retinal
How is retinal synthesised?
Retinal is synthesised from vitamin A, can also be made from alpha and beta carotene (a form of vitamin A found in carrots)
What happens when photons hit the photoreceptors (i.e. what happens in phototransduction)?
- Retinal hit with photon of light and activates the opsin molecule (1 photon is absorbed by 1 opsin protein)
- 1 opsin then activates transducin molecules
- Transducin molecules then activates phosphodiesterase enzymes
- PDE enzymes convert cGMP to GMP
- As cGMP levels fall, the cGMP sensitive ion channels close (these channels are the ones responsible for depolarisation in the dark)
- Causes hyperpolarisation and decrease in glutamate release
How does the density of photoreceptors affect visual acuity?
Photoreceptors are the retina’s equivalent to pixels; denser photoreceptors = clearer quality
How are rods and cones distributed across the retina?
- Generally high density of rod cells in the periphery
- Very low density of cone cells in the periphery
- No rod cells at the fovea
- Many cone cells at the fovea
How is daytime vision at the periphery of the retina?
Daytime vision is bad at periphery of retina, good in the centre
Can the fovea be used in low light level? Why?
Fovea cannot be used in low light level, due to only cones being present.
In the dark, what are photoreceptors continuously releasing?
Glutamate (bipolar cells sense this glutamate via their dendrites)
There are 2 types of bipolar cells based on how they respond to light. What are they?
- OFF bipolar cells
- ON bipolar cells
How do ON bipolar cells respond to light?
depolarise –> increasing AP firing
How do OFF bipolar cells respond to light?
hyperpolarise
What causes this difference in bipolar cells?
Types occur due to the differential expression of glutamate receptors they express
What glutamate receptors do OFF bipolar cells express?
express AMPA/Kainate receptors (ionotropic receptors)
In the dark, describe what happens to OFF bipolar cells
- Dark = lots of glutamate released by photoreceptors
- Glutamate binds to AMPA/Kainate receptors –> causes receptors to open
- Depolarising current flows into cell and bipolar cell depolarises
In the light, describe what happens to OFF bipolar cells
- Light = no glutamate released by photoreceptors
- Glutamate unbinds from AMPA/Kainate receptors and ion channels close
- Bipolar cell hyperpolarises
What glutamate receptors do ON bipolar cells express?
express mGluR6 and TRPM1 receptors
In the dark, describe what happens to ON bipolar cells
- Dark = lots of glutamate
- mGluR6 receptors bind to glutamate
- This inhibits ion channels in the membrane called TRPM1 channel (cation channel)
- Causes hyperpolarisation
In the light, describe what happens to ON bipolar cells
- Light = no glutamate
- Glutamate can’t bind to mGluR6 receptors which prevents inhibition of TRPM1 channels –> channels open
- Bipolar cells depolarise –> increase AP firing
In the fovea, how many bipolar cells is each photoreceptor connected to?
2 - 1 OFF and 1 ON
In the fovea, cone cells are connected to two bipolar cells. In the light, what happens to each bipolar cell?
- ON bipolar cells depolarise. There will be increased glutamate release at the depolarised ON cell, leading to signals at the ON ganglion cell.
- OFF bipolar cell will hyperpolarise.
What is each ON and OFF bipolar cell attached to?
An ON and OFF ganglion cell
In the dark, what happens to each bipolar cell/ganglion cell?
- ON bipolar cells will hyperpolarise
- OFF bipolar cells will depolarise and release glutamate to the OFF ganglion cell
What are ionotropic receptors?
Membrane-bound receptor proteins that respond to ligand binding by opening an ion channel and allowing ions to flow into the cell, either increasing or decreasing the likelihood that an action potential will fire.
What is a receptive field?
the area of the retina that causes any change in response of a neuron
In the outer retina, what shapes the receptive fields of bipolar cells?
horizontal cells
In the inner retina, what shapes the receptive fields of ganglion cells?
amacrine cells
Ganglion cells have a receptive field centre. What determines this?
