36 Questions
What is the primary function of the G protein transducin in the phototransduction cascade?
To activate a phosphodiesterase enzyme
What is the role of cGMP in the phototransduction process?
cGMP keeps sodium channels open in the dark
What is the primary function of the Na/K ATPase pump in the phototransduction process?
To pump sodium out of the inner segment and potassium into the inner segment
What is the primary structural component of the rhodopsin molecule?
All of the above
What is the initial event that triggers the phototransduction cascade?
The dissociation of rhodopsin from 11-cis-retinal
How does the closure of sodium channels in the outer segment membrane affect the photoreceptor cell's electrical state?
It causes the photoreceptor cell to hyperpolarize
What triggers phototransduction in photoreceptors?
Absorption of light by rhodopsin
What is responsible for the light-induced changes in the electrical activity of photoreceptors?
cGMP-gated channels
What maintains a slight negative electrical charge in photoreceptors in the dark?
Active transport of cations like sodium and calcium
What carries the message from activated ganglion cells to the brain?
Axon of the ganglion cell
Which process involves the conversion of cyclic GMP to GMP?
Phototransduction cascade
What initiates an electrical current flow through the retina in phototransduction?
Cell hyperpolarization
Which of the following is the key event during the phototransduction process that leads to the hyperpolarization of the photoreceptor cell membrane?
Closure of sodium channels
What is the role of the enzyme RPE65 in the visual cycle?
It oxidizes all-trans retinol to 11-cis retinal
How does the binding of light to rhodopsin in the photoreceptor cell lead to the activation of the G protein transducin?
The activated rhodopsin acts as a guanine nucleotide exchange factor (GEF) to promote the exchange of GDP for GTP on transducin
What is the role of cGMP in the phototransduction process?
cGMP levels decrease, leading to the opening of sodium channels
What is the primary function of the enzyme LRAT (Lecithin retinol acyltransferase) in the visual cycle?
It esterifies all-trans retinol to all-trans retinyl esters
How does the structure of rhodopsin contribute to its role in phototransduction?
All of the above
What is the purpose of the rapid decrease in cGMP levels during phototransduction?
To close the sodium channels and hyperpolarize the photoreceptor cell
How do horizontal cells contribute to the center-surround organization of receptive fields in the retina?
All of the above
What is the role of the enzyme 11-cis retinol dehydrogenase (11cRDH) in the visual cycle?
It oxidizes 11-cis retinol to 11-cis retinal
What is the primary function of the RPE cells in the metabolism of vitamin A?
RPE cells store vitamin A for use in the rod disc renewal system.
How do RPE cells contribute to the formation of the interphotoreceptor matrix (IPM)?
All of the above
What is the primary role of the phagocytosis of photoreceptor outer segment discs by RPE cells?
To remove undigested material that accumulates as lipofuscin in RPE cells.
What is the primary consequence of the accumulation of the substance A2E in the lipofuscin of RPE cells?
It contributes to the death and dysfunction of the RPE cells.
What is the primary purpose of the rapid shedding and phagocytosis of photoreceptor outer segment discs by RPE cells?
To facilitate the renewal and regeneration of the photoreceptor outer segments.
Which of the following is the primary role of glucose transporters located on the apical and basal membranes of the retinal pigment epithelium (RPE) and the endothelium of retinal capillaries?
To regulate the concentration of glucose in the retinal interstitial space
Which ion movement is primarily responsible for maintaining the slight negative electrical charge in photoreceptors in the dark?
Efflux of potassium ions
What is the primary function of the enzyme lecithin retinol acyltransferase (LRAT) in the visual cycle?
To esterify all-trans-retinol to all-trans-retinyl esters
Which process is responsible for the phagocytosis and degradation of photoreceptor outer segment discs in the retinal pigment epithelium (RPE)?
Lysosomal digestion
What is the primary role of anaerobic glycolysis in the retinal metabolism?
To generate ATP for the photoreceptor cells
What is the specific carrier protein involved in the transport of all-trans retinol from photoreceptors to the RPE?
ABCR
Which enzyme in the visual cycle is responsible for the conversion of all-trans retinol to 11-cis retinol?
atRDH
In the phagocytosis of photoreceptor discs, which protein plays a crucial role in the oxidation of all-trans retinol back to 11-cis retinal?
RPE65
What is the primary function of CRALBP in the metabolism of vitamin A within the eye?
Transporting all-trans retinol to RPE
Which protein is involved in the spatial summation and modulation of photoreceptor cells' responses through inhibitory feedback?
Horizontal Cells
Study Notes
Phototransduction
- Light absorption in photoreceptors triggers transformation of 11-cis retinal to all-trans retinal, which is moved into the cytoplasm and reduced to all-trans retinol
- All-trans retinol is transported to the RPE by specific carrier proteins within the interphotoreceptor matrix (IPM)
- RPE contains enzymes that convert all-trans retinol to 11-cis retinol, which is then oxidized back to 11-cis retinal and transported back to photoreceptors for incorporation into disc outer segments
Cycling of cGMP
- cGMP keeps Na+ channels open in the outer segment, allowing depolarization
- Light stimulation triggers a cascade that leads to a decrease in cGMP concentration, closing Na+ channels and hyperpolarizing the cell membrane
- When cGMP levels are restored, ion channels open, and the cell becomes depolarized again, releasing glutamate
Information Processing
- Photoreceptors maintain a slight negative electrical charge of about -40 mV in the dark due to active transport of cations such as sodium and calcium
- Light stimulation triggers a series of biochemical changes, leading to hyperpolarization of the cell membrane and a decrease in glutamate release
- Retinal neurons are designed as ON and OFF cells, responding differently to light conditions
- Receptive fields are arranged in a center-surround pattern, with light activation in the center and antagonistic response in the surround
Retinal Cells
- Bipolar cells have center-surround receptive fields and respond with graded potentials
- Ganglion cells have center-surround receptive fields and respond with action potentials
- Horizontal cells receive input from many photoreceptors, contribute to spatial summation, and impact surround responses of bipolar cells
- Amacrine cells have center-surround receptive fields and respond with action potentials, with AII amacrine cells playing a key role in the rod pathway
RPE Cells
- RPE cells phagocytose fragments from the continual shedding of photoreceptor outer segment discs
- RPE cells metabolize and store vitamin A, contributing to the biochemical process in the rod disc renewal system
- RPE cells contribute to the formation of the interphotoreceptor matrix (IPM) between the RPE layer and photoreceptors
Retinal Metabolism
- Glucose is the primary source of energy for retinal tissue, entering the tissue via facilitated diffusion
- Glucose metabolism switches from glycolysis to oxidative metabolism in retinal tissue
- High oxygen utilization in photoreceptors requires high blood flow in the choriocapillaris
Learn about the phototransduction process in photoreceptors where the Na/K ATPase pump on the inner segment plasma membrane plays a crucial role. Explore how the dark current influences the flow of sodium and other cations in and out of the cell, as well as the mechanisms behind depolarization and glutamate release.
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