Vision 1 Flashcards
Diagram of eye anatomy
Cornea
- Glassy transparent surface of the eye
- Continuous with the sclera (white part of the eye/wall of the eyeball)
Pupil
- The opening that lets light into the eye
- Surrounded by pigmented iris (gives eye its color, can alter size of pupil)
Lens
- Located behind the iris
- Suspended by ligaments called zonule fibers (connected to ciliary muscles)
Aqueous and vitreous humor
The lens divides the liquid in the eye into aqueous humor (behind cornea/in front of lens) and vitreous humor (behind lens, keeps eyeball spherical)
Labelled picture of retina
What is the retina?
The back of the eye, containing photoreceptors
Optic disk
- Where retinal vessels originate AND where optic nerve fibers exit
- No photoreceptors –> blind spot
Macula
- Yellowish region responsible for central vision
- Relatively lacks blood vessels
- Contains fovea
Fovea location
- Center of the retina
- Retinal surface closer to nose/medial relative to fovea is nasal
- Retinal surface closer to temple/lateral to the fovea is temporal
How is the retina specialized for high-acuity vision?
- Highest density of cone photoreceptors
- No blood vessels obscuring retinal surface
- Inner retinal layers pushed aside
What is the index of refraction (n) of air vs. water?
- Air: n=1
- Water: n=1.33
What is the index of refraction of all parts of the eye?
n > 1
Overview of refraction
- Ray of light moves through medium of lower n (e.g. air) to medium of higher n (e.g. water) –> ray of light bends closer towards the line, perpendicular to the surface between the two media
- The amount of bending is proportional to n (i.e. the greater the n, the greater the bending)
- The eye makes use of this to focus a vast amount of light through a small hole (pupil) onto a focused point on the retina
The greater the index of refraction (n), the ___ the bending
Greater
Indices of refraction for different part of the eye
- Cornea = 1.28
- Lens = 1.39
- Aqueous humor = 1.34
Refraction in lens vs. cornea
Although the lens has the highest n, the cornea contributes the most to light refraction because it is the first part of the eye that light hits (sharpest change in n)
Which part of the eye contributes most to light refraction?
The cornea
What is the purpose of the lens?
Accommodation: refractive changes bring light into focus on the retina by modulation of the shape of the lens
Accommodation of lens
- The fatter the lens, the more bending it accomplishes
- Light from a further object requires less bending to reach the retina because they are already very parallel
- Light from a closer object requires more bending to reach the retina because they are not as parallel
The fatter the lens, the ___ bending it accomplishes
More
Table of changes to eye with near and far objects
A near object requires ___ bending of light
More
A far object requires ___ bending of light
Less
Changes to the eye for near object
- Requires more bending of light
- To do this, you need a fat lens
- For this lens, you need loose ligaments/zonule fibers
- For those ligaments, you need contracted ciliary muscles
Changes to the eye for far object
- Requires less bending of light
- To do this, you need a thin lens
- For this lens, you need tight ligaments/zonule fibers
- For these ligaments, you need relaxed ciliary muscles
What is the result of contracting ciliary muscles?
Swelling of muscle decreases tension of zonule fibers (loosens them)
What is the result of relaxing ciliary muscles?
Relaxation increases tension of zonule fibers (tightens them)
Myopia
- Nearsighted
- Can’t see far object, which require less refraction
- Too much refraction
- You can get this problem if your eye is too long (horizontally)
- You need a concave lens to fix this (reduces refraction)
Hyperopia
- Farsighted
- Can’t see near objects, which require more refraction
- Too little refraction
- You can get this problem if your eye is too short (horizontally)
- You need a convex lens to fix this (increases refraction)
What type of lens would a person with myopia (nearsightedness) need?
Concave (reduces refraction)
What type of lens would a person with hyperopia (farsightedness) need?
Convex (increases refraction)
Diagram of layers of the retina
Describe the layers of the retina
- Topmost layer in picture is closest to center of the eye (“light enters backwards”)
- Direct pathway: Photoreceptors (outer nuclear layer) → bipolar cells (inner nuclear layer) → ganglion cells (ganglion cell layer)
- Horizontal cells (outer plexiform layer) get input from PRs and influence PRs + bipolar cells
- Amacrine cells (inner plexiform layer) get input from bipolar cells and influence bipolar + ganglion cells
Horizontal cells of retina
- Outer plexiform layer
- Get input from photoreceptors and influence PRs and bipolar cells
Amacrine cells of retina
- Inner plexiform layer
- Get input from bipolar cells and influence bipolar and ganglion cells
___ are the only light-sensitive cells in the retina (with one minor exception)
Photoreceptors
Exception are intrinsically-photosensitive retinal ganglion cells
Light-sensitivity of retina vs. other cells in the eye
- Photoreceptors are the only light-sensitive cells in retina (with one minor
exception) - All other retinal cells fire in response to light via direct or indirect synaptic connections
with photoreceptors - Contain light-sensitive photopigments in disks absorb light to trigger Vm changes
What are the two types of cells in the retina?
