visual system: the eye (I) Flashcards
T or F: The visual system acts like a camera
False! The eye does.
Eye anatomy: Fill in the blanks
Go!
Cornea
Focuses the light
Retina
-thin sheet/surface of neurons
-light is focused on the retina
-responds to light
Fovea
High-acuity center of the visual field
→part of the retina where everything is most focused
→has more precision than the rest of the retina
Lens
Adjusts the focus
→allows us to look near and far
Optic disc
-point of exit for ganglion cell axons
-no sensitivity to light
→blindspot!
Optic nerve
Axons of the output neurons of the nerve
→this will then project to the lateral geniculate nucleus in the brain
The retina consists of ____ cell types
Five
Name the retina’s cell types
- Photoreceptors
- Horizontal cells
- Bipolar cells
- Amacrine cells
- Retinal ganglion cells
Photoreceptors
- two types: rods & cones
- sensitive to light
→specialized to absorb photons of light that will then be transformed into electrical signals in the photoreceptors themselves - found at the back of the retina
(possibly due to sensitivity) - start of the “message”
- capable of synapsing onto bipolar cells
Bipolar cells
Bipolar cells connect photoreceptors to retinal ganglion cells
- relay the message given by photoreceptors
- capable of synapsing with retinal ganglion cells
Retinal ganglion cells
- their axons form the optic nerve
Amacrine cells & Horizontal cells
- produce horizontal connections
- enable communication between photoreceptors, bipolar cells and retinal ganglion cells
Light flow in the retina
- Photoreceptors absorb light
- Electrical signal is produced and relayed to the bipolar cells
- Bipolar cells generate an electrical signal that is relayed to the retinal ganglion cells
- Retinal ganglion cells fire APs that will propagate down their axons and form the optic nerve
- This message is relayed to the brain
throughout, the vertical information is modulated by amacrine and horizontal cells
The retina has _____ layers
Ten
→known as the neural retina!
Name the layers of the retina
(Outside to inside…)
1. Inner limiting membrane
2. Nerve fiber layer
3. Ganglion cell layer
4. Inner plexiform layer
5. Inner nuclear layer
6. Outer plexiform layer
7. Outer nuclear layer
8. Outer limiting membrane
9. Photoreceptor outer and inner segments
10. Retinal pigment epithelium
Nerve fiber layer
Axons of retinal ganglion cells
Ganglion cell layer
Cell bodies of retinal ganglion cells
Inner plexiform layer
Synapses between the bipolar cells, retinal ganglion cells and amacrine cells
Inner nuclear layer
Cell bodies of the horizontal cells, amacrine cells and bipolar cells
Outer plexiform layer
Synapses between photoreceptors, bipolar cells and horizontal cells
Outer nuclear layer
Cell bodies of the photoreceptors
From the 10 layers of the retina, which can be found in more abundance?
Photoreceptors!
→there are way more photoreceptors (input layer) than there are ganglion cells (output layer)
→this demonstrates that some processing must take place at the retina
→this demonstrates the notion of convergence
In the fovea, which cell type has direct access to light?
Photoreceptors!
→ganglion and bipolar cells are pushed to the side
Circuitry of the retina
-at least 17 distinct types of retinal ganglion cells
-10 types of bipolar cells
-more than 30 types of amacrine cells
Morphology of photoreceptors: Rods
(Fill in the blanks + explain!)
-MORE sensitive to light
-used for night vision
-do not contribute to colour vision
-mainly found outside the fovea
Morphology of photoreceptors: Cones
(Fill in the blanks + explain!)
-used to see the world
-responsible for colour vision
-concentrated in the fovea
-the discs of the outer segment are integrated in the membrane, forming a cone-like structure
T or F: The inner segment of photoreceptors is photosensitive
False!
The outer segment is the photosensitive part
T or F: Regarding peripheral vision, one has more sensitivity to light rather than colour
True!
Rods are more concentrated in the periphery, thus contributing to higher light sensitivity with peripheral vision
How does light affect the photoreceptor?
