Exam 3 Week 14 ppt 1&2 Retinal structure and function Flashcards
purpose of photoreceptors of the retina
convert photonic energy within the electromagnetic spectrum with wavelengths between 400 to 700 nm to electrical signals
What is generated from the signals of the retina sent to the brain?
a complex 3 dimensional world from a 2 dimensional input on the retina
One of three critical systems for balance
vision
Retina has how many major classes of cells?
7
Retina has how many histologically distinct layers
10
Two structures that make up the deepest cellular layer of retina
Rods and cones
Rods are photopic or scotopic?
scotopic
What does scotopic mean?
sensitive to low light levels but insensitive to color
Cones are photopic or scotopic?
photopic
What does photopic mean?
works at higher light levels but come in 3 forms with each sensitive to different color spectrum
How many rods and cones does each eye have?
80-110 million rods and 4-5 million cones
How are photoreceptors of rods and cones similar? (4)
- photosensitive outer segment with the photosensitive pigments
- Inner cytoplasmic segement (rich in mitochondria)
- Nuclear region
- Specialized presynaptic endings with invaginations which interfold with bipolar and horizontal cells
How are photoreceptors in rods different? (3)
- Not in the fovea/macula lutea
- Highest concentrations just lateral to fovea
- High concentrations maintained in periphery
How are photoreceptors in cones different? (2)
- Most concentrated in the fovea/macula lutea
2. Very low density elsewhere
Which area has the highest density of photoreceptors?
fovea/macula lutea
Significance of highest density of photoreceptors in the fovea/macula lutea
this is the area with the highest visual acuity
Significance of the lack of rods in the fovea and high densities in the retinal periphery
the periphery of the field of vision has a higher low light sensitivity
Cell types in fovea
direct cone to bipolar to ganglion cell
Cells in peripheral retina
additional cells to allow interaction between photoreceptor and ganglion cells
Bipolar cells- 3 points to note
- Analogous to primary afferents, receive input from photoreceptors
- Specific to types of photoreceptors
- in ON and OFF varieties
Horizontal cells- 3 points to note
- Laterally interconnecting interneurons which integrate and regulate the input from multiple photoreceptor cells
- Produce an integrated “lateral inhibition” on photoreceptor cells
- Enhance contrast
Amacrine cells- 3 points to note
- Laterally interconnecting interneurons which integrate and regulate the input from multiple bipolar cells
- Produce an integrated “lateral inhibition” on both bipolar and ganglion cells
- Enhance contrast
Ganglion cells- 3 points to note
- Analogous to 2° neurons of relay nuclei of other sensory systems
- Most superficial layer of neurons whose axons exit retina as the optic nerve
- > 100 million retinal receptor cells but only 1 million ganglion cells
Where is the greatest and least amount of convergence of photoreceptor cells onto Ganglion cells
greatest= peripheral rods least= foveal cones
two types of ganglion cells
M (magnocellular) type and P (parvocellular) type
P (parvocellular) type cells- 3 points to note
- Small receptive fields
- Specific to detail
- Sensitive to color and visual acuity (form)
(these are one of two types of ganglion cells)
M (magnocellular) type cells- 4 points to note
- Large receptive fields
- Not sensitive to color
- Very sensitive to low illumination
- Sensitive to gross patterns (location) and movement
(these are one of two types of ganglion cells)
How do Photoreceptors work?
- Membranes depolarized so release glutamate in dark
- Light reacting with membrane bound opsins hyperpolarizes the membrane so decreases glutamate release
How do bipolar cells work?
- Glutamate both depolarizes and hyperpolarizes bipolar cells depending upon type of glutamate receptor
- Produces OFF and ON bipolar cells
- OFF cells inactive in light (OFF is active in the dark)
- ON cells active in light (ON is active in the light)
Which cells respond in an annular way to light on the retina?
Bipolar and Ganglion cells
What happens to inner and peripheral region when cells respond in an annular way to light?
Inner “central” region responds one way and peripheral “surround” responds another.
