W4: Physiology Of The Eye Flashcards
Functions of the eye
Produce clean crisp images
Self cleaning through tears and blinking
Retina converts light into electrical signal
What is contained in the anterior segment of the eye?
Outer cornea, portion of white sclera, the iris, pupil, lens, ciliary bodies, zonule fibres.
What is the fluid in the anterior chamber?
Aqueous humour, AqH
Describe aqueous humour
Nourishes the cornea. Produced in the ciliary body and flows through from posterior to anterior chamber. Removes waste and debris.
What is in the posterior segment of the eye?
Vitreous body - gel-like substance
Retina
Choroid - heavily innervated by blood vessels
Outer tough collagenous sclera
What is the route of light through the retina?
Light passes all the way through the retina until it hits the photoreceptors at the back of the eye. Signal is transduced down to the retinal ganglion cells. This causes a hyperpolarisation of bipolar cells so we get signals in from photoreceptors to bipolar cells. They then converge on retinal ganglion cells (RGCs). Axons from these cells make up the optic nerve.
How do retinal cells connect?
Via chemical synapses
What cells generate action potentials in the retina?
Ganglion cells - direct pathway
What do horizontal and amacrine cells do?
They modulate transmission of information and enable lateral transmission. Information flowing through this region is the indirect pathway.
Where do signals go from the direct and indirect pathways?
Signals go through the optic disc = a portion of the back of the retina where all of the axons go through into the brain
What are photoreceptors?
Light sensitive cells in the retina. Rods or cones.
What are rods and cones?
Membranous disks which contain light sensitive photopigments - leads to the alteration in the membrane potential in the photoreceptor,
Which has more membranous disks?
Rods have more membranous disks than cones.
Structure of rods
Inside another membrane
Structure of cones
Made up of folds
What are the photopigment(s) in rods?
One photopigment = rhodopsin
What are the photopigment(s) in cones?
3 photopigments (opsins) - red, green and blue. Gives colour vision.
What are rods responsible for and how?
Since there are more discs which contain rhodopsin which is activated by light they are more sensitive to light than cones, so responsible for night vision.
What are cones responsible for and how?
Needs higher light levels in order to function because of the less discs. Hard to see colour in the dark. Gives night vision.
What is the blind spot?
Where the optic nerve leaves the eye. Has no rods or cones.
Where are the most cones found?
There are an increasing proportion of cones closer to the fovea (all cones)
Describe the peripheral retina
Has a high sensitivity to light
Has mostly rods so lots of photopigment
Many rods connect (via bipolar cells) to 1 ganglion cell, meaning there is a high sensitivity but low visual acuity (peripheral vision is more blurry)
Describe the macula
Central retina
Has more cones
Contains the fovea
What is the fovea?
A pit inside the macula with only and all cone cells.
Why does the fovea have a low light sensitivity?
Because it has all cones so less photopigment
1 cone converges via a bipolar cell to 1 ganglion cell. This means that a substantial light signal will be needed to be picked up by the neurone/RGC. This gives high acuity = fine detail. Different light levels on different cones allows fine detail.
Lateral displacement of cells other than photoreceptors.
Describe the channels orientation in the dark
- cGAMP-gated cation channels are open - so Na+ can enter the cell
- Na+ influx - dark current. Leads to a depolarisation due to more sodium coming in than potassium leaving = Na+ influx > K+ efflux
- Em = -30mV
- Depolarisation causes glutamate release (from photoreceptor) at the synapse with bipolar cells.
Describe the visual pathway
Light passes all the way through the retina until it hits the photoreceptors at the back of the eye. This causes a hyperpolarisation of bipolar cells, so signal sent to them. They then converge on the retinal ganglion cells (RGCs).
What cells make up the optic nerve?
Axons from retinal ganglion cells.
What is rhodopsin made from?
Rhodopsin (inactive) = retinal (chromophore) + opsin (G-protein coupled receptor)
Where is rhodopsin located and where is cyclic GAMP-gated cation channel located?
Within the disks is the rhodopsin. Within the rod cell membrane is the cyclic GAMP gated cation channel for sodium.
What happens in the direct pathway when there is light exposure?
Light reacts with rhodopsin and the rod becomes hyperpolarised, causing a decrease in glutamate release.
- retinal absorbed light
- opsin activated (G-protein coupled receptor)
- GTP binds to G-protein (transducin)
- G-protein activates phosphodiesterase, PDE (enzyme)
- PDE breaks down cyclic GMP from cyclic form to linear form so it can no longer bind to the sodium channel and thus it is closed, so sodium influx stops.
- so potassium leaving the cell leads to a hyperpolarisation. The increase in potassium efflux (compared with sodium influx) decreases the release of glutamate to bipolar and horizontal cells.
Do rods and cones have the same action in light and dark?
Yes, photoreceptors, rods or cones, will always hyperpolarise in the light and depolarise in the dark.
What are the subtypes of bipolar and ganglion cells?
ON: depolarise in response to light
OFF: hyperpolarise in respond to light
Thus a different response depending on whether the photoreceptor synapses with an ON or OFF cell.
Describe what happens with ON bipolar/ganglion cells which are depolarised by light.
- in the dark glutamate is released from photoreceptors leading to a hyperpolarisation = an inhibitory synapse (between photoreceptor and ON bipolar cell)
- in the light, less glutamate is released from the photoreceptors, so the hyperpolarisation is removed, allowing the ON bipolar/ganglion cell to depolarise.
- ganglion cells send APs in the light
Describe what happens in OFF bipolar/ganglion cells which are hyperpolarised by light.
- in the dark, glutamate released from the photoreceptor cells leads to depolarisation = an excitatory synapse (between photoreceptor and OFF bipolar cells)
- in the light, there is a decreased glutamate release, causing the bipolar/ganglion cells to hyperpolarise.
- ganglion cells send APs in the dark
Why do ON and OFF bipolar/ganglion cells have opposite electrical responses to light?
Due to different glutamate receptors in each cell type.
What cells are activated in the indirect pathway?
Horizontal and amacrine cells
What does the indirect pathway allow us to do?
Differentiate between different light levels
Each bipolar or ganglion cell has a receptive field. What is a receptive field?
A region of the retina that influences that cell.
What is the central part and surround part of the receptive field of a bipolar or ganglion cell?
Central part - involves direct connections from photoreceptors (leg,m PR to BC)
Surround part - indirect connections only via horizontal/amacrine cells to reach GC
What happens in ON a cells in the indirect pathway?
- hyperpolarisation of the photoreceptor and depolarisation of the bipolar cell
- hyperpolarisation of the photoreceptor can lead to hyperpolarisation of the horizontal cell which can have an influence on the ON cell to be hyperpolarised.
- we get activation through different parts of the retina, central and surround parts
Describe the light in a receptive field surround.
- has opposite electrical responses in BC/GC (compared to response via direct pathway)
- because of the influence of horizontal and amacrine cells
- allows us to see contrast at image borders, enhancing images we see
- central part has a greater influence than the surround part
What happens in OFF cells in the indirect pathway?
Light depolarises OFF cells - via the surround part in the indirect pathway
What causes the most action potentials fired?
Depolarisation in the centre and hyperpolarisation in the surround