Through sensory transduction.
→Light hyperpolarizes photoreceptors by closing a cGMP-gated cation channel
→Transient hyperpolarization of the photoreceptor membrane is the starting point
Signal transduction
Rhodopsin (specialized protein) absorbs photons of light, which initiates a series of biochemical events in the cell that will liberate a second messenger and that closes an ion channel, which then changes the electrical properties of the neuron.
T or F: Photoreceptors are neurons that fire action potentials
False!
It is TRUE that they are neurons, but they do NOT fire action potentials
Hyperpolarization of photoreceptors is dependent on _____
The amount of light
→low light, slight hyperpolarization
→bright light, great hyperpolarization
T or F: Bipolar cells are neurons that fire action potentials
False!
It is TRUE that they are neurons, but they do NOT fire action potentials
Explain the basis of the AP firing rate of a retinal ganglion cell in the dark + Draw it
☆slide 32-33☆
in the dark, the cone is depolarized such that it will release glutamate, which hyperpolarizes the bipolar cells,
causing them to not release much glutamate onto the ganglion cell, and as a result, the ganglion cell will only fire APs at a slow rate.
Explain the basis of the AP firing rate of a retinal ganglion cell when light comes in + Draw it
☆slide 32-33☆
when a light is shone, the cone hyperpolarizes and releases less glutamate (“stops releasing” would not be the correct term as there is a continuous change, it is not an all-or-none type of event), meaning that there is less hyperpolarization of the bipolar cell, such that the bipolar cell depolarizes (the bipolar cell was held at a negative potential by glutamate, but when the glutamate release decreases, it relaxes back to a new level –> depolarization), without firing an AP, but it will release more glutamate onto the ganglion cell, thus depolarizing the ganglion cell, and the ganglion cell will start to fire more APs
The frequency of action potentials in retinal ganglion cells is dependent on _____
The intensity of light
How does a low-intensity light affect the frequency of APs in retinal ganglion cells
-a dim light will only slightly hyperpolarize the cell, thus decreasing the amount of GLU it releases by just a bit
-the bipolar cell will depolarize by only a little bit and a small amount of GLU will get released onto the ganglion cell
-the ganglion cell will slightly increase its firing rate
How does a high-intensity light affect the frequency of APs in retinal ganglion cells
-a bright light will cause a strong hyperpolarization in the photoreceptor
-strong depolarization in the bipolar cell, causing a lot of GLU released onto the ganglion cell
-APs will come in at a very fast rate
T or F: A ganglion cell receives input from multiple cones
True!
→this is mediated by horizontal cells
Inhibitory horizontal cells
Inhibitory horizontal cells connect cones in the surrounding region to the central photoreceptor
☆read slide 36☆
Effect of horizontal cells when there is light on surrounding photoreceptors
- the surround photoreceptor gets hyperpolarized, releasing less GLU
- the connecting horizontal cell gets hyperpolarized, releasing less GABA
- center cell releases more GLU
- the bipolar cell gets hyperpolarized, releasing less GLU
- the ganglion cell fires less APs
For which cells is glutamate inhibitory or excitatory?
Inhibitory: bipolar cells
Excitatory: retinal ganglion cells, horizontal cells
When light shines on the central photoreceptors it _____ the ganglion cell
activates
When light shines on the surrounding photoreceptors it _____ the ganglion cell
inhibits
On-center off-surround ganglion cell receptive field
Retinal ganglion cells reflect convergent input from multiple photoreceptors.
→center light: excitation
→surrounding light: inhibition
→all around light: neutral response
Off-center on-surround ganglion cell receptive field
Occurs when photoreceptors make excitatory connections with bipolar cells.
→center light: inhibition
→surrounding light: excitation
→all around light: neutral response
T or F: In the fovea, each cone forms the center of an on- centered and an off-centered pathway.
True!