The lateral extent of the ganglion cell’s dendrites across the retina (which makes it sensitive to any visual stimuli across this region of photoreceptors) determines the receptive field centre
What is lateral inhibition?
- Lateral inhibition occurs from the bipolar cells and amacrine cells on either side of the central receptive field.
- The rods in the centre of the stimulus will transduce the “light” signal to the brain, whereas different rods on the outside of the stimulus will send a “dark” signal to the brain due to lateral inhibition from horizontal/amacrine cells.
- This contrast between the light and dark creates a sharper image.
What is;
a) receptive field centre
b) receptive field surround
a) Receptive field centre is due to direct connection to a glutamatergic neuron
b) Receptive field surround is due to ‘lateral inhibition’ from inhibitory neurons and is usually much larger than the dendritic field
What is the purpose of lateral inhibition and receptive field centre?
Allows comparison of light between centre and surround, so edges of objects can be identified. A shadow around an item will experience decreased firing due to lateral inhibition, bringing the object into clearer view.
Example of receptor field:
- Shadow in ‘surround’ RF only - hyperpolarisation & decreased firing
- As ‘edge’ moves over ‘centre’ get depolarization & increased firing
- When shadow completely covers ‘centre’ & ‘surround’ then firing decreases again
- Conclude that optimal stimulus is dark-light border across ‘centre’ & ‘surround’ RF i.e. edges
How is every colour in the rainbow obtained?
- Every colour in the rainbow can be obtained by mixing the proper ratio of red, green and blue light.
- Brain assigns colour based on the comparison of light falling on three cone types: blue, green, red.
When all 3 cone types are equally active, what colour do we perceive?
white
What are photoreceptor pigments called in;
a) rods
b) cones
a) rhodopsin
b) 3 types of opsins with different spectral sensitivity:
- Blue: 420nm
- Green: 530nm
- Red: 560nm
What is ‘colour opponency’?
Colour is coded with an opponent process: two colours are compared with one colour reducing ganglion cell activity and the other increasing it.
What are the 2 opposing colour pairs?
- Red vs green
- Blue vs yellow
This produced the colour wheel, as we cannot see a ‘yellowy-blue colour’ but we can see ‘greeny-blue’ or ‘bluey-red’
Colour opponency explained:
If there was just red light on the centre of the ganglion’s receptive field, we would get maximal firing. If this red light extended over the surround as well, we are going to get some weak inhibition from the green on bipolar cells (from amacrine cells).
If there was green light in the surround and red light in the centre, this would inhibit the excitation in the centre quite strongly.
What is melanopsin? What cell is it expressed in?
- 5th photopigment in the eye
- Sensitive to blue light
- Expressed in intrinsically photosensitive retino-ganglion cells (ipRGC)
Where do ipRGCs communicate blue light information directly to?
Communicate information directly to the suprachiasmatic nucleus (SCN) in the hypothalamus (situated directly above the optic chiasm) –> bypasses the thalamus
What is the SCN?
responsible for controlling circadian rhythm
How can blue light affect circadian rhythm?
Lots of blue light (e.g. phone screens) can disrupt circadian rhythm
What 2 aspects are ipRGCs involved in?
- Blue light
- Pupillary light reflex
What is the target of ipRGCs in the pupillary light reflex?
ipRGCs ganglion cells project directly to pre-tectal area (bypass thalamus) to the olivary pretectal nucleus
What occurs at the OPN?
Olivary pretectal nucleus: where the neurons of the ipRCGs converge and then connect to the Edinger-Westphal nuclei (parasympathetic preganglionic of the ciliary ganglia)
The lateral geniculate nucleus exhibits a layered structure. Describe this layered structure
There are two magnocellular layers, four parvocellular layers, and koniocellular layers between each of the magnocellular and parvocellular layers
Describe the parvocellular layers of the LGN. What ganglion cells do they receive input from?
