The Visual System Part 1 Flashcards
1
Q
Three Types of Eyes
A
1) Camera eyes (simple lens eye and corneal) 2) Compound eyes
2
Q
Difference between simple and compound eyes
A
Simple Eyes: A single lens collects and focuses light onto the retina of the eye.
Compound eyes: Multiple lenses are involved. Each of them focuses the light onto a small number of retinular cells
3
Q
Simple Eye Animals
A
Vertebrates, Cephalopod, jellyfish, gastropods, annelids and one type of copepod crustacean
4
Q
Simple Corneal Eye Animals
A
Arachnids, some vertebrates and some larval insects
5
Q
Compound eye Animals
A
Insects, crustaceans, some mollusks, and annelids
6
Q
Ciliary Muscle
A
Connects to the iris and changes the shape of the lens when your eyes focus on a near object
7
Q
Iris Function
A
Controls the opening of the pupil
8
Q
Lens Function
A
Connected to ciliary muscles transmit and focus the light onto the retina in order to create clear images of observed objects at various distances
9
Q
Cornear
A
Outer transparent layer at the front of the eye is specialized to allow incoming light rays to enter the eye
10
Q
Steps of Light Entering Eyes
A
1) Light enters the eye through the cornea
2) From the cornea, the light passes through the pupil. The iris, or the colored part of your eye, controls the amount of light passing through.
3) From there, it then hits the lens light is focused by the lens and passes through vitreous humor
4) It reaches the back of the eye where it is received by the photoreceptors of the retina, which are concentrated in the fovea.
5) Images are inverted in retina and the optic optic nerve is then responsible for carrying the signals to the visual cortex of the brain
6) he visual cortex turns the signals into images
11
Q
Branch of Ophthalmic Artery
A
Supplying blood to the retina and veins that are exiting out of the retina
12
Q
Optic Disck
A
1) Represents the beginning of the optic nerve and is the point where the axons of retinal ganglion cells come together
13
Q
Optic Nerve
A
Cranial nerve sends visual information from your retina to your brain
14
Q
Accommodation Def +Function
A
Describes the refractive power of the lens to ensure clarity of an image
15
Q
What is accommodation based on?
A
Contraction of ciliary muscles which releases tension of zonule fibres
16
Q
Zonule fibers
A
Allows for elasticity of lens to increase its curvature
17
Q
Myopia
A
an image of a distant object becomes focused in front of the retina, instead of the on the retina making distant objects appear out of focus.
18
Q
Hyperopia
A
An image of a distant object becomes focused behind the retina, making objects up close appear out of focus.
19
Q
Which layers do horizontal cells and amacrine cells mediate lateral interactions?
A
Outer and inner plexiform layers
20
Q
Horizontal Cells
A
Modulate information flow from photoreceptors to bipolar cells
21
Q
Where do bipolar cells exist?
A
Between photoreceptors (rod cells and cone cells) and ganglion cells.
22
Q
Which layer are amaricne, bipolar and horizontal cells and photorecpetors? located
A
Inner Nuclear Layer
23
Q
Bipolar Cells function
A
Bipolar cells receive input from photoreceptors (rods and cones) in the outer retina and transmit signals to amacrine and ganglion cells in the inner retina.
24
Q
Ganglion Cells
A
Carry information from photoreceptors received from interneurons into the brain via the optic nerve
25
Inner Plexiform Layer
Consists of synaptic connections between the axons of bipolar cells and dendrites of ganglion cells.
26
What does Outer Plexiform Layer contain?
Synapses between axons of photoreceptors and dendrites of bipolar and horizontal cells
27
Outer Plexiform Layer Function
The splitting of the visual signal into two separate channels of information flow, one for detecting objects lighter than background and one for detecting objects darker that background
28
What does outer nuclear layer contain
The cell bodies of the photoreceptor cells.
29
Nerve Fiber Layer
A layer that is formed by the expansion of optic nerve and includes the axons of the ganglion cells bodies also lie in this layer.
30
Where are rods found in the retina?
Found concentrated in the outer edges of the retina and are much more abundant than the cone cells.
31
Rod Function
Help in identifying the shape, size and brightness of images and cannot perceive light
32
Cones Function
Less sensitive to light and are responsible for perceiving colour and fine details of a visual image.
33
Where are cones found in retina?
concentrated in the fovea centralis which is a rods free area.
34
List the photopigment(s) found in rods.
