Visual System Flashcards
Immature sensory systems develop from a thickening of this germ layer
Ectoderm
Immature sensory regions in a developing organism are referred to as…
Placodes
Which sensory organ does not develop from a placode? What does it develop from?
Retina
Develops from the immature CNS tissue
Define intrinsic signaling
Non-receptor-mediated signaling in which signaling factors act on targets in the same cell as they were produced in
Define extrinsic signaling
Usually receptor-mediated signaling where signals act on cells other than the cell in which the factor was produced
If all cells have the same DNA, how do they develop differently?
Cells express different combinations of TFs which are often concentration gradient-specific
The epidermis, CNS, and Neural crest are all derived from this germ layer
Ectoderm
Formation of the 3 germ layers can be observed in this developmental stage
Late blastula
The first evidence of neural tissue appears at this developmental landmark
Neural induction
BMP is an (intrinsic/extrinsic) signaling factor
Extrinsic
When BMP reacts with its receptor, it induces the formation of this kind of tissue
Epidermis
When BMP binds to NIFs (e.g. noggin), it induces the formation of this kind of tissue
Neural tissue
NIFs are secreted from cells in this germ layer
The mesoderm
Noggin, chordin, and follistatin are all examples of
NIFs
BMP is part of this family of signaling molecules
TGF-Beta
In order for the retina to develop, the CNS has to make direct interactions with (1) during this stage of development (2)
- The lens placode
- The neurula stage
The main role of the cornea is…
Protect the eye
The main role of the lens is to…
Focus light onto the retina
The main role of the iris is…
To regulate the amount of light reaching the retina
Describe the retina
A neural tissue present in the eye where light information is detected
The retina sends information to the CNS via…
The optic nerve
Why are the cells of the retinal pigmented epithelium pigmented?
Melanin helps absorb light - protects cells of the eye from excess oxidation
What is the order of the tissues in the retina starting where the photoreceptor discs are? (6)
- Outer segment
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
Starting at photoreceptors, what are the cell layers of the retina?
- Photoreceptors
- Horizontal cells
- Interneurons (bipolar cells, etc.)
- Ganglion cells
What are the 2 kinds of photoreceptors and what properties of light do they detect?
Rods = Brightness
Cones = Colour
What are the three reasons the retina is an easy area of the CNS to study?
- Accessible - Isolation of the retina preserves most of its function
- Can be grown in explant culture - valid culture system to perform experiments on
- It is a “mini-brain” - A lot of the same neurotransmitters and receptors present in neurons are present in the retina
The retina is an extension of this brain region during development (“-cephalon”
Diencephalon
Define multipotent cells
Cells which are able to give rise to multiple different cell types
(Intrinsic/extrinsic) signaling leads to the development and differentiation of different sensory placodes
Extrinsic
Is a juvenile brain technically a sensory placode?
No
During later stage neural induction (neural tube stage), the interactions of the lens placode with the CNS start to bend tissues to create this structure
The optic vesicle
Which of these sensory structures is more deep (as opposed to cutaneous), the lens placode or the immature CNS?
CNS deeper than lens placode
The eye will not be properly formed in the absence of this signaling molecule
FGF
BMP and FGF are examples of…
Extrinsic signaling factors
Cell autonomous events typically involve signaling by these kinds of molecules
Regulatory transcription factors which are tissue/cell specific - intrinsic factors
Cell non-autonomous events typically involve signalling by these kinds of molecules
Extracellular signaling factors
What causes the change from multipotent retinal progenitor cells to different mature retinal cell types?
Intrinsic transcription factor signaling
What happens first during retinal development, intrinsic or extrinsic cell signaling?
Extrinsic first, intrinsic next
True or false: retinal cell types tend to arise at more or less the same time
False: different cell types have different “birth” days
What are the general cell types of the retina? (7)
- Rods
- Cones
- Muller
- Horizontal
- Bipolar
- Amacrine
- Ganglion
Which retinal cell types are born “early”
Cones, horizontal, amacrine, ganglion
Which retinal cell types are born “late”
Rods, muller, bipolar
How do BMP and FGF contribute to development of the retina?
