NEU Quiz 5 - Vision Flashcards
What is the fovea? What is its role in vision? How does the architecture of the fovea lead to its function?
Area of retina called fovea where there are many more cones than rods. Center of fovea called foveola there are no rods at all. Bc of high cone content fovea is part of the retina with the capacity for highest acuity vision. Thats why move eye a lot so that fovea can see.
Retina – part of eye responsible for actual vision that contains
Photoreceptors – receive light - phototransduction → specialized cells that respond to light
Fovea → area of highest acuity (sharpness) → certain photoreceptors called cones (color) & detail
Macula: Contains fovea → central vision
Fovea moves the layers of cells sideways (lateral). Basically, the ganglion cells, bipolar cells, are moved to the side, therefore creating a pit above the photoreceptors layer (cones). This allows light to strike the photoreceptor cells directly without passing through the others (ganglion & bipolar cells)–> leads to less light scatter.
Which feature is responsible for the superior acuity of the fovea?
Lack of retinal blood vessels
What are the two photoreceptors we studied in the retina that absorb light and change it into electrical signals?
When photoreceptors absorb photons causes changes in the amount of NT released this effects bipolar cells which is the next level.
Rods –Scotopic Vision (low light)
Converge multiple rods onto one bipolar cell
Cones-Photopic vision (high light)
Cones and rods depolarized in dark and hyperpolarize in light
Rods –Scotopic Vision (low light)
Converge multiple rods onto one bipolar cell
Retinal (light absorbing molecule)
Dim light
Black and White vision
Opsin: Rhodopsin
Outer segment rod shape
High Sensitivity to light
Low acuity (spatial resolution) - not good at seeing detail
Harder for us to see details when theres very little light due to the poor spatial resolution
Dim light can’t perceive color
Cones-Photopic vision (high light)
Retinal
Bright light
Color vision
Opsins: S, M, L
Outer segment cone shape
Low Sensitivity to light
High acuity (spatial resolution)
Opsins
4 proteins that bind retinal and allow different colors to be interpreted
Wavelengths Absorbed by Each Opsin
And what losses
S blue= short, M green = medium, L red = long wavelengths
Loss of M = deuteranopia
Loss of L = protanopia
Loss of either one leads to red green color blindness (tends to affect males more than females)
Color vision
Ability to detect differences in wavelengths of light
Distribution of Rods and Cones
High concentration of cones in fovea and high rods in peripheral
Blind spot no photoreceptors so no rods or cones
How do the opsins located in rods and cones transform light into electrical signals? When light hits the opsin located in Rods, what is the series of events that takes place to change the membrane potential of the photoreceptor?
Light enters the eye & strikes photoreceptors (rods or cones) → 11-cis-retinal, which is bound to opsin protein in the photoreceptor. This absorption of the photon→ change in the configuration→all-trans-retinal→ triggers a confirmation change in the opsin protein (making it active) → metarhodopsin II→ activated G-protein (GDP→ GTP by phosphorylation) called transducin (bound to the inner segment of the photoreceptor) →activates an enzyme called phosphodiesterase (PDE)-responsible for hydrolyzing cyclic GMP (cGMP) into 5’-GMP. There is a reduction in cGMP, which leads to ion channels closing→ decrease in influx of sodium & calcium into the photoreceptor→ membrane becomes hyperpolarized (inside of the cell becomes more negative compared to the outside) →decrease NT (glutamate) from the photoreceptor’s synaptic terminal → this decrease in NT is detected by ON-center bipolar cells (Inhibition of ON-center bipolar cell is reduced )→ on-center bipolar cells become depolarized→ On-center bipolar cell releases glutamate
→on-center ganglion cells depolarized→ action potential sent to the optic nerve to the LGN→optic chiasm→brain (V1).
ON-Center Bipolar Cell Overview
On Type Bipolar Cells: These bipolar cells are depolarized in the light and are hyperpolarized in the dark.
ON-Center Bipolar Cell: is maximally excited when there is light in the center of its receptive field and less light in the surrounding area. This means it responds most vigorously when there is a transition from DARKNESS TO LIGHT in the center.
- Light in the center: leads to an increase in NT release from the associated photoreceptor cell→ excite the ON-center bipolar cell
- Less light in the surrounding area: is typically in the shadow. This low light in the surrounding area→ excited ON-center bipolar cells
- Conclusion: ideal stimulus for an ON-center bipolar cell is BRIGHT light
OFF-Center Bipolar Cell Overview
Off Type Bipolar Cells: These bipolar cells are depolarized in the dark and hyperpolarize in light. These cells respond to more glutamate intake.
OFF-Center Bipolar cell: is maximally excited when there is reduced or no light in the center of its receptive field and more light in the surrounding area. It responds most vigorously when there is a transition from LIGHT TO DARKNESS in the center.
- Darkness in the center: leads to an decrease in NT release from the associated photoreceptor cell→ excite the OFF-center bipolar cell
- More light in the surrounding area: Typically well-lit in comparison to the darkness in the center. This increased lighting in the surrounding area→ excited OFF-center bipolar cells
- Conclusion: ideal stimulus for a OFF-center bipolar cell is DARK light
ON-Center Bipolar Cells - Rods in DARK
cGMP-gated channels OPEN→influx of cation→ photoreceptor depolarizes→Voltage gated Ca+2 channels open in synaptic terminals→ NT (glutamate) released→ IPSPs in bipolar cell→hyperpolarization→closes voltage gated Ca+2→inhibiting NT release→ No EPSPs occur in ganglion cells→no AP along optic nerve
on center in dark is hyper
ON-Center Bipolar Cells - Rods in the LIGHT
cGMP-gated channels CLOSED→influx of cation STOPS→ photoreceptor hyperpolarizes→Voltage gated Ca+2 channels close in synaptic terminals→ NO NT (glutamate) released→ lack of IPSPs in bipolar cell→depolarization→opens voltage gated Ca+2→NT release→EPSPs occur in ganglion cells→AP along optic nerve
on center in light is depo
Off-Center Bipolar Cells - Rods in DARK
Na+ & Ca+2 channels open→depolarization to -40mV→ glutamate release onto ganglion cells→ glutamate receptors are excited by NT binding→AP to optic nerve→ brain
off in dark depo
Off-Center Bipolar Cells - Rods in Light
Na+ & Ca+2 channels closed→hyperpolarization to -70 mV→no glutamate release=no excitation of ganglion cell→ NO AP
off in light is hypo
What types of retinal ganglion cells do we have? What type of information do they respond to?