Rhodopsin
35
List the photopigment(s) found in cones
opsin: short (blue), medium (green), long (red
36
Retinal Pigment Epithelium
Function is to provide nourishment to the retinal visual cells.
37
What type of vision occurs in levels of light at which only rods are activated?
Scotopic
38
What type of vision occurs in levels of light at which both rods and cones contribute (at twilight, for example)?
Mesopic
39
What type of vision occurs in levels of light at which only cones contribute—at twilight, for example?
Photopic
40
Important Property of Opsin
They can change their conformation from a resting state to a signalling state upon light absorption, which activates the G protein, thereby resulting in a signalling cascade that produces physiological responses
41
How does the phototranduction begin in a rod?
Begins with the absorption of a photon of light that causes the breaking of a double bond in 11-cis-retinal forming the isomer all-trans-retinal.
42
Where does the recycling of all-trans-retinal needs to be recycled into 11-cis-retinal occur?
retinal pigment epithelium,
43
Steps of Retinoid Cycle
1) All trans retinal is converted to all trans retinol (alchohol)
2) It is transported by the chaperone protein IRBP into the pigment epithelium
3) . There, in a series of steps, it is converted to 11-cis retinal and transported back to the outer segment (again via IRBP), where it recombines with opsin
44
Why is the retinoid cycle important?
Allows for continuous phototransduction in the retina.
45
What does light adaptation allow?
Allows the eye has to quickly adapt to the background illumination to be able to distinguish objects in this background.
46
What is the role of Ca2+ in photo adaptation?
- By modulating the cGMP-gated channels as well as cGMP synthesis and breakdown.
-Ca2+is involved in a negative feedback that is essential for photoreceptor adaptation to background illumination
47
Steps of Phototransudction
1) In the dark positively charged sodium ions flow into rod cells through ion channels that are activated by cGMP
2) This influx of psoitively charged ions causes cells to remain in a depolarized state, leading to the continous release of glutamate
3) When retinal absorbs light, its configuration changes from 11cist retinal to all transretina, an event that prompts opsin to activate a protien called transducin
4) Transducin activates PDSE which begin to break down cGMP
5) As cGMP levels fall, ions channels begin to close, thus less sodium enters hte cell and the cell becomes hyperpolaized due to potassium ions leaving the cell
6) Glutamate decsrease which acts as a signal that a light stimlus is present, rod cell returns to its normal state when activatd rhodsoin is inactived and arrestin (protien) binds to rhodsipin and blocks the ability of rhodsopin to activate transducin which makes the cascade unable to continue
7) The retinoid cycle occurs to restore retinal to its original configuration, making it ready to absorb light once again
48
Retinitis Pigmentosa
- Progressive vision loss results from degeneration of photoreceptors?
- A key feature of this disease is the formation of clumps throughout periphery from retinal pigments.
49
What happens to epiteletial cells during retinas pigmentosa?
The epithetical cells do not regenerate photopigment molecules but rather they begin to phagocytes (engulf) the outer segments of rods and cones
1) First tip separates from the rod
2)Tip becomes engulfed by pigment epithelium
50
The figure depicts an intracellular recording from a single cone. The amplitude increases with the intensity of the flash, proving that photoreceptors exhibit ______ potentials. Perhaps even more surprising, activation leads to ______, as evidenced by the change in membrane potential
graded; hyperpolarization
51
Which type of photoreceptor exhibits a sustained, inward current that can be likened to voltage-gated K⁺ channels?
Rods stay hyperpolarized longer
52
Which type of photoreceptor exhibits a transient, inward current that can be likened to voltage-gated Na⁺ channels?
Cones repolarize faster and regain sensitivity to light more quickly
53
Describe the molecular activity of photoreceptors under dark conditions?
In the dark, cGMP levels in the outer segment membrane are high; cGMP binds to the Na+-permeable channels in the membrane, keeping them open and allowing sodium and calcium ions to enter, and K+ leaving thus depolarizing the cell
54
Describe the molecular activity of photoreceptors under light conditions?
Absorption of photons leads to a decrease in cGMP levels, closing the cation channels and there is no influx of calcium and sodium but there is an efflux of potassium and resulting in receptor hyperpolarization
55
Rods possess high ______ sensitivity. In contrast, cones possess high ______ sensitivity and ______ acuity.
light; colour; spatial
56
Why do rods exhibit lesser spatial acuity than cones, based on structural features alone?