Rods and cones
What triggers Vm changes in photoreceptors?
They contain light-sensitive photopigments in disks that absorb light to trigger Vm changes
What makes retinal ganglion cells unique?
They are the only retinal cells to:
- Fire action potentials (the rest only experience changes in membrane potential through depolarization or hyperpolarization)
- Send axons out of the retina through the optic nerve (only source of input to the cortex from the retina)
Appearance of rods
Long, cylindrical segment with many disks
Light sensitivity of rods
1000x higher –> scotopic (night) vision
Rods and cones in mesopic conditions (mid-light)
Both types of PR contribute
Number of rods present in eye
92 million
Where are rods concentrated?
Peripheral retina
How many types of rods are there?
1
Can rods perceive color?
No
Appearance of cones
Short, tapered segment with fewer disks than rods
Light sensitivity of cones
Photopic (day) vision
Number of cones in the eye
5 million
Where are cones concentrated?
Central retina (fovea)
How many types of cones are there?
3, because there are 3 types of photopigments
Do cones have color perception?
Yes
You are able to save your vision by swatting the laser out of your
knucklehead roommate’s hand. To focus your vision on the laser, which was very close to your eye, you needed to have what combination of ciliary
muscle, zonule fibers, and lens?
Contracted ciliary muscle, relaxed zonule fibers, round/fat lens.
Rods and cones are receptor proteins found on light-sensitive
neurons. True or false?
False! Rods and cones themselves are light-sensitive cells, and they
do not fire action potentials.
What is visual acuity?
The ability of the eye to distinguish between two pints (spacial sensitivity of eye)
- Cones are concentrated at central retina
- In photopic light, greater spatial sensitivity at central retina (Fovea)
- Poorer at distinguishing colors in peripheral
retina
How does the concentration of rods and cones explain spacial sensitivity?
- Cones are concentrated at central retina
- In photopic light, greater spatial sensitivity at central retina (Fovea)
- Poorer at distinguishing colors in peripheral
retina - Rods are concentrated at peripheral retina → in scotopic light, greater spatial sensitivity at peripheral retina while central vision is blind
- Only rods are active in scotopic light → unable to perceive colors at night (b/c we need cones for this)
Blind spot
10 degrees in nasal direction
Where is acuity best for photopic light?
In the fovea
Where is acuity best for scotopic light?
In the periphery
What are the two reasons that photoreceptors have fundamentally different electrical activity?
- They have more Na+ leak channels (inward flow of Na+ is called dark current) –> Vm = -30mV
- Photoreceptors hyperpolarize in response to light
Photoreceptors ___ in response to light
Hyperpolarize
Phototransduction
- Secondary messenger cGMP keeps Na+ channels open
- Rhodopsin is rod photopigment and functions like a receptor for signal cascade
- Light changes conformation of retinal so it activates opsin (process called bleaching)
- Bleaching activates G-protein transducin
- Transducin activates effector enzyme phosphodiesterase (PDE) –> reduces levels of cGMP –> Na+ channels close –> hyperpolarization
Opsin and retinal
- Opsin is a receptor protein (has 7 transmembrane regions like G-protein coupled receptors)
- Retinal is a prebound agonist to opsin
- 3 different types of photopigments among cones –> react differently to certain wavelengths of light
Summary of phototransduction
- Receptor (roughly) corresponds to rhodopsin: Activated by bleaching (retinal changes conformation to affect opsin)
- G-protein is transducin: Opsin activates transducin
- Effector enzyme is PDE: Transducin activates PDE
- 2nd messenger is cGMP: PDE decreases levels of cGMP
- Final result is closing of Na+ channels: Stopping the dark current hyperpolarizes the photoreceptor
How do you see over an enormous range when photoreceptors can only change from -30mV to -60mV?
- The curve shows how the membrane voltage hyperpolarizes in response to increases in light intensity
- The curve can shift left or right, meaning that the photoreceptor can continue to respond to changes in light intensity (this is adaptation)
- The eye is not a camera (does not encode absolute light intensity) – it encodes changes
Light adaptation
- The membrane voltage hyperpolarizes in response to increases in light intensity
- The curve can shift left or right, meaning that the photoreceptor can continue to respond to changes in light intensity (this is adaptation)
- The eye encodes changes in light intensity, not absolute light intensity
- This is light adaptation because a higher light intensity is now required to produce a response
- The opposite direction would be dark adaptation
Does the eye encode absolute light intensity?
No, it encodes changes in light intensity
Does adaptation change the range of Vm that the photoreceptor can have?