BMP inhibition promotes development of nervous tissue (retina derived from CNS), FGF responsible for development into eyeball
Macular degeneration and retinitis pigmentosa are diseases of the retina which cause…
Death of photoreceptors
What kind of cells die in Glaucoma?
Retinal ganglion cells
Once photoreceptors are lost, can they be regenerated?
No, discs can regenerate but complete cells cannot
What does it mean to have a loss of visual acuity?
Loss of central vision, peripheral vision may be fine
What structures are in the outer segment of the retina?
Discs of rods and cones
What structures are in the outer nuclear layer of the retina?
Rod and cone cell bodies, dendrites of Muller cells
What structures are in the outer plexiform layer of the retina?
Dendrites/axons of bipolar cells and horizontal cells
What structures are in the inner nuclear layer of the retina?
Bipolar, Horizontal, Muller, and amacrine cell bodies
In what layer of the retina are the ganglion cell bodies?
The ganglion cell layer
An unproven theory exists where this kind of fiber of the retina may be transporting light to the outer segment
Muller cells/muller glia
The graded potential between the photoreceptors and the bipolar cells happens in this layer of the retina
In the outer plexiform layer
How many opsins do rods and cones have respectively?
Rods: 1 opsin = rhodopsin
Cones: 3 opsins, s-opsin, m-opsin, l-opsin (sml = short medium long, refers to wavelength)
If retinas are dominated by rods, what gives us the ability to see so well during the day?
The fovea: focusses light onto the retina
What kind of cell is densely packed into the fovea?
Cones
What retinal layers are missing from the fovea?
Inner nuclear layer or ganglion cell layer (no gc bodies, still has connections)
What is the difference between the macula and the fovea?
The macula makes up a larger area of the retina, the fovea is contained within this area
Why is macular degeneration so bad for vision?
Because most of the light is focussed onto the macula, particularly on the fovea. Degeneration of this area has a severe effect on vision
Central vision is mediated by (1), peripheral vision is mediated by (2)
- Fovea
- Rod-dominated eye (retina outside of the fovea)
Describe the brain structures information passes through from the retina to the primary visual cortex
Retina > Optic chiasm > Lateral geniculate nucleus > Primary visual cortex
Where is the site of phototransduction?
PR discs
When discs become old, they are shed and engulfed by…
Cells of the RPE
Where are opsins located in photoreceptors?
In the discs
Approximately how many stacked discs are there in a photoreceptor?
1000
What is the main light detector molecule in the mammal eye?
Rhodopsin
Describe the morphology of rhodopsin?
7TM protein, associates with 11cisretinal
Where is the retinal attachment site on rhodopsin (which TM region?)
TM region 7
What associates with the retinal attachment site of rhodopsin?
11-cis-retinal
11-cis-retinal is converted to all-trans-retinal by…
Light
Loss of this protein is a key factor in why retinitis pigmentosa patients cannot really see. Why is this?
Peripherin-2
Cannot support actin polymerization, so formation of the discs becomes impossible
Are photoreceptor discs formed gradually by transport of material via vesicles or are they formed all at once?
All at once
What happens to photoreceptor discs when the Arp2/3 complex is knocked out?
Because Arp2/3 promotes branching, there is no branching, get one big long curled disc instead of little stacked ones
In the dark, photoreceptors are (depolarized/hyperpolarized)
Depolarized
In the light, photoreceptors are (depolarized/hyperpolarized)
Hyperpolarized
When photoreceptors are depolarized, they release…
Glutamate
When photoreceptors are hyperpolarized, they release…
Nothing!
True or false: photoreceptors are more active in the dark
True
How many bipolar cell types make connections with rods?
1
How many bipolar cell types make connections with cones?
9
Bipolar cells can be divided into two categories, which are…
On and off
What special kind of synapse happens between photoreceptors and underlying bipolar cells/interneurons?
Ribbon synapse
When someone refers to the “inner retina”, they are referring to these cell types
Bipolar and ganglion cells
The mammalian eye is particularly attuned to these spatial qualities (2)
Directions (directional selectivity)
Contrast (edges, etc.)
