The Eye Flashcards
What are the first steps of vision?
- Transmission and refraction of light by the optics of the eye
- The transduction of light energy into electrical signals by photoreceptors
- The refinement of these signals by synaptic interactions within the neural circuits of the retina
Visual processing pathway 3 step diagram
Full visual processing pathway diagram
What do photoreceptors in the retina contain?
Pigments that absorb light reflected by objects
Wavelength defn
Distance between two peaks of the electromagnetic wave
What does wavelength relate to?
Light colour: when white light diffracts through a prism it splits into the separated l (rainbow effect)
Transparent media function
Will bend the light to focus it in the retina
Reflection defn and diagram
Refraction defn and diagram
Bending of light rays when they travel from one media to the other.
Absorption defn and diagram
Transfer of light energy to a particle. Black objects absorb all the l of visual light, green objects absorb all except green, etc.
Anatomy of the eye x 5
- Pupil
- Iris
- Cornea
- Extraocular muscles
- Optic nerve
What is the pupil?
Opening that allows the entry of light
What is the iris?
Pigmentation that provides eye colour.
What other than pigmentation does the iris contain and function?
Contain two muscles to regulate the size of the pupil
What is the cornea?
Transparent surface covering the front eye. It continues in the sclera, the white of the eye
Where does the extraocular muscle insert and function?
Inserts in the sclera, to control eye movement
Where does the optic nerve exit?
At the back of the eye
Anatomy of the eye diagram
Optics
Cornea and lens will diffract light to focus it in the retina. The lens will accommodate to different distances.
Far point optics diagram with focal distance draw
Near point optics diagram draw
Myopia diagram
Nearsightedness
Hyperopia diagram
Farsightedness
What is the retina a part of?
CNS
Most direct pathway to the brain
Photoreceptors ➡ Bipolar cells ➡ Ganglion cells
What are the only light-sensitive cells in the retina?
Photoreceptors
What are the only output cells in retina?
Ganglion cell axons (optic nerve) to higher CNS centres
What are horizontal and amacrine cells?
Local interneurons and modulate transmission onto bipolar and ganglion cells
Retina pathway to brain and cells diagram
Retina laminar organisation and layers
What do photoreceptors do?
Transform electromagnetic radiation into electrical signals
Rod and cone diagram
What does the outer segment of the retina contain?
Membrane disks, with light-sensitive photopigments that absorb light and will trigger changes in membrane potential
Rods make up
One photopigmentblack / white vision, lot of membrane disks (more sensitive to light)
Cones make up
3 photopigments-colour vision
Rods function
Night vision
Cones function
Daylight vision
What is the fovea enriched in?
Cones
What is the periphery enriched in?
Rods
Where are cones tightly packed?
In the fovea
Where are cones widely spaced?
In the periphery
Size of receptor fields in periphery and what does this mean?
Larger - lower resolution
Many photoreceptors feeding (convergence) into individual ganglion cells in the periphery, what does this mean?
More rods and greater input in the periphery (higher sensitivity)
Lateral displacement of the layers above the photoreceptor at the fovea →
Light hits directly without scattering
Describe fovea input
More cones and no convergence of input in the fovea: lower sensitivity
No convergence of input and direct light input in the fovea: higher resolution
(no convergence = one photoreceptor → one ganglion cell)
Rods light sensitivity
Low spatial resolution but very sensitive to light; saturated in very bright light → night vision
Cones light sensitivity
High spatial resolution but relatively insensitive to light; not operating in dim light → day vision
What do rods contain?
Contain photopigment rhodopsin
What are rods sensitive to
All visible wavelengths
Rods sensitivity to light
High sensitivity
When are rods used
Night vision
Where are rods located?
All over the retina except fovea
Total number of rods
100 million
What is each cone sensitive to?
One type of light
Do cones or rods require more energy to be activated?
Photopigments of cones require more energy to be activated
What do cones contain?
Cone opsins (3 types, with different absorption for red, green or blue light)
Sensitivity of cones to light
Low sensitivity to light
Function of cones
Daylight sight
Location of cones
Located mainly in the macula, and especially the central area of the macula, the fovea
How many cones in total?
6 million total
What is phototransduction?
