Topic 109 - Vision, Cellular mechanism of light perception, color vision Flashcards
1
Q
Words to include
A
- Optic system
- Photon
- Sensation of light
- Spatial orientation
Morphology of the eye
- Eye bulb
- Fibrous sclera
- Cornea
- Limbus
- Vascular choroid
- Retina
- 10 layers
- Light-sensing sensory epithelium
- Ganglion cells (neurons)
- Pigment cells
- Light absorbing
- Rods
- Cones
- Synaptic part
- Receptor cell
- Bipolar cells
- Ganglion cells
- Horizontal cells
- Amacrine cells
- Fovea
- Sharp vision
- Cones
- Macula lutea
- Blue light (ø absorbed)
- Optic disc
- N. opticus
- Blind spot
- Lens
- Accommodation
- Crystalline lens
- Optic axis
- Suspension of lens:
- Ciliary muscles
- Zonular fibers
- Pupil
- Sphinter muscles
- Dilator muscles
- Iris
- Anterior chamber
- Posterior chamber
- Aqueous humor
- Choroid plexus
- Accommodation
Retraction of eye
- Air - cornea
- Cornea - aqueous humor
- Aqueous humor - lens
- Lens - viterous humor
- Total refractive power
Imaging
- Rear focal point
- Beam intersect
- Hypermetropia
- Elasticity (ø)
- Myopia
- Near-sightedness
- Emmetropic
Cellular mechanism of the light perception
- Light sensation
- Rods
- Cones
- Darkness
- Cation channels
- Depolarization
- Dark current
- Rest
- Glutamate transmission
- Abolition of glutamate release (inhibition of dark current)
- cGMP
- Plasma membrane
- Sodium channels
- Sodium ion flow (→ cell)
- Sustained glutamatergic transmission in synapse
- Hyperpolarization
- Depolarization
- Glutamate release
- Bipolar cells
- Sodium channels
- Light
- Photopigment
- 11-cis-retinal bound
- All-trans-retinal
- Transretinal protein opsin
- Opsin
- Conformation change
- G-protein
- Phosphodiesterase enzyme (PDE)
- Membrane
- Hyperpolarization
- Glutamate transmitter release (↓)
- Photopigment
- Cation channels
Receptive fields
- Illumination state
- Ganglion cells
- ON-type ganglion cells
- Frequency of AP ↑
- OFF-type ganglion cell
- Frequency of AP ↓
- ON-type ganglion cells
Color vision
- Cone (at least 2)
- Blue
- Green
- Red
- Retina
- Photopigments
2
Q
Morphology of the eye
A
- The eye bulb is surrounded by fibrous sclera, within reaches the transparent cornea at a borderline called limbus
- Under this layer is the vascular choroid
- The inner layer of the eye is retina which contains light-sensing sensory epithelium and neurons (ganglion cells)
3
Q
Morphology of the eye
Retina
A
- 10 layers
- Innermost layer
- Cosists of:
- Pigment cells (light absorbing)
- Rods (light sensitive)
- Cones (light sensitive)
- Horizontal cells
- Bipolar cells (nervous element)
- Amacrine cells
- Ganglion cells (nervous element)
4
Q
Morphology of the eye
Uvea
A
- Middle layer
- Consists of:
- Iris
- Choroid
- Ciliary body
5
Q
Morphology of the eye
Sclera
A
- Outer layer (together with cornea)
- “White of the eye”
6
Q
Morphology of the eye
Cornea
A
- Outer layer (together with sclera)
- Avascular
7
Q
Morphology of the eye
Fovea
A
- The place for sharp vision
- Does not contain cones
- To ensure sharp vision, the fovea does not absorp blue light
- Location: macula lutea (humans)
8
Q
Morphology of the eye
N. opticus
A
- Optic disc / n. opticus
- Where the n. opticus leaves the eye
-
Blind spot
- Ø rods and cones
-
Blind spot
9
Q
Morphology of the eye
Lens
A
- Transparent
- Biconvex
- Role: helps to refract light to be focused on the retina
- Accommodation: By changing chape, functions to change the focal distance of the eye so that it can focus on objects at various distances
Suspension of the lens
- Ciliary muscles
-
Iris: regulates diameter of the pupil, separates the anterior chamber from the posterior chamber
-
Pupil
- Mm. sphinter
- M. dialator
-
Pupil
10
Q
Retraction
A
- The eye is able to receive stimulus from a relatively wide visual field by focusing the incoming light with retractive elements:
- Air - cornea
- Cornea - aqueous humor
- Aqueous humor - anterior surface of the lens
- Anterior surface of the lens - vitreous humor
11
Q
Imaging
A
- Rear focal point: where the beam intersect
-
Acommodation: the eye changes optical power to maintain a clear image or focus on an object as its distance varies
- Ciliary muscles can contract, it relaxes the zonula fibers and the convexity of the lens increases
- Emmetropic: eye with perfect accommodation
- Hypermetropia: loss of elasticity of the lens in elderly people
- Myopia: near-sightedness
12
Q
Cellular mechanism of light perception
A
- Light sensation is triggered in the rods and cones
- In darkness the continuously open cation channels (depolarization) maintain a dark current
- In rest both rods and cones manitain a dark current, which sustains a continuous glutamate transmission
In darkness:
- The system of the membranes of discs of outer segments is inactive (R, G, PDE)
- The continuously produced cGMP keeps the sodium channels open in the plasma membrane
- Through it sodium ions flow continuously into the cell: the dark current typical for cones and rods emerges
- This latest (rods?) generates a sustained glutamatergic transmission in the synapse
- The glutamate released activates or inhibits the bipolar cells, depending on the type of glutamate receptor in those bipolar cells
- Hence the glutamatergic transmission depolarizes some cells and hyperpolarizes others
Light activates the photopigment:
- This (rhodopsin or visual purple) is 11- cis-retinal bound to a transretinal protein, opsin
- In the photopigment light transforms 11-cis-retinal to all-trans-retinal. This change of conformation results in conformation change of opsin and that activates the G-protein
- G-protein activates the phosphodiesterase enzyme (PDE).
- PDE degrades cGMP, which holds open the sodium channels in the plasma membrane
- Hence the sodium channels close
- This hyperpolarizes the membrane:
- The glutamate transmitter release ceases
13
Q
Receptive fields
A
- The AP frequency of ganglion cells depends on the illumination state of that retinal section from which the ganglion cell receives information
- Two types of ganglion cells are known:
-
ON-type ganglion cells
- Freequency of AP ↑
-
OFF-type ganglion cells
- Freequency of AP ↓
-
ON-type ganglion cells
14
Q
Color vision
A
- One cone alone is unable to code the color. It contributes the increased contrast among objects and thus the better visual acuity:
- All colors can be generated by mixing the three colors, blue, green and red, sensed by the three photopigments of the retina
- The basis for the sensation of mixed colors is that the three types of cones detect the light not only of a single wavelength but in a wider range
- If from the three kinds of cones is only one type present, color vision is impossible
- The basis of color vision is the presence of at least two cones absorbing light rays of selected wavelengths with higher efficiency
- Scotopic vision: Rods react to the change of light intensity in the viewabel spectrum independently from the wavelength (color vision is the function of cones)
- Role of color vision: Increased contrast among objects → better visual acuity