Because several rod cells share a connection to the optic nerve. But this also improves the eye's ability to detect small amounts of light.
57
Why do cones have higher visual acuity?
Cones have a high visual acuity because each cone is connected singly to bipolar cells so brain receives nerve impulses from small area
58
What is sensitivity of light further amplified by?
By convergence of many rods onto a single bipolar cell
59
Why is there a region in the forve that does not have vasculature?
Allows light to pass unblocked into the photoreceptor outer segment
60
Macular Degeneration
Affects center of eye and causes blurred vision or blind spot in center of eye
61
Cause of macular degeneration
Blood vessels that leak fluid or blood into the macula (responsible for our central vision)
62
protanopia
Involves the loss of long (red)-wavelength-sensitive cones?
63
deuteranopia
loss of medium (green)-wavelength-sensitive cones).
64
Many deficiencies of color vision arise from alterations in the M- or L-cone pigment genes as a result of _______ _______ during meiosis.
Unequal crossing over
65
Amacrine cells
mediate transmission from bipolar cells to gangalion cells
66
How are on on-center bipolar cells activated and inhibited?
Activated when light falls in the center of their receptive fields and inhibited when light falls on the surrounding outer ring of their receptive fields.
67
How are off-center ganglion cells activated and inhibited?
Inhibited upon light falling on the center, and activated upon light falling on the surrounding outer rings.
68
What happens when light impacts the center of a recpetive field
In the forvea, each cone will contact and on and off center cell
69
The effect of light on glumate release?
Absence of light = depolarizing dark current causes the release of the transmitter glutamate.
Light= the photoreceptors are hyperpolarized and they release less glutamate.
70
What kind of receptors do on/off center bipolar cells express?
On center bipolar cells have metabotropic glutamate receptors
Off center bipolar cells have ionotropic glutamate receptors
71
OFF CENTER BIPOLAR CELLS: DARK
In the dark, glutamate released by the photoreceptor activates the ionotropic receptors, and sodium can flow into the cell, depolarizing the membrane potential.
72
OFF CENTER BIPOLAR CELLS: LIGHT
In the light, the absence of glutamate causes the ionotropic receptors (AMPA and Kinate) to close, preventing sodium influx, hyperpolarizing the membrane potential.
73
In OFF bipolar cells, the glutamate released by the photoreceptor is _______`
excitatory
74
In ON In ON bipolar cells, the glutamate released by the photoreceptor is __________
inhibitory
75
ON CENTER BIPOLAR CELLS: DARK
In the dark, glutamate released by the photoreceptor activates the metabotropic receptors, and the G-proteins close cation channels in the membrane, stopping the influx of sodium and calcium, hyperpolarizing the membrane potential.
76
ON CENTER BIPOLAR CELLS: LIGHT (express metabotropic receptors)
In the light, the absence of glutamate results in the ion channels being open and allowing cation influx, depolarizing the membrane potential.
77
Difference in response of ON/OFFbipolar and ON/OFF ganglion cells respond?
ON-centre ganglion cells and OFF-centre ganglion cells do not respond by depolarizing or hyperpolarizing, but rather by increasing or decreasing the frequency with which they discharge action potentials.
78
Relationship between OFF Bipolar Cells and OFF Center Ganglion Cells
OFF bipolar cells will also hyperpolarize in light, which will lead to a decreased firing rate in OFF-center ganglion cells.
79
Relationship between ON Bipolar Cells and ON Center Ganglion Cells
ON bipolar cells will depolarize in light, which will lead to an increased firing rate in ON-center ganglion cells.
80
What is the primary neurotransmitter used in the visual system for generating receptive field center responses of retinal ganglion cells?
Glutmate
81
Are photoreceptor synapses with off-center bipolar cells sign-conserving or sign-inverting?
sign-conserving (since the sign of the change in membrane potential of the bipolar cell (depolarization or hyperpolarization) is
the same as that in the photoreceptor.
82
Are photoreceptor synapses with on-center bipolar cells sign-conserving or sign-inverting?
sign inverting (because the change in the membrane potential of the bipolar cell is the opposite of that in the photoreceptor)
83
Are on-center synapses with on-center ganglion cells sign-conserving or sign-inverting?
sign-conserving (Both on- and off-center ganglion cells are connected to their respective bipolar cells via sign-conserving, ionotropic receptors. This is not to be confused with photoreceptor synapses with on-center bipolar cells, which are sign-inverting due to metabotropic receptors.)