→allows the cone to signal either an increase or decrease in the ganglion cell, thus this pathway can detect an increase or decrease in illumination
T or F: Outside the fovea, less photoreceptors feed into a single ganglion cell
False! There are MORE photoreceptors that feed into a single ganglion cell
T or F: Each type of retinal ganglion cell has its distinct area in the retina.
False! Each type tiles the entire retina.
→creates multiple parallel labelled lines to the lateral geniculate nucleus
→each labelled line conveys a distinct type of visual information
Name the three best characterized channels from the retina
- Midget ganglion cells (~ 70%)
- Parasol ganglion cells (~ 10%)
- Bistratified ganglion cells (~ 8%)
→each type is a separate labeled line (a separate channel carrying in info to the brain)
In daylight, each of the three retinal channels gets its main input from _____.
cones
Types of cones in the retina
- S (short λ) → blue (445nm)
- M (medium λ) → green (535nm)
- L (long λ) → red (575nm)
What do the different cone types respond to?
Each type is tuned to respond best to a distinct range of wavelengths within the visible spectrum.
→i.e. M cone will best respond to 535 nm (green) despite its absorption spanning a majority of the visible spectrum
Opsin
Opsins are the universal photoreceptor molecules of all visual systems in the animal kingdom.
→they absorb light to then transform it into an electrical signal
Colour blindness
-typically occurs in men as it is an X-linked disorder
-rare to have blue colour blindness
-the individual will have a defective gene for the proteins encoding the green/red cone opsin, such that they can tell the difference between shorter wavelengths of light and higher wavelengths (blue vs red, blue vs green), but not between the higher ones (red vs green)
-ability to discriminate colour is weak
Midget cells
-concentrated in the fovea
-convey the amount of red vs green in their receptive fields
-small receptive field
-good at resolving fine details
→differences in illumination/contrast (edges)
-not good at detecting rapid changes
-project to parvocellular layers in the LGN
P-Pathway
Synapses occurring in the parvocellular layers in the LGN from the midget cells, making a major contribution to cortical systems involved in processing colour and form.
Red-on green-off cell: red light on entire receptive field
Increase in firing.
→red light will activate the center, thus stimulating the firing of the ganglion cell, but not much activation of the inhibitory surround cells since they are maximally sensitive to green
Red-on green-off cell: green light on entire receptive field
Decrease in firing.
→inhibitory surround cones are now maximally sensitive
Red-on green-off cell: white light in the center
Increase in firing.
Red-on green-off cell: white light in its surround
Decrease in firing.
Red-on green-off cell: white light on entire receptive field
Neutral firing.
White light
It is a mixture of ALL colours.
Parasol cells
-larger receptive fields
-cannot discriminate colour (achromatic signal)
-lower spatial resolution
-better sensitivity to rapid changes in illumination
-project to magnocellular layers in the LGN
-center and surround get input from both red and green cones
M-Pathway
Synapses occurring in the magnocellular layers in the LGN from the parasol cells, which contribute to cortical systems involved in analysis of location and movement.
On-center off-surround parasol cell: white light in the center
Increase in firing.
On-center off-surround parasol cell: white light in the surround
Decrease in firing.
Bistratified cells
-detect the amount of blue vs yellow (red+green)
-have larger receptive fields
-do NOT have on/off center/surround system
-center and surround gets input from red, green and blue cones
-project to koniocellular layers of the LGN
K-Pathway
Synapses occurring in the koniocellular layers in the LGN from the bistratified cells, which contribute to cortical systems involved in colour perception.
Blue-on bistratified cells: blue light shining on entire receptive field
Increase in firing.
Blue-on bistratified cells: yellow light shining on entire receptive field
Decrease in firing.
T or F: A single type of cone cannot distinguish between different wavelengths of light.
True!
→i.e. M cones respond best to green light, but will also respond to wavelengths covering most of the visible spectrum
How is colour encoded in the retina?
Colour is encoded in the retina by the relative output of the parasol, midget and bistratified cells.
→combinatorial processing
☆slide 56 illustrates everything perfectly, take a break☆
