Phototransduction and Spectral Sensitivity (M1) Flashcards
What is the net movement of charges (ions) called?
current (I)
What is the difference in electrical charge across space called?
voltage (V)
Relative to outside of the membrane, is the inside of a neuron more negative or positive? 1. What is the voltage difference numerically? 2
- negative
2. -60 to -70 mV
A Na+/K+ ATPase pump moves what into the cell (and how much)? 1. What does it move out of the cell (and how much)? 2
- K+ (2)
2. Na+ (3)
What are the equations that govern the resting ionic gradient across a membrane?
- Nernst
2. Goldman-Hodgkin-Katz
What did Tomita’s intracellular recordings of membrane action potentials of photoreceptors reveal?
resting mem potential of PRs in the dark was less negative (depolarized) than that of a regular neuron (-40 mV)
Do cones depolarize or hyperpolarize in the response to light?
hyperpolarize
What type of potentials are light responses (and what does this indicate), making them different than most CNS neuron potentials?
graded (the more intense the light stimulus, the greater the hyperpolarization)
What is the numerical resting potential of photoreceptors? 1. What is the maximum light response numerically? 2
- -40 mV
2. -65 mV
What is the steady flow of ions in darkness in photoreceptors called?
the “dark current”
What is responsible for the light evoked hyperpolarization of photoreceptors?
Na-permeable channels in the outer segment closing
What is the rods visual pigment called?
rhodopsin
What is the cone visual pigment called?
opsins or photopsins
What does the visual pigment consist of?
- opsin protein (7-transmembrane domain receptor)
2. chromophore retinal
What is the retinal formation before absorbing a photon? 1. What is it afterwards? 2
- 11-cis-retinal
2. all-trans-retinal
What is the name for activated rhodopsin?
metarhodopsin II (R*)
What is the G-protein involved in phototransduction called? 1. Where is this protein located? 2
- transducin (Gt)
2. embedded in disk membrane
What is the molecule that keeps ion channels open in the photoreceptor?
intracellular cGMP
What ion movement in the photoreceptor continues while bright light is absorbed (where is it located)?
K+ efflux (in inner segment)
What aspect of the visual pigment is the same for rods and cones? 1. What is different? 2
- chromophore (11-cis retinal)
2. opsin protein
What is the type of opsin in rods called? 1. Cones? 2
- rhodopsin
2. S-cone opsin, M-cone opsin, and L-cone opsin
Where is opsin produced? 1. What is their lifetime and are they reused? 2
- inner segment
2. 10-14 days and they are reused
What phagocytizes disks that are at the end of the outer segment?
RPE
What chromosome is the gene for the S opsin found? 1. M opsin? 2. L opsin? 3. Rhodopsin? 4
- chromosome 7
- X chromosome
- X chromosome
- chromosome 3
What is the percent homology between S-opsin and rhodopsin?
40%
What is the percent homology between S-opsin and M-opsin?
45%
What is the percent homology between M-opsin and L-opsin?
98%
When is it thought that human trichromacy arose?
after New World and Old World primate split (about 60 million years ago)
How many cone pigments do most mammals have?
two, dichromacy (S and LM gene)
What does the differences in the amino acid sequences in the 4 different types of opsins affect?
the relative absorbance
What is the wavelength in which the absorption of rhodopsin is greatest/the spectral sensitivity is maximized?
500 nm
What is the wavelength in which the absorption of S-opsin is greatest/the spectral sensitivity is maximized?
425 nm
What is the wavelength in which the absorption of M-opsin is greatest/the spectral sensitivity is maximized?
530 nm
What is the wavelength in which the absorption of L-opsin is greatest/the spectral sensitivity is maximized?
560 nm
How does an increase in wavelength alter the phototransduction cascade?
it doesn’t (all wavelengths go through same phototransduction cascade)
What is the idea that once a light photon is absorbed, all info about its wavelength is lost called?
Principle of Univariance