- Small cell bodies
- Receive input from P type Retinal Ganglion Cells (RGC) smaller cell type making up 90% of the population
What are the parvocellular layers sensitive to?
Sensitive to stimulus from fine detail
Describe the magnocellular layers of the LGN. What ganglion cell do they receive input from?
- large cell bodies
- receive input from M type RGCs (large receptive field)
What are the magnocellular layers sensitive to?
Important for detection of stimulus movement
Describe the koniocellular layers. What ganglion cells do they receive input from?
- Very small cell bodies found in between each main layer
- receives input from K-type ganglion cells
Which retinal ganglion cell makes up the majority of the population?
P type RGC
how do neurons of the LGN project to the primary visual cortex?
via optic radiations
what is the primary visual cortex also known as?
V1 / Brodmann’s area 17 / striate cortex
Where is V1 located?
Located in the occipital lobe, on either side of the calcarine fissure
What are 3 components of the primary visual cortex?
- Orientation columns
- Ocular dominance
- Colour processing - blobs
What are orientation columns?
Organized regions of neurons located in the 1ary visual cortex that are excited by visual line stimuli of varying angles
What are ocular dominance columns?
Ocular dominance columns are stripes of neurons in the visual cortex that respond preferentially to input from one eye or the other.
- Inputs from the two eyes are still largely separate in V1
- Some inputs will be stronger from one eye over the other
What are ‘blobs’?
Sections of the visual cortex where groups of neurons that are sensitive to colour
- Pillars through the cortex are enriched with cytochrome oxidase – a mitochondrial enzyme (cell metabolism)
- Staining with cytochrome oxidase reveals “pillars” running through layers II, III, V & VI
- Each pillar is centred on an ocular dominance column
Striate cortex, or V1, is the first region of visual processing in the cortex. There are dozens of other areas of the cortex involved in extrastriate visual processing.
There are thought to be two cortical streams of visual processing., What are they?
- Striate cortex towards parietal lobe: visual motion
- Striate cortex towards temporal lobe: recognition of objects
What would a lesion of the visual cortex lead to?
Lesion of visual cortex can lead to complete blindness of perception, but not of sight.
What is ‘scotopic’ vision? Which type of photoreceptor is involved in this?
- Vision of eye under low-light levels
- Rods
What is ‘photopic’ vision? Which type of photoreceptor is involved in this?
- Vision of the eye under well-lit conditions
- Cones
Which photoreceptor is involved in colour determination?
Cones
Which photoreceptor is involved in determining different shades of grey?
Rods
The outer segment of a rod is filled with lots of membranous discs which are packed full of what?
A pigment called rhodopsin
What is rhodopsin made up of?
2 things;
- Retinal
- Opsin
What is retinal a derivative of?
Vitamin A
What is opsin?
A protein
Cones also consists of a pigment. What is this pigment?
Photopsin
How many different types of photopsin are there?
3; photopsin I, II and III
Photopsin pigment (found in cones) can detect different wavelengths of light to determine which 3 main colours?
- 1) Blue part of visible spectrum (lowest wavelength)
- 2) Red (highest wavelength)
- 3) Green
What does photopsin consist of?
- Iodine
- Opsin
Which neurotransmitter do horizontal interneurons secrete?
GABA (inhibitory)
Function of horizontal interneurons?
Located between photoreceptors and bipolar cells - inhibitory effect to help modulate activity as we move between bright and dark light
Describe shape of bipolar cells
1 dendrite extension and 1 axon extension both arising from cell body
Which neurotransmitter do bipolar cells secrete?