No, it changes the intensity of light that can cause a change in Vm
Adaptation in pupils
- Higher light intensity –> constriction of pupils
- Lower light intensity –> dilation of pupils
- Wider scope of effect than calcium (affects the whole retina and reflex is consensual (shining light in one eye while the other is covered will cause both to constrict))
- Constriction also increases depth of focus
In addition to limiting the amount of light that enters the eye, what effect does constriction have?
It increases the depth of focus
If you shine light in one eye while the other is covered, will the covered eye be affected?
Yes, both eyes will constrict
Adaptation: calcium
To understand this, remember:
- cGMP (synthesized by guanylyl cyclase from GTP) keeps cGMP-gated Na+ channels open
- Light –> decrease in cGMP
cGMP-gated Na+ channel also lets in calcium –> inhibits guanlyl cyclase –> reduces the amount of cGMP made –> reduces the number of open sodium channels –> reduces the concentration of calcium in the cell (negative feedback loop)
Light adaptation:
- Sodium channels close because of transducin signal cascade (from decrease in cGMP)
- Less calcium also let into cell
- More guanylyl cyclase activation
- More cGMP made
- Channels reopen, allowing activity at higher light intensity
Diagram showing the role of calcium in adaptation
Presence of light causes a ___ in cGMP
Decrease
Negative feedback loop with calcium (adaptation)
cGMP-gated Na+ channel also lets in calcium –> inhibits guanlyl cyclase –> reduces the amount of cGMP made –> reduces the number of open sodium channels –> reduces the concentration of calcium in the cell (negative feedback loop)
Adaptation in rods and cones
- Depending on how much light is available, the retinal circuitry changes.
○ If there is not much light available, the rods become higher input for ganglion cells
○ If there is a lot of light available, the cones become higher input for ganglion cells - Cones can contain one of 3 different types of opsin: Short (blue), medium (green), long (red) → Young-Helmholtz trichromacy theory states that perception of color comes from ratio of activation of 3 different types of cones
Young-Helmholtz trichromacy theory
Perception of color comes from ratio of activation of 3 different types of cones
What happens to the levels of cGMP if there if even more PDE is activated?
cGMP levels decrease even more
What is virtually the only difference between rod phototransduction and cone phototransduction?
Cones can have one of three different opsins, while for rods there is only one single opsin
What happens in the phototransduction pathway if the light increase is sustained (adaptation)?
- Inhibition of guanylyl cyclase is reduced
- So more cGMP is produced
- So ion channels reopen and Vm rises to the same level as before
Which of the following contributes to the dark current in photoreceptors
a) K+ leakage current
b) Na+ leakage current
c) voltage-gated Na+ current
d) more than one of the above
e) all of the above
b) Na+ leakage current
Which component of the visual pathway is partially located in the temporal lobe:
a) optic radiation
b) optic tract
c) optic chiasm
d) optic nerve
a) optic radiation
What is the cause of color blindness?
A mutation in the wavelength sensitivity of one cone type
The jelly-like substance that gives the eye its round shape is the ___
Vitreous humor
Photoreceptors have all the following ionic currents EXCEPT
a) K+ leakage
b) Na+ leakage
c) G-protein dependent K+
d) G-protein dependent Na+
c) G-protein dependent K+
What does light go through just before it reaches the lens?
a) Cornea
b) Pupil
c) Sclera
d) Vitreous humor
b) Pupil
Synapses between bipolar cells, amacrine cells, and ganglion cells are in which retinal layer
a) Inner nuclear layer
b) Inner plexiform layer
c) Outer nuclear layer
d) Outer plexiform layer
b) Inner plexiform layer
What is the first effect that light has on a photoreceptor
a) Activates transducin
b) Activates phosphodiesterase
c) Closes Na+ channels
d) Changes the shape of retinal
a) Activates transducin
Considering the three types of cones and rods in the human eye, which absorbs the light with
the longest wavelengths
a) Red cone
b) Green cone
c) Blue cone
d) Rod
a) Red cone
Red cones (also known as L-cones for “long-wavelength” cones) are most sensitive to longer wavelengths, peaking around 564–580 nm, which corresponds to the red part of the visible spectrum.
Green cones (M-cones for “medium-wavelength” cones) have a peak sensitivity around 534–545 nm, in the green part of the spectrum.
Blue cones (S-cones for “short-wavelength” cones) are most sensitive to shorter wavelengths, peaking around 420–440 nm, in the blue part of the spectrum.
Where is the blind spot located?
On the nasal retina
Suppose you’re roommate shines a new military-grade laser into your eye to demonstrate key
points about receptive fields. Which of the following components of the eye does the light go
through immediately after the lens?
a) aqueous humor
b) vitreous humor
c) iris
d) retina
b) vitreous humor
High visual acuity in the fovea is associated with
a) High density of blood vessels
b) High density of rods
c) Displacement of inner retinal layers
d) Large photoreceptors
c) Displacement of inner retinal layers
What type of retinal neuron sneds axons to the LGN?
Ganglion cells
