Vision: The Eye Flashcards
Anatomy of the human eye
- fluid filled sphere
- 3 layers of tissue
- only the retina contains neurons
- ciliary body encircles the lens: ciliary muscle adjusts the lens and ciliary processes supply eye with fluid
- iris: coloured region that contains two sets of muscles to adjust pupil size
- pupil: the opening in the centre
- sclera: outer most tissue layer
- cornea: transparent tissue that permits light rays to enter the eye
accommodation in the human eye
retina contains all the neurons
being able to adjust that lens gives us the ability to make sure the image falls perfectly on the retina
accommodation and disorders
- emmetropia (normal): relaxed ciliary muscles results in focused image in the distance
- myopia (nearsighted): light rays are focused in front of the retina
- hyperopia (farsighted): light rays are focused beyond the retina
macular degeneration =
the creation of blind spots, reduced acuity, etc
photoreceptors for colour are only found in
the small foveal region
patients with glaucoma
the mind fills in the blindspot - but if you have a glaucoma in both eyes, it is more difficult for the brain to fill this in
structure of the retina 1
FIVE BASIC CLASSES OF NEURONS
- Photoreceptors: rods and cones (they synapse onto bipolar cells which synapse onto ganglion cells which head out the optic nerve to the brain)
- Bipolar cells
- Ganglion cells
- Horizontal cells: modulate interaction between photoreceptors and bipolar cells
- Amacrine cells: modulate interactions between photoreceptors and ganglion cells
2 TYPES OF PHOTORECEPTORS
- rods
- cones
Order of the retinal process
Photoreceptor
(horizontal cell modulation - allows sensitivity to a wide range of illumination)
Bipolar cell
(amacrine modulation - modulates the visual response by allowing the response to be more sophisticated)
Ganglion cell
(ganglion axons form optic nerve)
Brain
Structure of rods v cones
Rod: found more in the periphery and are not sensitive to one specific wavelength of light - they support our low light vision
Cones: found primarily in the central foveal region –> 3 types of cones that detect light at different wavelengths, giving us colour vision
the physiological scotomoa is
a blind region where the ganglion cell axons leave the retina in the optic nerve
Why are the photoreceptors in the deepest layer?
- Photoreceptor disks ‘wear out’ (occurs over 12 days) and so continuously move up the photoreceptor until they reach the top, are pinched off, and then recycled (phagocytosed)
- Pigment epithelium regenerates the photopigment after exposure to light.
- Blood flow in pigment epithelium supplies photoreceptors
Phototransduction: absorption of light by the photopigment results in change in membrane potential
- photoreceptors do not exhibit action potentials (graded response)
- light activation causes a graded change in membrane potential
- shining a light on a photoreceptor results in hyperpolarization (meaning less transmitter is released onto the post synaptic neurons) –> as in the dark, the receptor is naturally depolarised
What changes the membrane polarisation?
The same thing happens in rods and cones
IN THE DARK:
- large amount of cGMP floating around the outer segment –> resulting in a lot of open channels that depend on cGMP (cations)
- cations (particularly Na) enter into the cell (depolarising influence)
- K flows out consistently (hyperpolarising) but the overall cell is DEPOLARISED
LIGHT HITS:
- in the outer segment, photons are absorbed –> decreases cGMP
- many cGMP gated channels close
- cations can’t enter cell
- reduced inward current (less depolarising)
- the inner segment isn’t affected by cGMP therefore K keeps flowing out of the cell
- therefore HYPERPOLARISED cell
What happens when a photon is absorbed?
Biochemical process when light hits
- photon –> absorbed by retinal –> changes the configuration –> causes changes to opsin which:
- activates transducin
- activates PDE
- hydrolyses cGMP therefore less cGMP
Signal Amplification
1 photon of light results in 1 activated photopigment molecule that activates up to 800 transducing molecules which each activates a single PDE and breaks down 6 cGMP, which closes up to at least 200 ion channels
the retinoid cycle
restores the retinal to the form that it can capture a photon
activated photopigment is phosphorylated by a kinase:
- allows arrestin to bind
- prevents the activation of transducin
- stops the process
Rods and cones differ in:
- shape
- photopigment
- pattern of synaptic connections - which determines spatial acuity
- distribution across retina (rods in the periphery, cones in the fovea)
2: Photopigment (and response characteristics)
rods = rhodopsin –> illumination
cones –> colour, acuity
- pattern of synaptic connections (spatial acuity)
Rods:
- convergence from rods to rod bipolar cells: pools the signal –> allows for low light vision
- rod bipolar cells synapse on amacrine cells, which synapse on bipolar cells and ganglion cells
whereas
Cones:
- one cone to one cone bipolar cell - and then straight to ganglion cells (less sensitive, but high spatial acuity)
- distribution of photoreceptors in the human retina
Cones: throughout at low density, packed in fovea, high spatial acuity
Rods: throughout periphery, none in centre, low spatial acuity
Photopic vision is
vision supported by photoreceptors
The fovea is
a region of the retina packed densely with cones
Cones and colour vision
- rods contain a single photopigment (rhodopsin)
- ocnes can contain 3 different types of photopigment (3 opsins)
- normal human colour vision is trichromatic
Photoreceptors and bipolar cells are unique because
they don’t fire action potentials but instead a graded response (where depolarisation results in more release of transmitter and hyperpolarisation with less release)
Responses of retinal ganglion cells
Each ganglion cell responds to stimulation in a small patch of retina, which reflects a place in the visual field.
Ganglion cells respond to changes in illumination in specific locations of absolute space
ON-centre neurons: increase firing rate with increases of luminance in the receptive field
OFF-centre neurons: increase firing rate with decreases in luminance in the receptive field - i.e. if it is darker in the centre, action potentials occur, but will hyperpolarise when light is in the centre
Therefore photoreceptors and off centre neurons
react in the same way (hyperpolarise with light)
What leads to the centre-surround receptive fields of retinal ganglion cells?
Photoreceptors reduce release of transmitter (glutamate) when photons are absorbed.
The bipolar cells are going to also change the amount of glutamate they release in a graded way, then the ganglion cells decides whether or not that is worth an action potential.
Tree for the centre-surround receptive fields of retinal ganglion cells
Cone gets input from the centre of a ganglion cell receptive field
Photoreceptor absorbs light (hyperpolarises) = less transmitter released
2 OPTIONS
a) (sign inverting) Depolarises ON-centre bipolar, which activates on-centre ganglion
b) (sign conserving) Hyperpolarises OFF-centre bipolar, deactivates off-centre ganglion
How our ganglion cells adapt
- increase in background illuminance means an adaptive shift in the ganglion cells so that you need more photons to result in the same number of action potentials heading to the brain
- ganglion cell firing therefore reflects general illuminance and the specific field
- ganglion cells respond more vigorously to a small spot of light
Ganglion cells signal luminance contrast
t1: cone with an on-centre ganglion cell: light in centre hyperpolarises cell
- -> less transmitter released on bipolar cell
- -> on centre depolarises
- -> releases more transmitter
- -> ganglion cell fires action potential
t2: cone in surroudn also absorb light - hyperpolarise cells
- -> less transmitter released
- -> hyperpolarises horizontal cells
- -.> reduces activity on synapses with central photoreceptor
- -> depolarises centre cone
- -> increases transmitter relase
- -> on centre bipolar cell hyperpolarises
- -> ganglion cell fires less action potentials
