L19 Rods vs Cones

rhodopsin

photon receptor made up of retinal + opsin

transmembrane G protein coupled receptor located in the disk membrane

When is rhodopsin inactive?

inactive when bound to 11-cis retinal

What type of photoreceptors are found in the fovea?

cones only, rods are absent

What is the intracellular concentration of Na+ and Ca2+ of photoreceptors in the light?

both have very low concentrations.

Why does intracellular [Na+] concentration decrease in the light?

drops due to CNG closure

Why does intracellular [Ca2+] concentration decrease in the light?

drops due to continued NCX and PMCA activity

What effect does light have on synaptic ribbons?

decreases in size with illumination

occurs via the formation of synaptic sphere that becomes disassociated with ribbon

facilitates the reduction in the tonic glutamate release

What is the state of rhodopsin in a non-illuminated environment?

rhodopsin is inactive

What is the state of transducin-GDP in a non-illuminated environment?

inactive

What is the state of phosphodiesterase in a non-illuminated environment?

inactive

What is the state of synaptic voltage gated channels in a non-illuminated environment?

inactive

What is the rate of glutamate vesicle fusion in a non-illuminated environment?

high rate

What is the state of metarhodopsin II in an illuminated environment?

active

What is the state of transducin GTP in an illuminated environment?

active

What is the rate of glutamate vesicle fusion in an illuminated environment?

rate is reduced

What are the three mechanisms needed to reset the system and allow another light stimulus?

inactivation of metarhodopsin II

cGMP restoration

transducin and phosphodiesterase 6 inactivation

What are the steps to inactivate metarhodopsin II?

1. phosphorylation by rhodopsin kinase

2. arrestin binding

3. Schiff's base of hydrolysis of all-trans retinal

How is cGMP restored?

guanylyl cyclase activity increases with lowering of cytoplasmic [Ca2+] levels

What molecule is responsible for transducin and PDE6 inactivation?

RGS9-1

What opsin is found in rods?

rhodopsin

What options are found in cones?

S-opsin-blue

M-opsin-green

L-opsin-red

What type of vision do rods allow?

scotopic viison

What type of vision do cones allow?

photopic vision

What is red green color vision defects due to?

due to alternations in L-opsin and/or M-opsin genes

found on the X chromosome

What are blue-yellow color vision defects due to?

alterations in S-opsin gene

protan

loss/limited L-cone

deutan

loss/limited M-cone deutan

tritan

premature destruction of S cones or the production of defective S cones

cone monochromacy

2/3 cone photopigment genes don't work

rod monochromacy

functional disruption of all cone types

due to mutations in cone-specific phototransduction protein encoding genes

What are the three reasons that rods are more sensitive than cones?

1. fewer photons are necessary to elicit similar response in rods

2. cone response must "rise above the noise"

3. convergence of photoreceptors to a single retinal ganglion cell is greater in rods than cones

Which opsin has a greater rate of spontaneous isomerization?

spontaneous isomerization rates of cone opsin are greater than rhodopsin

T/F: cones have a higher spatial and temporal acuity

true

Why do cones have higher spatial acuity than rods?

due to the convergence of cones on RGCs

Why do cones have higher temporal acuity?

due to speed of phototransduction

What photoreceptor dominates in the peripheral retina?

rods

Which part of the retina has the highest spatial acuity? Lowest?

highest: fovea

lowest: peripheral retina