What causes inner and peripheral regions to respond differently?
Due to action of horizontal and amacrine cells
Which cells have an “On” center - “Off” surround
Bipolar and Ganglion cells
“On” center and “Off” surround
When light hits on the center of annulus on surface of retina then the bipolar or ganglion __________________
cell fires, but is then quiet when light off
“On” center and “Off” surround
When Light on surround of annulus then bipolar or ganglion________
cell is inactive until light turned off on that outer region, when the cell then becomes active
“On” center and “Off” surround
When Light shines on both center and surround then_____
there is some activity in the cell but less activity than when light is only on the center
“Off” center and “On” surround
When Light shown on center of annulus on surface of retina then__________
ganglion cell is quiet then active when light goes off
“Off” center and “On” surround
When Light on surround of annulus then_________
cell active until light turned off then quiet
“Off” center and “On” surround
When Light shines on both center and surround then______
there is some but less activity than when light is only on the outer annulus
“On” center - “Off” surround
When light hits on the center of annulus on surface of retina then the bipolar or ganglion __________________
cell fires, but is then quiet when light off
“On” center - “Off” surround
When Light on surround of annulus then bipolar or ganglion________
cell is inactive until light turned off on that outer region, when the cell then becomes active
“On” center - “Off” surround
When Light shines on both center and surround then_____
there is some activity in the cell but less activity than when light is only on the center
“Off” center - “On” surround
When Light shown on center of annulus on surface of retina then__________
ganglion cell is quiet then active when light goes off
“Off” center - “On” surround
When Light on surround of annulus then_________
cell active until light turned off then quiet
“Off” center - “On” surround
When Light shines on both center and surround then______
there is some but less activity than when light is only on the outer annulus
In what way do Bipolar and Ganglion cells provide enhanced contrast?
- With sharp delineations between active and not active at borders of light and dark
- This particularly true in peripheral vision
- In macular degeneration see contrasts but not detail
What is the blind spot?
- Circular, elevated region where ganglion cell axons gather to leave the eye as the optic nerve
- Devoid of rods and cones, represents the blind spot in the visual field
Where does neural retina receive its blood supply? (2)
- Outer retinal layers via choroidal capillaries
- Inner retinal layers via central retinal artery, a
- Branch of the opthalmic artery from the internal carotid
What would result from blockage of central retinal artery?
Sudden blindness
Age-related macular degeneration (AMD)- 4 points to know
- Increases in prevalence in sixth decade
- Leading cause of blindness in the elderly
- Loss of central vision and acuity
- Preserved peripheral vision and ability to determine contrasts
Two types of Age-related macular degeneration (AMD)
- Dry
2. Wet
Most common type of Age-related macular degeneration
Dry
Which type of AMD is caused by detachment of retinal pigmented epithelium due to hypervascularization?
Wet
Which type of AMD is caused due to atrophy of retinal pigmented epithelium (RPE)?
Dry
Optic nerve- 4 points to know
- Arises from ganglion cells of the ipsilateral eye, is part of the CNS
- Myelinated by oligodendrocytes
- The optic nerve is ensheathed by pia, arachnoid and dura, and thus has subarachnoid space with CSF.
- Increased intracranial pressure causes bulging of the optic disc called papilledema
from where do the outer retinal layers receive blood supply?
via choroidal capillaries
from where do the inner retinal layers receive blood supply?
via the central retinal artery, which is a branch of the opthalmic artery, which is from the internal carotid artery
Retinal blood supply: what artery does the opthalmic artery supply, and where does it come from?
the opthalmic artery supplies the central retinal artery (which supplies the inner retinal layers)
The opthalmic artery comes from the internal carotid artery
retinal blood supply: what artery does the internal carotid artery supply?
the opthalmic artery (which supplies the central retinal artery which supplies the inner retinal layers)
what is papilledema?
bulging of the optic disc because of increased intracranial pressure
retinal blood supply: what do choroidal capillaries do?
it supplies the outer retinal layers
