Lecture 11 Flashcards
Sight Signalling
How does light reach photoreceptor cells in the vertebrate eye?
Light passes through the neural layer and cell bodies of rods and cones, acting as a signal in the outer segment of the retina.
What is the primary function of rod cells?
Rod cells are responsible for non-color vision at low light intensity.
What is the function of cone cells?
Cone cells detect color and function at higher light intensities.
What is rhodopsin?
Rhodopsin is a specialized GPCR composed of opsin (a GPCR protein) and 11-cis-retinal (a light-absorbing chromophore).
What happens to retinal when it absorbs light?
It undergoes cis-trans isomerization, triggering a conformational change in rhodopsin.
What is metarhodopsin II?
The activated form of rhodopsin after retinal isomerization.
What happens when metarhodopsin II is activated?
It stimulates nucleotide exchange on transducin (a G-protein).
What are the key G-proteins and their functions?
Gs: Stimulates adenylate cyclase (e.g., glucagon, epinephrine). Gi: Inhibits adenylate cyclase (e.g., adenosine, prostaglandins). Gt (Transducin): Stimulates cGMP phosphodiesterase (activated by photons). Gq: Stimulates phospholipase C. G12: Activates ion channels (e.g., Na+/H+ exchange).
What is the role of cGMP in sight signaling?
cGMP keeps ion channels open; its breakdown causes channel closure and changes in membrane potential.
How is rhodopsin inactivated under high light intensity?
Through phosphorylation by rhodopsin kinase, reducing transducin activation.
Arrestin binds to phosphorylated rhodopsin, stopping transducin activation.
Why does temporary blindness occur in bright-to-dark transitions?
It takes 20-30 minutes to reset rod cells due to signal termination mechanisms.
What are the three types of cone cells and their peak absorbance wavelengths?
Blue: 414-426 nm
Green: 530-532 nm
Red (Yellow-Green): 560-563 nm
How do cone cells achieve color tuning?
Amino acid differences in opsins alter the electronic environment around 11-cis-retinal.
How do birds and mantis shrimp differ in color vision?
Birds: Tetrachromatic, some UV-sensitive.
Mantis Shrimp: 12+ color receptors and polarization sensitivity.
Why is cephalopod color vision puzzling?
They lack cone cells but can change colors.
How do cephalopods perceive color?
By processing chromatic aberration (light diffraction) using their large optic lobes.
What are the three types of color blindness?
Protanopia: Missing red (L-cone).
Deuteranopia: Missing green (M-cone).
Tritanopia: Missing blue (S-cone).
Why does color blindness persist in human evolution?
Dichromats excel at detecting camouflaged objects, offering a survival advantage.
How did John Dalton contribute to color blindness research?
He hypothesized that a blue pigment in his eye caused his condition. Genetic testing later confirmed he lacked the green opsin gene.
How does sildenafil (Viagra) affect vision?
It inhibits PDE-6, disrupting blue-green color discrimination.
Why does the FAA advise pilots to avoid sildenafil before flying?
It impairs night vision and recognition of cockpit/runway lights.
