Lecture 5 - The eye and the retina Flashcards
What are the main components of the eye
- Cornea
- Pupil
- Iris
- Lens
- Vitreous Humor
- Retina
- Optic Nerve
- Optic disk (blind spot)
- Fovea
What are the possible refractive errors in vision?
- Emmetropia
- Myopia (nearsighted)
- Hyperopia (farsighted)
What causes Myopia?
When the eyeball is two small, or the cornea is too curved, the focal plane is in front of the retina, so light focuses then keeps going, resulting in a blur
Nearsighted is when things that are close are clear and things that are far are blurry
What causes Hyperopia?
When the eyeball is too long, or cornea is not curved enough the focal plane lands behind the retina, resulting in a blur
Farsighted is when things that are far are clear, but things that are close are blurry
How do images land on the retina?
Upside-down, but our brain better presents the information to us
What does n represent in Snell’s law?
n=refractive index
What is Snell’s law?
n1sinθ1 = n2sinθ2
What is the refractive index of a vacuum?
n = 1.000
What is the refractive index of air?
n = 1.003
What is the refractive index of water?
n = 1.33
What is the refractive index of the cornea?
n = 1.38
What is the refractive index of the lens?
n = 1.42
What is the refractive index of diamonds?
n = 2.42
What does n1/n2 mean?
The ratio of medium densities, going from medium 1 to medium 2
What is the link between the value of n and the density of the medium?
The bigger n is, the more dense the material
What happens if n1/n2 >1
Light is moving from a more dense medium to a less dense medium
Light bends away from normal
What does a smaller n1/n2 mean?
If n1/n2 is smaller, more light is being bent
Corresponds to a bigger n2/n1
Between the cornea and the lens, which has the greater focusing power (i.e. bends more light)?
The cornea bends more light since the light is coming from air and not water like in the case of the lens (air is less dense than water)
What happens if n1/n2<1?
Light is moving from a less dense medium to a more dense medium
Light bends towards the normal
What happens when n1/n2=1?
No refraction occurs, light just continues into the second medium in a straight line
What is the link between a curved lens, and light bending?
The more the lens is curved, the more light it bends
What is the difference between cornea and lens accommodation?
The cornea has a fixed shape
Lens is adjustable (automatic) –> Presbyopia happens when people get old and their lens hardens and can no longer bend, so they can only see things clearly when they are far from them
Why do we say that the retina is inside out?
When light first enters, it must pass through all the layers of the retina to make it to the photoreceptors, and then once light is captured by the photoreceptors and converted to an electric signal, it is sent back to bipolar cells and then ganglion cells (note that their axons form the optic nerve)
Ganglion cells exit through the optic disk towards the thalamus
What are the two halves of the retina?
- Temporal half
- Nasal half
On which half is the blind spot located?
Nasal half
Why does the optic disk cause a blind spot?
It has no photoreceptors
What are the key components of the retina structure?
- Pigment epithelium (maintains retina health, and is responsible for photoreceptors being in the back of the retina explaining the inside out configuration)
- Rods and cones
- Bipolar cells
- Amacrine cells
- Ganglion cells
What is the typical configuration of a photoreceptor?
- Inner segment (where proteins are made)
- Outer segments (where light detection happens, has discs)
- Pigment epithelial cell
What was the goal of the pulse chase experiment?
To observe the lifetime of a photoreceptor, i.e. the shedding of the photoreceptor disk into the pigment epithelium
How was the pulse chase experiment done and what did it show?
There was the use of 3H-Methionine to be incorporated with the new proteins (made in the inner segments), and the lifecycle of the photoreceptors could be observed
The pulse chase showed that over time, the newly made protein would move from the inner segments of photoreceptor, to the outer segment and then shed into the pigment epithelium where it was engulfed
What are the 4 steps of phagocytosis of photoreceptor into the pigment epithelium?
- Disk curled
- Tip became spherical
- Tip detached from photoreceptor
- Tip engulfed by pigment epithelium
What happens to cell membrane potential when it is dark vs when there is light?
Light hyperpolarizes the cell
Dark depolarizes the cell
What does cGMP stand for, and what does it do?
Cyclic Guanosine Monophosphate, it is degraded by light and ligand for the ions
What is the phototransduction process in the dark?
In the dark cGMP ion channels are opened letting in sodium, causing depolarization in the cell that opens the calcium channels, that later causes the release of neurotransmitters
What it the phototransduction process in the light?
In the light cGMP ion channels are closed, as a result sodium cannot get in the cell, and potassium continues to leave through the leak channels, causing a hyperpolarization that does not let calcium in the cell, and does not allow the release of neurotransmitters
What is photoisomerization and what two events does it lead to?
