M3 L5 Flashcards
What is an important part of the retina and what does it do?
The retina contains photoreceptors that convert light energy into neural activity
whats signal transduction
the process by which a cell converts an external signal into a functional response (electrical activity)
What Are Transducer Cells
Transducer cells are specialized sensory cells that convert external stimuli (such as light, sound, pressure, or chemicals) into electrical signals that the nervous system can process.
What is inside the retina?
neuronal cells
Where are photoreceptors found?
in macula
What is located inside the macula?
the fovea is a specialized region located within the macula
What is the fovea and what does it do?
A small, central pit within the macula that contains the highest concentration of cone photoreceptors, making it the point of sharpest vision.
- plays a big role in visual acuity
What are photoreceptors
Photoreceptors are specialized sensory cells in the retina that detect light and convert it into electrical signals, which are then processed by the brain to create vision. They are the primary transducer cells of the visual system.
What does the retina do?
captures visual information and converts it into electrical signals for the brain to interpret.
what are ganglion cells in the visual system
* where synapse/project?
Ganglion cells are the only output neurons of the retina in the visual system. They collect and process visual information from bipolar and amacrine cells, then send it to the brain via the optic nerve.
- project to the lateral geniculate nucleus (LGN) of the thalamus
Which cells are sensitive to light?
Photoreceptors (Rods & Cones)
* Rods detect dim light and are used for night vision.
* Cones detect color and fine detail in bright light.
What are Bipolar cells?
* function?
* types?
Interneurons in the retina
Function: Carry signals from photoreceptors (rods & cones) to ganglion cells.
Types:
* ON bipolar cells – Activate in response to light.
* OFF bipolar cells – Activate in response to darkness.
What are Horizontal Cells?
* function
* how work
Interneurons in the retina
- Function: Modify signals between photoreceptors and bipolar cells to enhance contrast and sharpen edges.
- How? Lateral inhibition—helps us see clear borders between light and dark areas.
what are Amacrine Cells?
* function?
Interneurons in the retina
Function: Regulate signals between bipolar and ganglion cells to refine motion detection, brightness adaptation, and temporal processing.
Laminar Organization of the Retina:
What is the function of the ganglion cell layer?
Contains ganglion cells, which send visual signals to the brain via the optic nerve.
Laminar Organization of the Retina:
What happens in the inner plexiform layer?
It contains synapses between bipolar, amacrine, and ganglion cells, helping refine motion, contrast, and brightness detectio
Laminar Organization of the Retina:
What does the inner nuclear layer?
contains the cell bodies of bipolar, horizontal, and amacrine cells
Where do photoreceptors synapse with bipolar and horizontal cells?
Outer plexiform layer.
What is found in the outer nuclear layer?
The cell bodies of photoreceptors (rods and cones).
Which layer contains the light-sensitive parts of rods and cones?
Layer of photoreceptor outer segments
What is the function of the pigmented epithelium?
Absorbs excess light, prevents scattering, and provides nutrients to photoreceptors
Why are the outer segments of rods and cones embedded in the pigmented epithelium?
to facilitate essential functions like nutrient supply, waste removal, and the regeneration of visual pigments (such as regenerating rhodopsin and cone opsins for continuous light detection)
Which cells does light hit first?
Light hits the ganglion receptors first then bipolar cells then photoreceptors.
Light must pass through several layers before reaching the photoreceptors (rods & cones), where it is detected.
What is the inside out arrangement?
The signal is transmitted backward through the retina:
Photoreceptors → Bipolar cells → Ganglion cells (where they send the final processed signal to the brain via the optic nerve)
The inside-out arrangement of the retina refers to the fact that light-sensitive photoreceptors (rods and cones) are at the back of the retina, while neuronal processing cells (bipolar, amacrine, and ganglion cells) are in front. This means that light must pass through several layers of cells before reaching the photoreceptors, where visual information is first detected and processed.