An “off” bipolar cell will synapse to a (1) ganglion cell
Off
Why do on and off center bipolar cells respond differently?
They have different kinds of glutamate receptors
On: mGluR
Off: iGluR
On bipolar cells (depolarize/hyperpolarize) in response to glutamate
Hyperpolarize
Off bipolar cells (depolarize/hyperpolarize) in response to glutamate
Depolarize
Ionotropic glutamate receptors when activated cause (depol/hyperpol) of the cell
Depolarization (cation channels open)
Metabotropic glutamate receptors when activated cause (depol/hyperpol) of the cell
Hyperpolarization (cation channels [TRPM1] close)
Why do the photoreceptors use graded potentials instead of action potentials to convey information?
Info only has to travel a short distance, allows more nuance to be transferred from cell to cell
Why do ganglion cells use action potentials instead of graded potentials?
Info has to travel all the way to the primary visual cortex - would degrade if using GPs
Describe what kind of protein a mGluR is
7-TM g-protein coupled receptor (not a channel!)
When mGluRs bind to glutamate, TRMP1 channels are (open/closed)
Closed = cell hyperpolarized
When mGluRs is not bound to glutamate, TRMP1 channels are (open/closed)
Open (cell depolarized)
When a photoreceptor is hyperpolarized causing an on bipolar cell to be depolarized, this is called a (1)-synapse
Sign-inverting synapse
Describe the kind of protein that TRPM1 is
6TM region cation channel
What does TRP of TRPM1 stand for?
Transient receptor potential
Why do we have the on/off bipolar cell systems?
Maximizes the efficiency and speed of visual perception. Specifically, detection of edges and contrast can be maximized by organizing visual fields
Describe a simple receptive field
When multiple photoreceptors are arranged in a center-surround manner to input to a single ganglion cell
What kinds of ions does GABA allow into the cell?
Primarily anions, specifically Cl-
What neurotransmitter do horizontal cells primarily release?
GABA
What are the 3 forms of retinoid in the visual cycle?
Aldehyde
Alcohol
Ester
What is another name for all-trans-retinol?
Vitamin A
The little units that make up the compound drosophila eye are called…
Ommatidia
Each ommatidia has 8 photoreceptive cells which are known as…
Rhabdomeres
R1-R6 (rhabdomeres) are analogous to human…
Rods
R7 and R8 (rhabdomeres) are analogous to human…
Cones
Each ommatidia is made of… (3)
- Lens
- Pigment cells
- R1-R8 rhabdomeres (one of each)
Where are opsins in the fly eye?
On the apical surface of the rhabdomeric cells in microvilli (similar to human discs)
Photoreceptors in drosophila are described as (1), and in mammals they are described as (2)
- Rhabdomeric
- Ciliary (because the discs extend from a modified cilium)
What is the role of the phosphodiesterase in the mammalian phototransduction cascade?
Cause the conversion of cGMP to GMP
When phosphodiesterase is active, what happens to cGMP levels?
Decrease
When cGMP is low, what happens to CNG channels? As a result, the cell is (depolarized/hyperpolarized)
Not able to open = Cell hyperpolarized
What is Ca2+’s role in the phototransduction cascade?
Comes in through CNG channels, acts as a second messenger which activates guanylyl cyclase. Guanylyl cyclase converts GTP to cGMP to open CNG channels
Describe the direction of light relative to the direction of light perception in mammals versus insects
Mammals: light travels opposite processing
Insects: light and processing travel the same direction
Which has more extensive visual processing, mammals or drosophila?
Mammals
How can on and off bipolar cells be distinguished from one another?
Have slightly different morphologies and terminate in different layers of the retina
Which tends to terminate deeper, on or off bipolar cells?
On
What is the role of the lateral geniculate nucleus?
Relay information from the retina to the cortex
In the absence of photoreceptors, the pupillary light reflex will be mediated by these cells
Intrinsically photosensitive retinal ganglion cells