Glutamate
Describe the phototransduction pathway in the light in rods only (N.B. is identical in cones)
- Photon enters eye and hits photoreceptor
- Hits rhodopsin molecule in membranous disc of rod
- Causes retinal to become separated from opsin protein
- Opsin goes off and activates transducin protein
- Transducin activates phosphodiesterase enzyme (PDE)
- PDE converts cGMP to GMP (cGMP is then no longer active so cannot bind to channels in rod membrane)
- Channels in membrane close (Na+ and Ca2+ cannot move into photoreceptor)
- Cell hyperpolarises (more negative)
- Very few/no APs travel down axon of rod
- Little/no glutamate is released
What does opsin activate in the phototransduction pathway?
Transducin protein
What does transducin activate in the phototransduction pathway?
PDE
Action of PDE in the phototransduction pathway?
Converts cGMP to GMP
What is the effect of cGMP being bound to channels on rods membrane?
When bound it opens up the channels and allows influx of Na and Ca (making inside of cell positive response).
In the dark, is cGMP bound to channels in rods membrane? What is effect of this?
cGMP is bound to channels in rod membrane; Na+ and Ca2+ entering cell (inside of cell more positive) and depolarisation occurs
In the light, how much glutamate is released?
Very little
What is the effect of very little glutamate being released in the light on bipolar and ganglion cells?
- Very little glutamate released by photoreceptor stimulates bipolar neuron
- Very few cations leave cell and cell becomes more positive
- Causes increase in glutamate released by bipolar neuron onto ganglion cells
- Increase in glutamate from bipolar cells causes increase in actions potentials moving down optic nerve
In the light, describe the glutamate levels released by;
a) photoreceptors
b) bipolar cells
a) very little
b) lots
How do horizontal interneurons inhibit bipolar cells?
Glutamate from photoreceptors is released onto bipolar cells but can also be released onto horizontal interneurons as well:
- Release of glutamate onto horizontal cells stimulates horizontal cells
- Horizontal cells then release GABA; they are inhibitory interneurons
- GABA then inhibits photoreceptors
Purpose of inhibitory effect of horizontal interneurons?
In bright light, horizontal cells function to adapt bipolar cells in depending level of light (e.g. desensitise photoreceptors in bright light)
Purpose of inhibitory effect of amacrine cells?
Inhibitory effect on ganglion cells; modulates action potentials and visual pathway (makes sure it’s very precise)
Describe the phototransduction pathway in the dark in rods only (N.B. is identical in cones)
- Retinal and opsin join back together
- Transducin not activated
- PDE not activated
- cGMP not broken down and remains bound to channels in rod membrane (levels of cGMP increase)
- Na+ and Ca2+ enter cell –> depolarisation
- More glutamate released
- Lots of glutamate released by photoreceptor inhibits bipolar cell
- Causes decrease in glutamate released by bipolar cell
- Very little action potentials moving down ganglion cells (optic nerve)
There are 2 types of bipolar cells based on how they respond to light. What are they? How does each respond to light?
- ON –> depolarise in response to light, increasing AP firing
- OFF –> hyperpolarise in response to light, decreasing AP firing
Which glutamate receptors do ON bipolar cells express?
Express mGluR6/TRPM1 receptors
Which glutamate receptors do OFF bipolar cells express?
Express AMPA/Kainate glutamate receptors
In the dark, describe the glutamate levels released by;
a) bipolar cells
b) photoreceptors
a) little
b) lots
In the dark, lots of glutamate is released by photoreceptors. Describe the effect of this on ON vs OFF bipolar cells
- ON:
- mGluR6 receptors bind to glutamate
- This inhibits ion channels
- Causes hyperpolarisation
- OFF:
- Glutamate binds to AMPA/Kainate receptors
- Ion channels open
- Bipolar cell depolarises
In the light, no glutamate is released by photoreceptors. Describe the effect of this on ON vs OFF bipolar cells
ON:
- Glutamate can’t bind to mGluR6 receptors
- Prevents inhibition of ion channels – they open
- Bipolar cell depolarises and increases AP firing
OFF:
- Glutamate unbinds from AMPA/Kainate receptors
- Ion channels close
- Bipolar cell hyperpolarises