84
Match Bands
Optical phenomenon from edge enhancement due to lateral inhibition of the retina
- This is an inbuilt edge enhancement mechanism of the retina, where the edges of darker objects next to lighter objects will appear darker and vice versa, creating a false shadow.
85
Light stimulation of the surround leads to (DEPOLARIZATION / HYPERPOLARIZATION) of horizontal cells and a(n) (DECREASE / INCREASE) in the release of GABA onto the photoreceptor terminals.
hyperpolarization, decrease
86
Fill in the structures of the visual pathway, from where ganglion cell axons exit the retina at the optic disk to the visual projection areas in the brain.
1) Optic Nerve
2) Optic Chaism
3) Optic tract
3) LGN (lateral geniculate nucleus
4) Optic Radiation
5) Visual (straied) Cortex
87
Optic Chiasm
The place in the brain where some of the optic nerve fibers coming from one eye cross optic nerve fibers from the other eye.
88
Lateral geniculate nucleus (thalamus)
From here, visual information is organized from the retina and sent off to the primary cortex.
89
Optic Radiation
Contains axons from the neurons in the lateral geniculate nucleus to the primary visual cortex.
90
Visual Cortex
This is where images received from your retina begin to get processed and then sent out to other brain regions
91
Pretectum
Responsible for reflex control of the pupil and lens
92
superior colliculus
receives input from the retina and the visual cortex and participates in a variety of visual reflexes (head and eye movements)
93
Pupillary light reflex
Automimic reflex that constricts the pupil in response to light, thereby adjusting the amount of light that reaches the retina.
94
Course of Optic Radiation to Striate Cortex
- Axons carrying information about the superior portion of the visual field sweep around the lateral horn of the ventricle in the temporal lobe (Meyer’s loop) before reaching the occipital lobe.
95
Calcrine Suclus Location and Function
Located at the caudal end of medial occipital lobe separates the upper and lower visual fields in the striate cortex
96
Fovea
an area in the middle of the retina that gives the sharpest vision of any part of the retina and contains only cones
97
Where is the upper and lower visual vield in relation to the calcarine suclus?
The upper visual field = below the calcarine sulcus
The lower field =above the calcarine sulcus
98
How many layers does the LGN have?
Six cell body layers in the LGN
- The inner two layers, (1 and 2) are magnocellular layers
- outer four layers, (3,4,5 and 6), are parvocellular layers.
99
Where do layers 1,4 and 6 recieve input from? LGN
Contralateral (the eye located on the opposite side of the body) nasal hemiretina,
100
Where do layers 2, 3 and 5: receive input from LGN?
From the ipsilateral (the eye located on the same side of the body as another structure) temporal hemiretina.
101
What happens at the LGN?
There is a mix and blend of info coming from the left and right eye and this info is mixed and blended/distributed across these six layers at the visual cortedx
102
Occular Dominance
tendency to prefer visual input from one eye over the other
103
M cells supply which layer of LGN
M cells have large-diameter cell bodies and large dendritic fields. =supply the magnocellular layers
104
P cells characteristic and supply which layer of LGN
P cells have smaller cell bodies and dendritic fields = supply the parvocellular layers
105
K cells charactersitic and supply which layer of LGN?
K cells have small cell bodies and intermediate-sized dendritic fields =supply the koniocellular layers (present between the layers)
106
Bitemporal (heteronomous) hemianopsia
Results in blindness of the temporal visual field of each eye, the loss of half of the visual field on different sides in both eyes.)
107
Left homonymous hemianopsia.
loss of sight in the left visual field
108
Left superior quadrantanopsia
loss of vision) affecting a left quater of the visual field
109
Left homonymous hemianopsia with macular sparing
A partial loss of vision in the left half of both eyes with the center, the macula, spared.
110
How many principal cellular layers is the primary visual (striate) cortex divided into?
6 (nissil staining, dendritic morphology, LGN inputs, interlaminar, outputs (ascending red and descneidng (green, blue)
111
Ocular dominance columns:
Respond preferentially to input from one eye or the other
112
Orientation columns
Preferentially respond to orientation
113
Neurons in the primary visual cortex response
Neurons in the primary visual cortex typically respond vigorously to a bar of light oriented at a particular angle (vertical edges) and less strongly—or not at all—to other orientations.
114
Scelera
The supporting wall of the eyeball. It helps maintain your eyeball's shape, and protects it from injury.
