1
Q
where are photoreceptors located?
A
the retina
2
Q
what are photoreceptors and what do they do
A
photoreceptors are light-sensitive neurons that convert light energy into electrical energy in cells (phototransduction)
3
Q
2 main types of photoreceptors in the retina
A
cones and rods
4
Q
each retina contains —— cones
A
6 million
5
Q
each retina contains —— rods
A
120 million cods
6
Q
what are rods and cones and how do they function?
A
they are neurons, though they do not fire action potentials, but instead respond to stimuli with graded membrane potential
7
Q
why is the usage of graded membrane potential by rods and cones important?
A
it can be adjusted in a continuous manner based on the light they detect
8
Q
describe the basic structure of cones and rods
A
outer segment: the membrane folds into dis-like layers which contain the visual pigments that respond to light
inner segment: the nucleus and organelles for protein synthesis
basal layer: base of photoreceptor cell, a synapse that releases glutamate
9
Q
cones and rods point towards which direction?
A
the back of the eye
10
Q
layers of the adult retina
A
retinal pigment epithelium (RPE)
outer segments (OS)
outer nuclear layer (ONL)
outer plexiform layer (OPL)
inner nuclear layer (INL)
inner plexiform layer (IPL)
ganglion cell layer (GCL)
nerve fibre layer (NFL)
11
Q
retinal pigment epithelium
A
supports, nourishes, and regenerates photoreceptors
12
Q
outer segments
A
contains stacks of membrane discs with visual pigments - the site of phototransduction
13
Q
outer nuclear layer
A
houses the cell bodies of rods and cones
14
Q
outer plexiform layer
A
synapses between photoreceptors and bipolar cells
15
Q
inner nuclear layer
A
contains the cell bodies of bipolar, horizontal, amacrine, and Muller glial cells
16
Q
inner plexiform layer
A
site of synapses between bipolar cells, amacrine cells, and ganglion cells
17
Q
ganglion cell layer
A
contains the output neurons of the retina - the ganglion cells
18
Q
nerve fibre layers
A
contains axons of the ganglion cells, heading toward the optic nerve
19
Q
why do the laters of the retina appear to have a backwards structure?
A
- Proximity to the Retinal Pigment Epithelium, which is responsible for nutrient and oxygen supply, regenerating retinal after it has been exposed to light, and waste removal and protection
- Enhanced Vision via Müller Glial Cells: span the depth of the retina act as living optical fibers, channeling light directly to the photoreceptors and minimizing light scattering
20
Q
how do photoreceptors detect light?
A
using membrane-bound visual pigments
21
Q
pigments in rods and cones
A
- rhodopsin in rods
- 3 other pigments in 3 types of cones
22
Q
function of rods vs cones
A
Rod photoreceptors detect light only, while cones detect colors.
23
Q
phototransduction - dark
A
- Inactive rhodopsin: opsin + 11-cis retinal
- Because rhodopsin is inactive, the G-protein transducin (Tαβγ) is GDP-bound/inactive and phosphodiesterase (PDE) remains inactive.
- There is therefore no increased cGMP hydrolysis.
- Guanylyl cyclase actively synthesizes cGMP, convert GTP → cGMP.
- High intracellular cGMP concentration.
- cGMP binds and opens cyclic-nucleotide gated (CNG) channels. Binding keeps the CNG channels open, allowing a steady inward current of Na⁺ (and some Ca²⁺) — this is the dark current.
- This partially depolarizes the photoreceptor membrane. Rods are typically ~ −40 mV in the dark.
- Because the photoreceptor is depolarized, voltage-gated Ca²⁺ channels at the synaptic terminal are active and the cell continuously releases glutamate onto bipolar and horizontal cells.
- Calcium that enters through the CNG channels is forced out by the NCKX (sodium-calcium-potassium exchanger), which exchanges Ca²⁺/K⁺ out for Na⁺ in to maintain calcium homeostasis. But the influx>efflux so intracellular Ca²⁺ remains elevated.
24
Q
phototransduction - light
A
- Light converts 11-cis retinal to all-trans retinal, which activates opsin, forming metarhodopsin II (active rhodopsin).
- Activated rhodopsin causes the G-protein transducin (Tαβγ) to exchange GDP → GTP, activating the Tα-GTP subunit.
- Tα-GTP activates phosphodiesterase (PDE).
- Active PDE rapidly hydrolyzes cGMP → GMP, lowering intracellular cGMP concentration.
- With less cGMP available to bind, CNG (cyclic nucleotide–gated) channels close, stopping Na⁺ and Ca²⁺ influx.
- K⁺ efflux continues through leak channels, but the inward “dark current” stops so the photoreceptor hyperpolarizes to about −65 to −70 mV.
