Lecture 30: Visual System I, Retina Transduction Flashcards
What is the equation for focal length? Power of lens?
1/focal length = 1/di + 1/do
1/focal length (m) = something diopters
How does lens have to shift or contract when dealing with near and far sight?
Far vision, eye is least spherical
Near vision, eye is most spherical
What is responsible for contracting or relaxing the lens?
Zonules of Zinn (Zonular fibers)
When you need to accommodate, what do zonule fibers do? When looking at something from afar?
They relax (making lens more spherical) They contract, making lens more flat
How does light travel through the eye?
Light goes through cornea
Anterior chamber
Lens
Vitreous humor
Hits the photoreceptor of the retina first (which is the layer closest to the BRAIN)
-once the photoreceptor is activated, it sends signals to either an on-center or off-center bipolar cell
-then bipolar sends shit to off-center or on-center ganglion cell
-ganglion cell sends shit to optic nerve
What is the macula?
An oval-shaped highly pigmented yellow spot near the center of the retina of human eye
Function: specialized for acuity vision
Contains the fovea
-central vision
What is the fovea?
It is the center most part of the macula.
Is responsible for our central sharpest vision
Lacks inner nuclear and ganglion cell layers
Contain ONLY cones
What are the two main structures that refract light?
i. cornea (does bulk of refraction)
ii. lens (is the only one that can be adjusted)
Rhodopsin
protein that receives the photons and are excited
What are the three layers of cells in the retina?
i. Outer nuclear layer (ONL)
-Photoreceptors: cell bodies of rods and cones
-closest to brain
ii. Inner nuclear layer (INL)
-bipolar cells
-horizontal cells
-amacrine cells
iii. Ganglion cell layer (GCL)
-innermost layer of the eye (closest to vitreous humor)
Significance: ganglion cells are only cells in the retina that produce action potentials
-send axons to optic nerve
What is the point of the choroid (choroid plexus) that are closest to the brain?
They absorb light that did not get absorbed in photoreceptors…sharpens acuity and prevents reflection
What are the two types of photoreceptors? Are they technically axons?
Rods and Cones
-they are not axons because they don’t carry action potential
What is the function of a rod?
They are very sensitive to light (in that only one photon is needed to activate a rod cell)
-therefore they can function well at night (when there is not a light present)
-not color vision because only has one type of light-sensitive pigment
No rods in fovea so you cant see shit at night…
Rod used for starlight vision
What is the function of a cone?
Responsible for color vision
Not very responsive to light (so need to use them during the day to get enough light to stimulate these fuckers)
Are rods and cones distributed uniformly in the retina?
No! Most cones in fovea and more rods in periphery
Cat
How is acuity preserved in the central retina?
Because a cone in fovea goes to a nucleus in the occipital lobe via two unique and monogamous pathways (two bipolars for each cone)
How does photon activate a signal (in a rod?)
Photon stimulates rhodopsin (GPCR) GTP GMP cGMP-gated channel channel opens and Na flows in
- In the dark, cGMP is at high concentration leave the sodium channel open and allow the rods to do its thing and GLUTAMATE is continually released
- paradoxically light inhibits this activation because rods are not used during the day…so light hyperpolarizes rods…actually makes sense
What does light do to a rod?
Causes hydrolysis of cGMP permitting these channels to close, leading to hyperpolarization and decreased glutamate release of Off-cells
What are the two types of biopolar cells?
Off bipolars = depolarize with glutamate in DARK
-carries information about light decrements
On bipolars = depolarize with glutamate in LIGHT
-carries info about light increments
How many bipolar cells does one photoreceptor have?
One on-postsynaptic bipolar and one off-postsynaptic bipolar
When one turns on, the other turns off (either light or dark)
How does brain perceive contrast?
Cones send signals to the brain of how bright something is COMPARED to its surroundings, not how much absolute light it is receiving
-this is known as CONTRAST
Mechanism of contrast in vision?
One center pathway (the signal that the photon directly elicits
And a circular surrounding pathway
-surrounding photoreceptors are INHIBITED by horizontal cells around the center pathway
-Remember that horizontal cells are GABAergic so always inhibitory
What are the two types of ganglion cells we have to know?
P-cells (midget)
M-cells (parasol)
What is the function of P-ganglion cells?
P-ganglion cells (midget cells) receive from SINGLE cone bipolars (small Receptive Fields, RF) and convert the input to firing rates.
Project to the LGN
Lamina 3 and 6
High spatial resolution
What is the function of M-ganglion cells?
M-ganglion cells (parasol cells…shits OPPOSITE) receive MANY bipolars (large receptive fields, RFs) and are especially responsive to motion
Project to LGN
Lamina 1 and 2
How are optical images represented in the cortex?
Optical images are represented by the firing rates of population of ganglion cells. A single ganglion cell can tell the brain remarkably little. Population coding