Conversion of light energy into membrane potential changes
Rods x 6 points
- Contain photopigment rhodopsin
- Sensitive to all visible wavelengths
- High sensitivity to light
- Night vision
- Located over all retina (except fovea)
- 100 million in total
Cones x 6 points
- Contain photopigments cone opsins (3 types, with different absorption for red, green or blue light)
- Low sensitivity to light
- Daylight sight
- Located mainly in the macula, and especially the central area of the macula, the fovea
- 6 million in total
Describe phototransduction membrane potentials
Graded changes in membrane potentials (the only neurons firing action potentials are ganglion cells) → change in rate of transmitter release
Phototransduction in dark
Membrane depolarised, Ca2+ channels open, rate of transmitter release is high
Phototransduction in light
Hyperpolarisation, Ca2+ channels close, decrease in rate of transmitter release
Stimulus of classic GPCR
Chemical
Receptor activation of classic GPCR
G protein binds GTP
Enzyme of classic GPCR
Activation
Second messenger of classic GPCR
Increase in second messenger levels
Ion channel of classic GPCR
Increase / decrease conductance
Stimulus of rhodopsin
Light
Receptor activation of rhodopsin
G protein binds GTP
Enzyme of rhodopsin
Activation
Second messenger of rhodopsin
Decrease in second messenger levels
Ion channel of rhodopsin
Decrease Na+ conductance
Classic GPCR vs Rhodopsin diagram
What happens to photoreceptors in darkness and why?
Depolarised (-30mV) due to a “dark current” of Na+
Phototransduction of rods in darkness process
What happens to rods with light?
Photoreceptors will hyperpolarize with light
Rhodopsin =
Opsin + Retinal
Rods absorption of light process
Rods hyperpolarisation with light process
Phototransduction- signal amplification
What does light do to photoreceptors?
Hyperpolarises
What does darkness do to photoreceptors?
Depolarises
What do responses of in the bipolar and ganglion cells varying depend on?
On the receptors they express and lateral modifications (horizontal and amacrine cells)
What can bipolar cells be depolarised or hyperpolarised by?
Glutamate
Hyperpolarisation expression
AMPA/Kainate R expression
Depolarisation expression
mGluR6 expression
Depolarisation = mGluR6 expression pathway
Bound to glutamate (during dark), mGluR6 causes closure of Na+ channels. Hyperpolarisation of photoreceptor by light reduces glutamate release, reduces mGluR6 binding and allows Na+ channels to open on the bipolar cell.
Dark and light responses diagram
What is the bipolar receptive field?
An area of retina where a stimulus will evoke a response in that bipolar cell
Receptive field centre =
Direct connection from photoreceptors
Receptive field surround =
Connection from photoreceptors through horizontal cells
Response triggered in centre =
Opposite will be triggered by the surround
How are bipolar cells classified?
According to their responses to light
Types of bipolar cells
- On-centre bipolar cells
* Off-centre bipolar cells
ON-centre Bipolar cells
Depolarized by light, expressing mGlur6
OFF-centre Bipolar cells
Hyperpolarized by light (as Photoreceptors), expressing AMPA / Kainate
What is the area of retina where a stimulus will evoke a response in that ganglion cell called?
The receptive field of a ganglion cell
What does each ganglion cell
A ‘centre’ and ‘surround’ to it’s receptive field
Effect of centre vs surround
Light in the centre will have the opposite effect to light in the surround
What do ganglion cells generate?
AP in the retina
What are the only cells that produce APs in the retina?
Ganglion cells
When do on-centre ganglions produce APs?
When light is shone on the photoreceptor that directly innervates it.
What happens to off centre ganglions when light is shone on photoreceptor that innervates it?
Decreases APs
When does the photoreceptor get hyperpolarised?
ALWAYS by light
What can happen to AP firing in ganglion cell when hyperpolarised?
May increase or decrease
What decreases APs when light is turned off?
On-centre ganglion cells
What increases APs when light is turned off?
Off-centre ganglion
When is there no change in firing rate?
When both centre and surround are in same level of illumination: A, C and E
When is there the greatest difference in firing?
When you have most contrast between the centre and surround: B and E
What is the antagonistic centre/surround effect mediated by?
Horizontal cells
What do horizontal cells regulate?
Amount of transmitter released by photoreceptor onto bipolar cell
Light at centre causes depolarisation of bipolar cell: process
- Light hyperpolarises centre cone
- Decreased release of glutamate from centre cone
- Depolarisation of bipolar cell (mGluR6)
What does addition of light to surround cause?
Reduces firing rate of on-centre
Horizontal cell hyperpolarisation
Surround cone releases less glutamate onto horizontal cell
Addition of light to surround process
- Strong hyperpolarisation of horizontal cells
- Reduced release of GABA from horizontal cell onto centre cone
- Reduces hyperpolarisation of centre cone
Reduced hyperpolarisation of centre cone ➡
Increased release of glutamate from centre cone ➡
Hyperpolarization of bipolar cell ➡
Hyperpolarization and reduced firing of GC
Opsin ➡
Transducin → PDE → CNG Channel → Glu
Light energy conversion
Converted to membrane potential changes in photoreceptor cells, through Opsin → Transducin → PDE → CNG Channel → GluC
Membrane potentials in bipolar pathway
Glutamate information will be converted into membrane potential changes in bipolar (centre ON/OFF) and horizontal (surround ON/OFF) cells