Advantages of the Inside Out Arrangement in the Retina
- Epithelial cells are important
– To regenerate pigments
– Absorb light to minimize scattering
Discuss the 3 structures in a photoreceptor:
- Synaptic Terminal – Connects photoreceptors to bipolar cells, allowing signal transmission
- Inner Segment – Contains organelles like mitochondria for energy production
- Outer Segment – Contains light-sensitive pigments within membranous disks, where phototransduction occurs
How many rods and cones are in the retina?
~92 million rods and ~5 million cones.
What is the main function of rods?
Low-light (scotopic) vision, detecting black & white.
What is the main function of cones?
Color and daylight (photopic) vision, detecting red, green, and blue light.
What type of proteins absorb visible light in photoreceptors? and explain what they do
Opsins (rhodopsin in rods, cone opsins in cones).
Opsins determine the wavelength of light absorbed, allowing color vision.
What is 11-cis retinal, and why is it important?
11-cis retinal is a chromophore (prosthetic group) that absorbs light and is essential for initiating vision.
- needs to be in a certain conformation to do this
What happens when 11-cis retinal absorbs light?
It undergoes photoisomerization, changing shape to all-trans retinal, which activates opsin.
This structural change activates opsin, triggering a biochemical cascade that leads to phototransduction (conversion of light into an electrical signal).
What is rhodopsin?
A 7-transmembrane protein (pigment) in rods, consisting of opsin and 11-cis retinal, responsible for light absorption.
- sensitive to light but not color
What is the natural absorption peak of amino acids? How does opsin affect retinal’s absorption?
Around 360 nm (ultraviolet range).
It shifts absorption 200 nm toward longer wavelengths, allowing detection of visible light.
Why is the absorption shift important for vision?
This is important because most vision occurs between 400 and 700 nm (blue to red). The shift allows rhodopsin to detect visible light instead of ultraviolet, making vision useful in normal lighting conditions.
How many visual pigments are found in human photoreceptors?
Four – rhodopsin (rods), and three cone opsins (blue, green, red).
Rods Vs Cones: Rods
- type of photopigment?
- sensitivity?
- where located?
- when useful?
- effort to activate?
- all contain same photopigment (rhodopsin)
- not sensitive to color (more for night vision)
- scattered in the periphery of the retina
- useful for night/dim
- easy to activate/fire bc highly sensitive to light (do not need much light to be triggered).
Rods Vs Cones: Cones
- type of photopigment?
- sensitivity?
- where located?
- when useful?
- effort to activate?
- 3 diff types w 3 diff photopigments
- sensitive to diff wavelengths (responsible for color vision)
- mostly located in the fovea
- useful for daylight
- low sensitivity to light, need more stimulation (brighter light) to respond
Why is the thinning of the retina in the fovea important?
The fovea is a thinning of the retina, meaning that the layers of retinal cells (like the ganglion cells, bipolar cells, and others) are displaced to the side in this area.
It allows light to pass directly to the photoreceptors (mainly cones), minimizing scattering and producing sharper, clearer images.
Whats the resting membrane potential in rods? What is it due to?
about -30 mV. This is the baseline voltage across the rod’s membrane when it is not exposed to light (i.e., in the dark).
Due to Na+ ions continuously entering the rod through cation channels in the outer segment of the photoreceptor. This influx of Na+ ions causes a depolarization, resulting in -30 mV.
what is the Dark Current
the flow of positively charged ions (mainly Na+) into the rod cell in the dark
dark current keeps the rod depolarized and is driven by the sodium-potassium pump, which actively pumps out Na+ to maintain the inward flow. This process is essential for maintaining the rod in its “on” state (ready to respond to light).
Why are rods good at night?
bc at night there isn’t that much light and rods dont need much light to respond so they can take over
What is the role of cGMP in rods?
level of cGMP affects Na+ channel being open or not
high levels of cGMP = cGMP-gated sodium channels open in the rods = Na+ ions flow into the cell = depolarization
low levels of cGMP= the sodium channels close = Na+ ions stop entering the rod cell = hyper polarization
What happens when rods are depolarized?
the rod is constantly active and continuously releasing neurotransmitters (like glutamate) onto bipolar cells.
