The Eye and Vision Flashcards
What are the 2 two compartments of the eye?
Where are they?
What is their function?
Vitreous humour = between the lens and the retina, gives eyeball shape
Aqueous humour = between cornea and lens, nourishes cornea
Where is aqueous humour produced and reabsorbed?
It is secreted into the posterior chamber and reabsorbed in the anterior chamber
What is glaucoma?
Increased pressure in the aqueous humour due to increased production or blocked resorption
What are the two types of glaucoma?
What are their causes?
Which is more serious?
- Angle Closure —> blindness
- iris adheres to cornea, blocks reabsorption from anterior chamber - Open Angle - common
- sclerosis of veins where humour is reabsorbed
What are the treatment options for glaucoma?
- Surgery? - angle closure only
- Medication
- beta blocker, decrease production of aqueous humour
- prostaglandin analogues –> vasodilation, increased reabsorption
What are the five cell types in the retina?
How are they organised?
What are their functions?
Deep to superficial
- photoreceptors = detect light, photosensitive pigment
- horizontal = modilate transmissoin
- bipolar = transmit signal from photoreceptors to ganglion cells
- amacrine = modulate transmission
- ganglion = generate action potentials
What is the optic nerve made up of?
axons of ganglion cells in the retina
- all the axons converge at the optic disc to form the optic nerve
What are the two types of photoreceptors?
Rods (rhodopsin)
Cones (3 x opsin, red, blue, green)
Describe the structure of a rod cell
Intracellular membrane discs stacked in the outer segment of the cell.
They do not interact with each other or the cell membrane
High density –> large SA for absorption
- V sensitive (night vision)
Describe the structure of a cone cell
Invaginations of surface membrane of outer segment of the cell
Less dense than in rod cell –> smaller SA, need a high light intensity
–> colour vision (daytime)
Describe the regional organisation of the retina and the properties of each area
- Blind spot = optic nerve leaves eye, no photoreceptors
- Increasing proportion of cones closer to fovea
- Fovea = all cones
Peripheral Retina - high sensitivity to light, low acuity - Rods (lots of photopigment)
- Many rods connect to one ganglion cell
Macula/Central Retine
Fovea - low sensitivity, high acuity - cones (less photopigment)
- 1 cone : 1 ganglion cell
- lateral displacement of cells other than photoreceptors
What is the definition of focusing in terms of light hitting the retina?
Moving eye structures to focus the image on the fovea
What are the 2 components of rhodopsin?
What is the role of each component?
Retinal (chromophore)
Opsin (GPCR)
Describe phototransduction in rods in the dark
- cyclic GMP gated cation channel = OPEN
- Na+ influx (dark current) –> depolarisation
- so Em = -30mV
- Depolarisation –> glutamate release onto the synapse with a bipolar cell
Describe phototransduction in rods in the light
Retinal absorbs light
- hydrogen ion flips at 11-cis position(11-cis –> all trans)
Opsin activation
GTP binds to Gprotein (transducin)
G protein activates PDE
PDE breaks down cyclicGMP –> GMP
- cyclic GMP gated cation channel now closed
–> hyperpolarisation of rod
–> decrease glutamate release onto bipolar cell
What are the 3 cell types in the direct pathway of transmission through the retina
Photoreceptor –> Bipolar cell –> Ganglion cell
Direct Pathway
- Photoreceptors always …………. in the light
- Response of the other cells is dependent on their …….. and their different ………
- There are two classes of bipolar cells
ON (…………… in light)
and
OFF (………………. in light)
Photoreceptors are always hyperpolarised in the light
Response of other cells is dependent on their type and their different receptors
There are two classes of bipolar cells
ON - depolarised in light
OFF - hyperpolarised in light
Describe the electrical position of the following cells in the following situations:
- ON bipolar cell in the light
- ON bipolar cell in the dark
- OFF bipolar cell in the light
- OFF bipolar cell in the dark
- ON bipolar cell in the light = decreased glutamate –> depolarised
- ON bipolar cell in the dark = increased glutamate –> hyperpolarised
- OFF bipolar cell in the light = decrease glutamate –> hyper polarised
- OFF bipolar cell in the dark = increased glutamate –> depolarised
Describe the indirect pathway of transmission through the retina
Each bipolar or ganglion cell has a receptive field (region of the retina that influence that cell)
Centre = direct connections
Surround = affect Horizontal/Amacrine cells
Light in receptive field surround
- opposite electrical responses to B/G cells
- because of the influence of Horizontal/Amacrine cells
- important for contrast at image borders
OFF-centre ganglion cell output:
- where can you see the greatest response?
Dark centre and light surround
What can the visual field be divided up into?
- left and right visual hemifield
- nasal and temporal
- right/left optic nerve
- left/right optic tract
- optic chiasm
Look at visual field defects
…
Explain the 3 responses of OFF ganglion cell to light in receptive field
a) centre = light --> hyperpolarised surround = light --> depolarised = overall hyperpolarised b) centre = dark --> depolarised surround = light --> depolarised = greatest response c) centre = dark --> depolarised surround = dark --> hyper polarised = overall depolarised, but less so