Ganglion cells - conduct electrical signals – receive signals from bipolar cells - form optic nerve → axons leave the retina/eye though optic nerve
- Optic disk is blind spot with no photoreceptors
- Types: M - Magnocellular AND P - Parvocellular
Horizontal and amacrine cells - allow for combination laterally between neurons (exist as modifiers of activity in bipolar and ganglion cells) - do not directly send information to the brain
P&M ganglion cells:
- P = color, acuity - primarily are getting information from cones
- M= motion, achromatic, low acuity, lack color information – receive inputs from many rods and a few cones
- Based on information they receive from the network of photoreceptors, bipolar cells, and amacrine cells
How is the LGN organized? How many layers are there, and what type of cells are present in each? For these different cell types, what type of information do they typically carry?
6 Layers of LGN
Magnocellular: Bottom 2 thinner layers(#1&2)
- mLGN cells
- Synapse with M retinal ganglion cells
- No color
- Movement
Parvocellular: Top 4 thicker layers(#3-6)
- pLGN cells
- Synapse with P retinal ganglion cells
- Contrast – allows detection of edges
- Shape and edges, Color
Koniocellular: Unable to see
- kLGN cells
- Synapse with P retinal ganglion cells
- Color
LGN Cross Over Thing
Right eyes to LEFT LGN: visual info from the right half of the visual field of each eye is transmitted to the left side of the brain. Basically, optic nerve fibers from the RIGHT eye carry info from the right visual field to the left LGN→ LEFT BRAIN.
Left Eye to RIGHT LGN: opposite of above
Draw the pathway that light entering the eye travels throughout the brain. Include how the left and right visual field is taken in by the retina, and how information crosses over. Include where axons from the retina synapse, and where in the cortex they are sent. Label each part of the pathway. And Orders And The Major Visual Areas and Pathways - once we leave retina
cornea - pupil - iris - lens - vitreous humor - retina - optic nerve.
cornea - lens - retina - fovea - optic disc (in ganglion cell axon leave eye get ready to go the brain and they go optic to do this, optics disc ganglion go to leave eye so no photoreceptors here)
all inside retina –> photoreceptors (cone or rod) - horizontal cells - amacrine cells - ganglion cells
optic nerve extends to region below hypothalamus called optic chiasm. right visual vield travels to left side of brain, and vice versa. After optic chiasm visual vibers no longer optic nerve now optic tract. this goes to different parts of brain.
The Major Visual Areas and Pathways - once we leave retina
Optic nerve → axons of the ganglion cells
Optic Chiasm: where optic nerve fibers cross over
Lateral geniculate nucleus: visual relay in the thalamus (grand central station for all senses except for olfactory smell)
Striate (striped) cortex: early visual cortex Also known as V1 or primary visual cortex
Only axons from the nasal half of the retina cross in the optic chiasm
Left vision field: hits nasal field of left eye and hits temporal part of right eye - right LGN
Right vision field: hits nasal part of right eye and temporal of left eye - left LGN
Both visual fields are processed by both
Everything crosses over
Information from both visual fields hit both eyes
Only left visuals field to right has to cross over and vice versa
What layer of the primary visual cortex (V1) do the axons travel from the LGN synapse onto, and what type of cells do they form synapses with?
From LGN to V1
Primary visual cortex:
V1, Striate cortex
LGN axons synapse in deep layer 4 of V1
Synapse on stellate neurons
Signaling from stellate to pyramidal neurons
Still monocular
From layer 4, signals radiate to other layers
Converge so image is binocular
Retinotopic – neurons organized in V1 like a map of the retina; so they receive input from specific regions of retina
cortical magnification
Extrastriate – Color, Motion, What and Where
From V1:
V2/V3- further processing; combination of signals
V4- color interpretation
V5/MT – motion detection; direction processing
To WHAT and HOW/WHERE:
What – temporal lobe
ID the object
How/where – parietal lobe
Determine position in space
ID how to use
When does the image converge, becoming binocular?
From layer 4, signals radiate to other layers
Converge so image is binocular
Convergence refers to this media movement of the two eyeballs so they are both directed toward the object being viewed. The nearer the object, the greater the degree of convergence necessary to maintain a singular binocular image.
Convergence – when looking at a close-up object, your eyes angle inwards towards each other (you become slightly cross-eyed). The extra effort used by the muscles on the outside of each eye gives a clue to the brain about how far away the object is.
Binocular convergence is when both eyes rotate inward at different angles to focus on an object. The degree to which the eyes turn is sent to the brain to determine how far away an object may be. Binocular convergence creates a three-dimensional image that helps with depth perception and the location of objects.
What is the most direct path that light information travels on its way to the optic nerve?
Photoreceptor cell; bipolar cell; ganglion cell; optic nerve
Which of the following correctly matches rods and cones with their properties?
Rods: high sensitivity to light; cones: high spatial resolution
- each cone connects to one bipolar cells, 1 to 1 - many rods converging onto one bipolar cells, rods have a greater reaction to being hit with it