Bipolar cells are inhibited by this continuous glutamate release, which prevents them from sending signals to ganglion cells.mAs a result, no visual signal is sent to the brain about the surrounding environment because the signal is being blocked at the bipolar cell level.
What happens when rods are hyperpolarized?
When rods are hyperpolarized, it means that the rod cells in your eyes are responding to light.
There is decreased glutamate release from rods to the bipolar cells which means the bipolar cells are now active. The rods then begin to hyperpolarize which provides the brain with an indication that light levels are increasing and that the system should shift from relying on rods for dim-light vision to relying on cones for bright-light vision.
explain this graph
It starts off as dark and sustained depolarization, then you shine light and the photoreceptor hyperpoarizes and we sift to -60mv due to a lack of Na+ bc the cGMP channel closes
In the dark, photoreceptors =
in the light, photoreceptors =
In the dark, photoreceptors = depolarized
In the light, photoreceptors = hyperpolarized
Retinal Active vs. Inactive: *discuss form
Inactive Retinal (in the dark): It is in the cis form, tightly bound to opsin (in rhodopsin), and unable to activate the phototransduction cascade.
Active Retinal (when exposed to light): It undergoes a cis-to-trans isomerization, changing to the trans form, which activates rhodopsin and starts the phototransduction process.
whats bleaching
bleaching refers to the loss of color or change in the structure of visual pigments (like rhodopsin) when they absorb light.
Phototransduction Cascade in Rods:
What happens in the first step when light shines on the pigment in rods?
Leads to the second step where light causes retinal to change conformation, activating rhodopsin.
What happens after rhodopsin is activated in step 2?
Activated rhodopsin binds to and activates the G-protein transducin (step 3).
What happens once the G-protein transducin activates (step 3)?
Leads to the alpha subunit of the G-protein dissociating (step 4).
What occurs when the 𝛼-subunit of transducin dissociates(step 4)?
The 𝛼-subunit of transducin activates phosphodiesterase (PDE) which will breakdown cGMP to GMP, reducing the concentration of cGMP in the rod cell (step 5)
What is the result of decreased cGMP in the rod cell?
The Na+ channels close stopping the influx of sodium ions into the cell (step 6)
The rod cell hyperpolarizes (becomes more negative), shifting its membrane potential from -40 mV (depolarized) to about -60 mV (hyperpolarized). step 7
Outer segment of rods:
* what type of channel does it contain?
* higher affinity to what ion
* purpose of channel
Contains cGMP-gated channels that allow Na+ and Ca2+ to enter.
These channels are more permeable to Ca2+ than Na+ (but Na+ has a higher concentration gradient so it diffuses faster)
The Na+ influx is key to keeping the cell depolarized in the dark.
Inner segment of rods
* channel type + purpose
* purpose of K channel
* how do Ih channels move
Contains Na+/K+ ATPase, which actively pumps Na+ out and K+ in to maintain ionic gradients.
K+ selective channels allow K+ to leak out, helping maintain the resting potential.
Ih channels move ions by diffusion.
Synaptic terminal of rods
* channel type + purpose
Has voltage-gated Ca2+ channels, which control neurotransmitter (glutamate) release to bipolar cells.
How does light affect the membrane potential of a photoreceptor?
Light closes Na+/Ca2+ channels, reducing inward current and causing hyperpolarization.
Small amount of light → mild hyperpolarization.
More light → deeper hyperpolarization.
Stronger hyperpolarization means a greater reduction in glutamate release.
Ih channels when photoreceptor is:
* depolarized
* hyperpolarized
depolarized: Ih channels are permeable to both sodium
and potassium equally
hyperpolarized: driving force for sodium exceeds the driving force of potassium. This makes the sodium move inward slowly. The cell gains positive gradually and the membrane starts to repolarize
Big idea of this graph?
The brighter the light, the higher the hyperpolarization (light intensity correlates with hyperpolarization)
More light closes cGMP-gated Na+/Ca2+ channels, reducing inward current, leading to hyperpolarization.
This results in a more negative membrane potential (up to ~ -60 mV).
What is saturation in photoreceptors and how is it prevented?