Photoisomerization is the process by which the retinal inside the opsin absorbs light, and changes from an 11-cis configuration to an all-trans configuration
This change in the shape of the retinal inside opsin causes leads to phototransduction and then when light is on for long enough, adaptation
Describe the phototransduction process
This is the process that helps explain why light hyperpolarizes the cell
- Light enters changing the configuration of retinal in ospin
- This activates the enzyme transducin
- Transducin activate PDE (phosopdiesterate)
- PDE degrades cGMP causing hyperpolarization
Describe the process of photoadaptation
Guanylate cyclase produces cGMP
Ca++ inhibits the production of guanylate cyclase
- Light enters changing the configuration of retinal in opsin
- This activates transducin
- This activate PDE
- However, since when light is left on for a long time there is no Ca++ (due to constant hyperpolarization), and so there is production of Guanylate cyclase
- This cause the opening of the cGMP ion channels letting in Na+ and then Ca++ depolarizing the cell
How do the transduction loop and adaptation loop go together?
TRANSDUCTION LOOP:
Light enters –> PDE activated –> cGMP degraded —> Na+ and Ca++ cannot enter —> hyperpolarization
ADAPTATION LOOP:
From: Na+ and Ca++ cannot enter –> Guanylate cyclase increases –> cGMP levels increase –> Na+ and Ca++ enter cell –> depolarization
When looking at a black bulb against a blank canvas for a long time, after, why do we see a similar light shape in a completely white canvas?
The photoreceptors for the white parts have adapted whereas the part with the black bulb have not adapted and when we are back looking at just a white canvas, there is hyperpolarization for the photoreceptors that were looking at the black bulb so the brain just infers that the spot with hyperpolarization must be where the light is
Brain infers colors from activation of three different cone types, what are they?
- Short wavelength cone
- Medium wavelength cone
- Long wavelength cone
Which wavelength cone is constantly the least distributed?
Short wavelength cone (blue)
What technique was used by scientists to observe the distribution of the 3 cone types?
Distribution does vary from person to person, but the technique they used to observe it followed three steps:
- Flash 550 nm; red and green absorb it, blue reflects it (observe blue’s response)
- Bleach 650 nm to eliminate red function, then flash 550 nm (observe green and blue’ response)
- Bleach 470 nm to eliminate green function, then flash 550 nm (observe blue and red’s response)
What are three reasons that rods are more sensitive than the cones?
- Rods are longer (more disks and so more molecules that can have photopigments)
- Rods converge more onto a single bipolar cell (they need very little light to trigger a response in the brain making them sensitive to low lights)
- Rods have greater amplification: with just 1 photon of light absorbed, it actives 800 transducin enzymes, activates 800 PDE, degrades 4,800 cGMP and closes 200 Na+ channels…for the same results in cones, 100 photons of light are required
What are the retinal location of rods and cones?
Rods are eccentric (90 million rods), Cones are central (4.5 million cones)
What is the main trait of off-center ganglion cells
The fire more APs in the center of receptive field when light is off in the center (since light hyperpolarizes cell)
What are two reasons that Cones have better spatial acuity than rods?
- They are more concentrated in the fovea which has no overlaying of axons and blood vessels
- They have less convergence (more precise)
What is the main trait of on-center ganglion cells
They fire more APs in the center of receptive field when light is on in the center
Explain the reason behind the AP firing patterns in off-center ganglion cell
- They have normal ionotropic glutamate receptor (AMPA R)
- When light causes hyperpolarization, glutamate is not released
- Ionotropic glutamate receptor’s Na+ channels close
- Off-center bipolar cells hyperpolarize, releasing less glutamate onto ganglion cells
- Ganglion cells releases less APs
Explain the reason behind the AP firing patterns in on-center ganglion cell
- They have metabotropic glutamate receptors (mGluR6)
- When light causes hyperpolarization, glutamate is not released
- mGluR6 actually open when no glutamate is released causing Na+ to enter
- On-center bipolar cells therefore depolarize, causing glutamate to release onto ganglion cells
- Ganglion cells fire APs
What are the steps of Biolistic transfection
- Loas DNA into gold particles
- Place cold particles into tubing
- Rotate the tubing
- Cut tubing into cartridges
- Load cartridges in gene gun
- Shoot DNA in neuron hoping that in lands in nucleus and neuron can begin making the protein that the shot DNA codes for
What is the purpose of Biolistic transfection?
To see what retinal ganglion cells look like
How did investigators use Biolistic transfection?
They shot mouse ganglion with two fluorescent protein:
1. td-Tomato (red) –> fills cytoplasm making it visible in orange
2. PSD95-YFP –> protein found at synapse marked in green to help identify synaptic sites
Both proteins were made in the neuron (DNA shot in neuron coded for them)
What was the use of a dot finder in biolistic transfection?
Dot-finder was a computer system that set threshold for what should be considered a synapse (puncta) and would color it in blue