- Because CNG channels are closed, intracellular Ca²⁺ levels drop.
- Voltage-gated Ca²⁺ channels at the synaptic terminal close, so glutamate release decreases.
- Even though less Ca²⁺ is entering, the Na⁺/Ca²⁺–K⁺ exchanger (NCKX) continues pumping Ca²⁺ out, further reducing intracellular Ca²⁺.
- Low Ca²⁺ activates guanylyl cyclase (via GCAPs), which starts regenerating cGMP — an important negative feedback to help the cell recover and adapt to light.
25
distinguish between cones and rods
- cones are less sensitive than rods; they are responsible for vision in bright light and for distinguishing colours, but they don't operate in dim conditions
- rods can detect single photons, but they only operate in low light; in daylight they are 'bleached out' (ie their rhodopsin is broken down so they can't sense light)
26
what happens to rods when lights go dim?
the rods dark adapt, rebuilding their stores of rhodopsin over 30 minutes
27
how are photoreceptors distributed in the retina?
- they are most densely packed in the macula, a central disk, and especially in the fovea (its central pit)
- there are no receptors in the blind spot
28
define the blind spot
the hole where axons carrying visual information exit the eyeball to form the optic nerve
29
define visual acuity
measure of how clearly or sharply you can see (ability to distinguish fine details or small objects).
30
what is the fovea important for?
visual acuity/detailed vision:
- when line of sight is 5° from the fovea's centre, acuity is quartered
- when line of sight is 20° from the fovea's centre, it falls below the standard for legal blindness
31
why do you not normally notice your blind spots?
because objects in one eye's blind spot aren't in the other eye's blind spot, and because the brain fills in the gaps
32
draw and label a graph for the distribution of cones and rods
33
fovea almost exclusively contains
cones
34
more-peripheral retina mainly contains
rods
35
which is more sensitive to light; your peripheral retina or your fovea?
peripheral retina
36
what are the three layers of neurons in the retina? what cells are between these?
back of eye
receptors
(horizontal cells)
bipolars
(amacrine cells)
ganglion cells
37
up to ----- photoreceptors may converge on a single bipolar cell; thus, in each eye, the signal from 126 million receptors is condensed into ---- ganglion cells
45; 1 million
38
where is convergence greatest? where is convergence least?
convergence is greatest in the peripheral retina and least in the fovea (where some receptors project 1:1 to bipolar)
39
what is a receptive/visual field of a neuron?
the region of the retina where light affects the neuron's activity (ie the set of photoreceptors which affect cell)
40
describe bipolar cells
- have centre-surround receptive fields
- these contain a round centre region and a doughnut shaped surround
41
what are two types of bipolar cells?
on-centre cells
off-centre cells
42
on-centre bipolar cells
- excited by light in the centre of their field, and then inhibited by light in the surround
- these cells respond most when a light spot fills their centre and the surround is dark
43
off-centre bipolar cells
- inhibited by light in the centre of their field, and excited by light in the surround
- these cells respond most when a dark spot fills their centre and the surround is light
44
how do horizontal and amacrine cells refine signals?
both use lateral inhibition
horizontal: refine space (contrast, edges)
amacrine: refine time (timing, motion, adaptation)
45
What types of glutamate receptors do ON-center and OFF-center bipolar cells use, and how do they respond to light and dark?
- on-center bipolar cells have metabotropic (mGluR6) glutamate receptors.
- off-center bipolar cells have ionotropic (AMPA) glutamate receptors.
46
describe what happens to on-centre and off-centre cells in the dark
photoreceptors are depolarized and release glutamate continuously: -
glutamate inhibits on-center cells via mGluR6 → they become hyperpolarized (turn off).
- glutamate excites off-center cells via AMPA → they become depolarized (turn on).
47
describe what happens to on-centre and off-centre cells in the light
photoreceptors release less glutamate:
- reduced glutamate removes inhibition from on-center cells → they become depolarized (turn on).
Reduced glutamate removes excitation from off-center cells → they become hyperpolarized (turn off).
48
both types of bipolar cell react to
contrast
49
bipolar cells - uniform lighting
neither type of bipolar cells responds, because the effects of the centre and surround cancel, leaving the cell at its resting level of activity (only weak active)
50
bipolar cells - uneven lighting
respond with graded membrane potentials (do not fire action potentials)
51
bipolar cells synapse onto
retinal ganglion cells
52
how do ganglion cells communicate?
they fire action potentials
53
how are ganglion cells similar to bipolar cells?