Saturation occurs when a photoreceptor is hyperpolarized to the point where further increases in light do not cause additional changes in membrane potential. Essentially, the cell cannot respond to brighter light because it has reached its limit.
Ih channels prevent prolonged saturation bc they open when the cell becomes too hyperpolarized, allowing Na+ and K+ to flow in. This counteracts extreme hyperpolarization and brings the membrane potential back toward -35 mV, preventing it from getting “stuck” in an inactive state.
What happens to calcium in light?
The concentration drops bc the cGMP channels close (due to light activating PDE) so there is less Na+ and Ca2+ coming in.
As Ca2+ levels drop (due to channel closure), it increases guanylyl cyclase activity, producing more cGMP, helping reopen some cGMP-gated channels. This reduces photoreceptor sensitivity, ensuring the cell continues to detect relative changes in light intensity, preventing saturation.
Explain how Calcium Levels Regulate the cGMP Channel Activities
- high levels
- low levels
High calcium levels (dark):
* high Ca2+ activates GCAP which inhibits guanylyl cylclase - leading to reduced cGMP production
* cGMP-gated channels remain open due to existing cGMP
Low calcium levels (light):
* low Ca2+ inhibits GCAPs → meaning guanylyl cyclase is not blocked and can converting GTP into cGMP → more cGMP is produced
* Helps reopen cGMP-gated channels after initial closure
Purpose of epithelium cells in regeneration?
Epithelial cells (RPE cells) are responsible for converting 11-trans retinol → 11-cis retinal
What is the bleaching step?
Light Exposure (Bleaching Step)
* 11-cis retinal (active form) is bound to opsin in photoreceptors.
* Light converts 11-cis retinal → 11-trans retinal, causing it to detach from opsin (this is photobleaching).
What happens to retinal in dark conditions?
Retinal is in the 11-cis form, which is attached to opsin (part of the photopigment rhodopsin).
What happens to 11-cis retinal when light hits the photoreceptor?
It absorbs light and is converted to 11-trans retinal (photoisomerization).
This change activates opsin, leading to the phototransduction cascade.
Why does 11-trans retinal need to be recycled?
11-trans retinal cannot detect more light, so it must be converted back to 11-cis retinal to restore sensitivity.
What does retinol dehydrogenase do when 11-trans retinal needs to be recycled?
Retinol dehydrogenase converts 11-trans retinal → 11-trans retinol, a storage form that can be transported.
How is 11-trans retinol transported between photoreceptors and epithelial cells?
Interphotoreceptor Retinol Binding Protein (IRBP) shuttles it through the extracellular space.
What happens to 11-trans retinol in the epithelial cells?
It is converted into 11-cis retinol.
Then, 11-cis retinol is converted into 11-cis retinal, the active form.
How does 11-cis retinal return to photoreceptors?
It is transported back to the outer segment via IRBP, where it reattaches to opsin, restoring the photoreceptor’s ability to detect light.
What does vision in the day depend on?
only on cones whose photopigments require more energy to become bleached
What are the 3 pigments cones contain and what wavelength are they activated at?
– Short (blue) maximally activated at 430 nm
– Medium (green) maximally activated at 530 nm
– Long (red) maximally activated at 560 nm
What is the receptive field?
The area on the retina that trigger the firing of certain ganglion cells
- Ganglion cells respond best when bright light is shinning on the retina
what determines color perception
the ratio of cone activation
What are the two types of bipolar cells
– Bipolar cells with ionotropic receptors (OFF)
– Bipolar cells with metabotropic receptors (ON)
Bipolar cells are intermediate neurons in the retina that transmit signals from photoreceptors (rods and cones) to ganglion cells. They are classified into ON and OFF types based on how they respond to light.
What happens if the photoreceptor is in the dark?
Depolarized → Continuously releasing glutamate (Glu)
The Glu will have diff effects to ON and OFF cells
ON Bipolar cells in the dark
* receptor type
* status in dark
* Glu effect 1 and 2
* effect on ON ganglion cell?
* AP fire or no?
- metabotropic
- hyperpolarized (less active)
- Glu inhibits the ON bipolar cells bc metabotropic is inhibited - causes ON bipolar cells to become (1) hyperpolarized and then they (2) release less glutamate.
