Lecture 28-Visual System I Flashcards
Which two parts of the eye control focusing power?
cornea and lens
Which part of the eye accounts for 2/3 of the focusing poers?
cornea
In addition to the cornea, which structure is involve in “fine tuning” the focus of an image based on distance from the eye?
lens
How is the curvature of the lens controlled?
contraction/relaxation of the ciliary muscle
What is the function of the ciliary muscle?
controls curvature of the lens (accommodation)
How do the ciliary muscle and lens adjust when looking at distant objects?
- ciliary muscle relaxes
- lens flattens
How do the ciliary muscle and lens adjust when looking at near objects?
- ciliary muscle contracts
- lens is rounded
When is the ciliary muscle at its largest diameter?
when looking at distant objects (flatten lens-tighten suspensory ligaments)
What happens as the lens looses elasticity with age?
-focusing power on nearby objects declines (presbyopia)
What is presbyopia?
- happens as the lens looses elasticity w/age
- focus on nearby objects declines
What are the 2 types of photoreceptors?
Rod and Cones
Which photoreceptors are more receptive to lights?
rods
Which photoreceptors are more receptive to color?
cones
What is scotopic vision?
night vision
What would happen if you only functioned on “rod” vision?
you would only see in shades of gray
What is photopic vision?
day time vision
Which photoreceptors have a higher density in the peripheral retina?
rods
Which photoreceptors have a higher density in the fovea?
cones
Where in the retina are the cones more highly concentrated?
cones-FOVEA
What is the light absorbing pigment found in photoreceptors?
rhodopsin
What are the 2 parts of the rhodopsin protein?
- 11-cis retinal (chromopore-Vit A derivative)
- opsin (protein part)
Which part of the rhodopsin molecule is responsible for catching light?
11-cis retinal (chromopore)
What happens to the 11-cis retinal part of the of the rhodopsin molecule in the presence of light?
isomerizes to all trans-retinal
How does light affect the photoreceptor cells
causes isomerization of the chromopore (11-cis retinal) to all trans-retinal
What happens to the all-trans retinal portion of the rhodopsin molecule after photoisomerization?
It detaches from the opsin protein
What causes the G-protein transducin to be activated?
activated rhodopsin (after photoisomerization and separation of the 2 parts)
What does the G-protein transducin activate? What does this cause?
Transducin activates phosphodiesterase (PDE) which hydrolyzes cGMP (decreased cGMP)
How does the phosphodiesterase (PDE) affect cGMP levels?
decreases them
What is the role of cGMP in darkness?
keeps ion channels in outer segment open
What is the “dark current”?
constant flow of ions in the dark that keeps the photoreceptor more polarized and leads to a continuous release of glutamate
What is the resting membrane potential during the dark current for a photoreceptor? Why is this so?
-35mV, more depolarized because cGMP keeps ion channels open and maintains a constant ion current
What neurotransmitter is released at the synaptic ending of a photoreceptor?
glutamate
What happens to the photoreceptor membrane potential in the presence of light?
hyperpolarizes
What does the reduction in cGMP by the PDE cause in the photoreceptor?
-closure of membrane channels-> reduction in dark current -> hyper polarization -> decreases glutamate release
Do photoreceptors generate action potentials?
No- they send receptor potentials based on the intensity of light
What are the 2 advantages to having a phototransduction cascade?
- amplification
2. modulation
How does the phototransduction cascade provide amplification?
1 rhodopsin activates 100s of transducins which activate 1000s of PDEs which significantly affect cGMP levels
Why is modulation of the phototransduction cascade important? How is it controlled?
- important for light and dark adaptation
- controlled by Calcium feedback mechanism
Where do photoreceptors send info to be pre-processed before signals reach the brain?
bipolar cells
What are the two types of bipolar cells?
H= off-center bipolar cells D= on-center bipolar cells
What type of receptors do H-bipolar cells express? How do they respond when stimulated?
- ionotropic GLU receptors
- hyperpolarize when stimulated
What type of receptors do D-bipolar cells express? How do they respond when stimulated?
- metabotropic GLU receptors
- depolarize when stimulated
Where do bipolar cells synapse?
retinal ganglion cells
What is contained within the optic nerve?
axons of many retinal ganglion cells
Where do reinal ganglion cells project?
to the brain (lateral geniculate nucleus of the thalamus) via the optic nerves/tract
In what plane would the horizontal cells be found?
Between the photoreceptors and bipolar cells
What cells mediate lateral inhibition in the retina?
horizontal cells (and amacrine cells)
What is the purpose of the lateral inhibition provided by the horizontal cells?
enhance contrast
Which cells provide lateral inhibition at the bipolar retinal ganglion cell synapse?
amacrine cell
What are the 3 forms of rhodopsin that cones contain?
blue (short-420nm)
green (medium-496nm)
red (long-560nm)