- most also have centre-surround receptive fields, and these fields can be on-centre or off-centre
- detect contrast
54
describe how ganglion cells in different parts of the retina differ
- a ganglion cell near the fovea gets input (via bipolars) from only a few photoreceptors, mostly cones
- farther out, each ganglion cell gets input from many receptors, mostly rods
55
describe how the functionality of ganglion cells differs in different parts of the retina
- near the fovea, ganglion cells are less sensitive to light but have better spatial resolution because each one gets input from just a few densely packed cones
- in the periphery, each ganglion cell is very sensitive to light but poor at reporting spatial detail because it blends information from a wide range of receptors
56
how are ganglion cells classified?
based on how their signals are used in the brain
57
3 types of ganglion cells
magnocellular (M cells)
parvocellular (P cells)
melanopsin ganglion cells
58
magnocellular (M cells)
- provide information that is used by the brain to infer the movement of objects
- phasic
- 10% of retinal ganglion cells
59
parvocellular (P cells)
- provide information that is used to infer form and fine detail, such as texture
- 70% of retinal ganglion cells
60
melanopsin ganglion cells
- photoreceptors with their own visual pigment (melanopsin)
- they project to the suprachiasmatic nucleus, a centre for circadian rhythms
- 1% of retinal ganglion cells
61
draw a diagram of how visual fields can be divided into right and left hemifields
62
what two zones does the visual field have?
monocular and binocular zones
63
how does visual information leave the retinas?
- in the optic nerves
- each of the million ganglion cells in each retina sends its axon out the back of the eye through the blind spot
- these million fibres from each eye form its optic nerve
64
optic nerve =
cranial nerve
65
how do visual pathways sort information?
by hemifield
66
describe how optic-nerve fibres are arranged
- when each optic nerve reaches the optic chasm, half of its fibres cross to the other side of the brain
- fibres from the nasal half of each retina cross; those from the temporal retinals do not
67
why do the optic nerve fibres cross?
- in the eye, the right visual hemifield projects onto the left side of each retina
- because the nasal fibres cross, all the information from the right hemifiedl comes together in the left cerebral hemisphere, and vice versa
68
label nasal/temporal side of each retina
69
how does information move after the chiasm?
- nerve bundles emerge from the chiasm are called optic tracts
- they end in the 2 lateral geniculate nuclei (LGN) in the thalamus, which project via the optic radiations to primary visual cortex, V1
70
together, the 2 LGN have ----- nuclei, the same as ----; whereas V1 has -----
2 million; the number of ganglion cells in the 2 retinas; 300 million neurons
71
where is V1 located
in the occipital lobe, concentrated in the calcarine sulcus
72
how are visual areas in the brain organised?
retinotopically
73
what is retinotopic organisation
- neurons close to each other in the brain get information from close-together parts of the retina
- this arrangement is found in the LGN, V1, and many higher visual processing areas
74
does the retinotopic map preserve ares?
no
75
give an example of lack of area preservation in the retinotopic map
- the fovea, which covers only a small area of the retina, projects to large areas in V1 and other cortical regions and LGN
76
why does the fovea data get processed from such a large proportion of visual cells in the brain?
because it has many photoreceptors, bipolar, and ganglion cells, so it carries a lot of information
77
anopia
general term for vision loss
78
scotoma
focal area of lost vision
79
monocular
affecting one eye
80
binocular
affecting both eyes
81
hemianopia
loss of half the visual field
82
quadrantanopia
loss of one-quarter of the visual field
83
homonymous
same side lost in both eyes
84
bitemporal
outer halves of both eyes lost
85
contralateral
lesion is on the opposite side of the lost visual field
86
how does the Hermann grid work?
lateral inhibition in the retina:
- at the intersections of the white grid lines, each retinal ganglion cell’s receptive field receives more overall light from its surround region than cells looking at single white lines.
- the surround inhibits the center response via horizontal cells (lateral inhibition).
- because the inhibition is stronger at intersections (more bright surround), those spots appear dimmer or gray.
- when you look directly at an intersection, it falls on the fovea, where receptive fields are small → no illusion.
PSL300
flashcards
Decks in class (55)
# Cards
-
L140
-
L216
-
L340
-
L431
-
L532
-
L638
-
L733
-
L857
-
L930
-
L1041
-
L1138
-
L1227
-
L1362
-
L1453
-
L1546
-
L1646
-
L1756
-
L1824
-
L1971
-
L2053
-
L2174
-
L2286
-
L2344
-
L2457
-
L2563
-
L2644
-
L2748
-
L2832
-
L2926
-
L3042
-
L3127
-
L3248
-
L3375
-
638
-
733
-
857
-
930
-
1041
-
1138
-
1228
-
1453
-
1546
-
1824
-
1971
-
2053
-
2174
-
2286
-
2563
-
2644
-
2748
-
270
-
2832
-
3127
-
3248
-
3375