- lack of stimulation prevents ON ganglion cells from being excited - no fire AP
OFF Bipolar cells in the dark
* receptor type
* status in dark
* Glu effect 1 and 2
* effect on OFF ganglion cell?
* AP fire or no?
- inotropic (AMPA)
- depolarized
- glutamate excites OFF bipolar cells bc ionotropic receptors are excited, causing them to (1) depolarize and (2) release more glutamate to OFF ganglion cells.
- OFF ganglion cells depolarize and send AP
What happens if the photoreceptor is in the light?
Hyperpolarized → releasing less glutamate (Glu)
ON Bipolar cells in the light
* receptor type
* status in dark
* Glu effect 1 and 2
* effect on ON ganglion cell?
* AP fire or no?
- metabotropic receptors
- depolarized
- metabotropic receptors get inhibited by Glu BUT in light there’s less Glu released by photoreceptor so inhibition is lifted → ON bipolar cells (1) depolarize and (2) releases more glutamate onto ON ganglion cells
- ON ganglion cells get excited and send AP
OFF Bipolar cells in the light
* receptor type
* status in dark
* Glu effect 1 and 2
* effect on ON ganglion cell?
* AP fire or no?
- ionotropic
- hyperpolarized
- ionotropic receptors get excited by Glu BUT less Glu in light means less excitation → OFF bipolar cells are (1) hyperpolarized (not excited) which means they (2) release less glutamate onto OFF ganglion cells.
- OFF ganglion cells remain inactive - no AP
Where do photoreceptors synapse?
on bipolar cells directly and via horizontal cells indirectly
What are horizontal cells?
* what receive and what release?
retinal interneurons that help modify the signal from photoreceptors (PRs).
receive glutamate (Glu) from PRs and release GABA, which is inhibitory (more hyperpolarization so more bipolar cell excitation)
How are horizontal cells amplifiers for sending an AP?
in the dark the photoreceptors are depolarized and releasing more glu THEN the horizontal cells receive this Glu which excites them so this makes them release GABA onto nearby PR which in this case is the light PR. This GABA inhibits the PR to release even less glu (it was hyperpolarized and not sending much glu to start) which means the metabotropic receptor doesn’t get activated by Glu so she can’t block the ON bipolar cells which means the ON bipolar cells depolarize and releases glu which then excites the ON ganglion cell to send an AP
the receptive field of a neuron (typically in the retina or visual cortex) consists of two regions:
Center – This is the central part of the receptive field, where light stimulation has the strongest effect on the neuron’s firing rate.
Surround – This is the outer area surrounding the center, which has an opposite effect compared to the center when stimulated.
- One bipolar cell is receiving direct input from center photoreceptor cells and indirect input from surround photoreceptor cells
How do bipolar cells respond to the 2 diff receptive fields
🔹 The direct pathway depolarizes the ON-center bipolar cell.
🔹 The indirect pathway hyperpolarizes the ON-center bipolar cell.
🔹 The response in the center (direct) is opposite to the response in the surround (indirect) due to horizontal cell interactions.
What are the 3 types of ganglion cells?
1) P (parvo) type
2) M (magno) type
3) non P non M cells
Ganglion Cells: Parvo Type
* percentage?
* response to stimulation
* sensitivity
- make up 90% of ganglion cells
- continue firing action potentials as long as the stimulus remains.
- sensitive to differences in wavelengths of light (respond to specific color contrasts rather than just brightness)
Ganglion Cells: Magno Type
* percentage?
* receptive field size
* AP fire speed
* response to stimulation
* sensitivity
- make up 5%
- larger receptive fields than P-cells, meaning they receive input from a bigger area of the retina.
- M-cells fire more rapid action potentials than P-cells
- M-cells fire a brief burst of action potentials when stimulated and then stop.
- M-cells detect things in low contrast conditions (e.g., shadows, dim lighting).
Ganglion cells: Non P non M
* sensitivity
Sensitive to differences in the wavelengths of light, meaning they are involved in color processing (like P-cells) but in a different way.
