How We See 2 - Transduction Flashcards
What is visible light an example of?
Electromagnetic wave
What is phototransduction?
Conversion of light energy to an electrochemical response by photoreceptors (rods and cones)
Where are photoreceptors?
On the retina
What do the phototransduced rods and cones activate and how?
Optic nerve cells by generating an action potential
What is each lamellae made up of?
Cell membrane which has the visual pigment rhodopsin in rods and coneopsins S, M and L in cones
What is the cell in the retina called where rods and cones exist?
Muller cell
What is integrated into the cell membrane of lamellae in rods?
Rhodopsin
What is integrated into the cell membrane of lamellae in cones?
Coneopsins S, M and L
What do the molecules rhodopsin, conopsins S, M and L differ in?
Spectral sensitivity
Where on the Muller cells in the retina are photoreceptors found?
On stacks of lamellae in the outer segment
What wavelength does the blue cone best absorb?
437nm
What wavelength do rods best absorb?
498nm
What wavelength do green cones best absorb?
533nm
What wavelength do red cones best absorb?
564nm
What is the structure of rhodopsin?
Integral transmembrane helical protein
What is chromosphore?
Part of a molecule responsible for changing its colour
What is opsin?
Protein which forms part of the visual pigment rhodopsin and is released by the action of light
Where is chomosphore found in rhodopsin?
Nesting in the opsin
Why is vitamin A important for sight?
It is used to form opsin within rhodopsin
What are the two importnat parts of rhodopsin?
Opsin and retinal
What happens to 11-cis retinal when light falls on it?
Isomerises to all trans retinal
Why does rhodosin split from all trans retinal
All-trans retinal cnnot fir into the opsin, resulting in bleaching of the visual pigment purple
Explain the process of rhodopsin causing the phototransduction cascade?
1) Chomophore nests in the protein opsin which is part of retinal
2) When light falls on 11-cis retinal it isomerises to all-trans retinal
3) All-trans retinal cannot fit into the opsin so rhodopsin splits
4) Results in bleaching of the visual purple which causes photoconduction due to the phototransduction cascade
What vitamin is responsible for visual pigment regeneratoin?
When are photoreceptor cells at rest, and in what polar state are they?
At rest in th edark in a depolarised state by open Na/Ca channels
What maintains at rest photoreceptor cells in a depolarised state?
Open Na/Ca channels
Explain the phototransduction cascade?
1) Photo of ligh hits rhodopsin
2) Which becomes activated rhodopsin
3) Becomes transducin-GTP, then phosphodiesterase (acitvated) then GMP
4) Na channel closes
5) Relative hyperpolarisation of photoreceptor cell
6) hyperpolarisation transmitted by a flux of Ca ions to the synapse with bipolar cell
7) Stimulates ultimately the retinal cell
Explain visual pigment regeneration?
What does vitamin A deficiency lead to in relation to the eyes?
Since vitamin A is supplied through the diet any condition that affects vitamin A absorption will affect vision, causing blindness
Also essential for healthy epithelium so conjunctiva and corneal epithleium are also abnormal
What are examples of conditions where vitamin A deficiency can occur?
Malnutrition
Malabsorption conditions such as coeliac disease or sprue
What are signs of vitamin A deficiency?
Bitot’s spot in conjunctiva
Corneal ulceration
Corneal melting which leads to opacification of the cornea
