Lecture 5 Flashcards
Anatomy of the human eye
Cornea
Pupil
- expands or dilates depending on light due to iris
Lens
- curved and brings light to focus
Retina
- sensory region of the eye
Optic nerve
- 1 million axons
- goes back to thalamus
Emmetropia, myopia, hyperopia
My friends are near, they are around
Emmetropia
- normal vision
- focus point on retina
Myopia
- nearsighted
- lens too curved or eyeball too long
- focus point in front of retina
Hyperopia
- farsighted
- lens not curved enough or eyeball too short
- focus behind the retina
= think of cornea and eye as entire eyeball squished
Friends far = you long for them
Snell’s law
n1Sinθ1 = n2Sinθ2
θ is the angle relative to the normal / vertical line
Snell’s law - sample n
if n2 > n1
θ2 is smaller
Smallest n
vacuum
air
water
cornea
lens
diamond
Biggest n
Bending line with a curved surface - cornea
more curved / round cornea
= light bends more
Lens vs Cornea
- Cornea bends light but it is not adjustable
- Lens is the one that brings the light into focus by moving and bending light
- Cornea bends light more than the cornea bc the density of the cornea is compared to the density of air → much bigger difference
But once it gets into the eye, it’s in the vitreous humour (basically water) - Cornea bends the light a lot, lens fine tunes it
- Cornea has greater focusing power
Flippy retina
- What hits your retina is upside down and left to right switched
- Your brain knows how to interpret the image and flip it back
Inside-out retina
Light
1. Retinal ganglion cells = neurons
2. Bipolar cells
3. Photoreceptors = rods and cones absorb light
Optic nerve
The blind spot
- no photoreceptors where 1 million axons converge
= optic disk
near the nasal retina
Structure and function of the retina - photoreceptors and RPE
- Photoreceptors make a voltage change
- 2 main types rods and cones
- Outer segment and cell body
- In outer segment a bunch of disks filled with molecules called photopigments that absorb light
RPE
- pigment epithelium cells
- absorbs light so it doesn’t bounce = prevents blurring
- nourishes the photoreceptors
- photoreceptors at the back so they can access RPE
Pulse chase experiment - disks move over time
Injected a radioactive AA (3H methionine)
→ looked at different time point
→ could see where the radiation was
→ gets taken up into proteins
→ over the course of a few week migrate into retinal epithelium
→ shed their disks into retinal epithelium and add new ones in
3H: tritium
methionine: amino acid
Why do we shed photoreceptors disks
- light is damaging to tissue
- damages molecules in the disks
- phagocytosis swallowed by the RPE, degraded and recycled
Phototransduction
general
- physical stimulus (light) converted into nervous system response
- done by photoreceptors
Whole-cell patch clamp for hyperpolarization of photoreceptors
Result
- light causes photoreceptor hyperpolarization (depolarization in the light)
hyperpolarization = inward current
brighter light = hyperpolarizes more
- doesn’t matter for AP, just a signal
Photoreceptor ______ in the dark
depolarized
+ charge enters the cell
Na, Ca enter, K leaves
- current
current reduced by the light flash
Ca/Na ions enter through channels controlled by an intracellular ligand
K through leak channels
cGMP = cyclic guanosine monophosphate
- opens channel and lets ions flow in
- neurotransmitter glutamate released in dark and binds to bipolar cells
Photoreceptor ______ in the light
hyperpolarized
same as dark but less Na and Ca enter, K still leaves
- this is because cGMP degraded when the photoreceptor absorbs light
- cGMP usually binds to let in Ca and Na
graded action potential
Ca closed so no/little neurotransmitter release
Photoisomerization
what in which protein
- opsin proteins
- rhodopsin in rods
retinal in opsins
- ret absorbs light
- conformational change forming an isomer
- is a form of vit. A
- absorbs light = chromophore
changes shape of retinal cis to trans
Phototransduction
- photoisomerization of retinal in opsin
- activates transducin enzyme
- transducin activates phosphodiesterase (PDE)
- breaks down cGMP
= ion channels close
Photoadaptation
- hyperpolarize when light first turns on but slowly depolarize
- reduction due to sustained constant response
- when cGMP not there and ion channels close, less Ca inside cell
- Ca usually inhibits guanylate cyclase function
- guanylate cyclase is an enzyme that produces cGMP
no more inhibition = produces cGMP again = channels reopen
- causes depolarization
Phototransduction and adaptation loop
Transduction
- Light
- phosphodiesterase activated
- cGMP down
- close Na and Ca channels
- hyperpolarization
Adaptation
- guanylate cyclase activated
- cGMP up
- open Na and Ca channels
- depolarization
Colour - rods and cones graph
three different types of cones
smallest wavelength / purple
- short cones (blue)
- rods (greyscale)
- medium cones (green)
- long cones (red)
red / largest wavelength
brain infers colour from the relative activation of the three cone types
Rods vs. cones - 3 reasons for light sensitivity
Most sensitive to light
= rods
Length
- longer so they contain more photopigment
- sensitive enough to see at night
Convergence
- many rods converge onto the same bipolar cell so there is a higher probability that one of them will see the light
Amplification
- have greater amplification, close more Na channels in response to same amount of light absorbed
1 photon = 1mV hyperpolarization
- cones need 100 photons to have the same effect
Rods vs. cones - 2 reasons for spatial acuity
Better spatial acuity
= cones
- cones densely packed at the fovea which lacks blood vessels and axons on top of it = no light scattering
- less convergence, knows exactly where signal came from
General rod/cone differences
Rods
- eccentric
- 1 type
- grayscale
- high sensitivity
- low spatial acuity
- 90 million
Cones
- central
- 3 types
- colour
- low sensitivity
- high spatial acuity
- 4.5 million
Discovering distribution of cone types in the retina
- flash with yellow = reflected with blue, absorbed red and green
- bleach with red to eliminate red function, flash with yellow and only green will absorb
On-center and off-center retinal ganglion cells
On-center
- turns on when light is on in their center
- does the opposite of cone
- weird glutamate receptor
- metabotropic glutamate receptor mGluR6
- glutamate causes mGluR6 to close Na channel
= glutamate is inhibitory
= depolarize in light
Off-center
- turns off when light is off in their center
- does the same as cones
- normal ionotropic glutamate receptor (glutamate makes Na channel open)
= hyperpolarize in light
Ionotropic versus metabotropic glutamate receptors
Ionotropic
- neurotransmitter binds to channel
- channel opens
- ions flow across membrane
ex. glutamate (AMPA) receptor, GABA (A) receptor
Metabotropic
- nt binds to receptor
- activates G-protein
- G-protein subunits go to effector proteins
- then send intracellular messengers
- cause channel to open or close
- ion flow
Biolistic transfection
biology + ballistic
- like a BB gene gun shooting DNA particles into a neuron
- gold particles covered with DNA, in tubes
- shoot into neuron
- some happen to land in nucleus and get transcribed
= neuron will make the protein
Biolistic transfection - example with retinal ganglion cells
morgan, schubert, wong
- put genes for 2 fluorescent proteins into mouse retinal ganglion cells
- using biolistic transfection
- used to show what the cells look like and where their glutamate receptors are
Fluorescents
1. td-tomato fills cytosol (orange)
- done with gene gun
- PSD-95 found around glutamate receptors (green)
PSD = postsynaptic density
- can be digitized using a computer program to get rid of confusing dots, only show probable receptor dots with specific traits
