How we see - part 2 Flashcards
What is visible light?
Visible light is an electromagnetic wave
What is phototransduction?
Phototransduction is defined as the conversion of light energy to an electrochemical response by the photoreceptors (rods and cones)
What activates optic nerve cells?
The phototransduced rods and cones need to activate optic nerve cells (generate an action potential)
What are 2 photoreceptors present in the retina?
rods
cones
What is the function of rods?
function in less intense light
responsible for vision at low light levels (scotopic vision). They do not mediate color vision, and have a low spatial acuity
entierly responsible for night vision
What is the function of cones?
responsible for colour vision
function best in brighter light
What is the fovea centralis?
small, central pit in the retina composed of closley packed cones
responsible for sharp central vision
What are the different types of cones?
S-cones
M-cones
L-cones
each cone is sensitive to visible wavelengths of lgiht that corrospond to short, medium and longer wavelength
colour blindness is due to the abscence of one of the following types of cones
What is the strucutre of rods and cones?
Where is the visual pigment found in photoreceptors?
Outer segments
What are the photosensitive chemicals in rods and cones called?
The outer segment of a rod or a cone contains the photosensitive chemicals:
in rods, this chemical is called rhodopsin
in cones, these chemicals are called color pigments
What makes up rhodopsin?
Opsin + 11-cis Retinal
What is shown here?
protein in the membrane of the outer segement of a photoreceptor
What is 11-cis retinal?
a vitamin A compound
Where does the vitamin A compound of 11-cis retinal sit?
inside opsin
What happens when light hits the vitamin A?
When light falls on 11-cis retinal, it isomerises to all-trans retinal
it becomes straight and no longer sit in the opsin t so it releses it and no longer looks purple as rhodopsin looks purple but now looks bleeched
What happens when light hits rhodopsin?
when light hits retina, vitamins A in opsin changes shape and rhodosin slipts and bleeches and when rhodopsin splits this causes a who series of reactions
ultimatley leading to a Na+ channel in the cell membrane to close and when this closes this causes hyperpolarisation and by the time it reaches the optic nerve it becomes depolarisation and this is the action potential which is taken to the brain to see the outside world
then all resets for next photon of light
what happens to all-trans retinal after rhodopsin splits?
leaves the photoreceptor
Where is all-trans retinal taken up?
taken up by a layer underneath the rods and cones called the pigment epithelial layer
Where is all-trans retinal changed by to 11cis-retinal?
pigment epithelial layer
During the process of all-trans retinal changing back to 11cis-retinal some vitamin A is lost, what does it go on to form?
need to be replenished in order to continue to see normally and comes form the diet
retinyl ester
How does bleaching of the visual pigment result in phototransduction?
Phototransduction cascade
What role does Vitamin A play in the visual pigment?
visual pigment regeneration
How does vitamin A deficiency affect the eyes?
Vitamin A is supplied through the diet, any condition that affects vitamin A absorption will affect vision - (night)blindness
Vitamin A is also essential for healthy epithelium so conjunctiva and corneal epithelium are also abnormal
Vitamin A deficiency can occur in conditions such as malnutrition, malabsorption syndromes such as coeliac disease, sprue
What things can vitmin A deficiency cause in the eye?
Bitot’s spots in conjunctiva are sometimes the first indication of Vitamin A deficiency - silver traingles
Corneal ulceration ( green colour is dye to show up extent of ulcer)
Corneal melting which leads to future opacification of the cornea
What is the main difference between rods and cones?
they respond to different wavelengths
What is the process of an action potential forming?
Cascade of reactions within rod/cone - hyperpolarises - impulse is transmitted - becomes an action potential in the optic n cell
How is cis-retinol reformed?
All trans-retinol converted into cis-retinol in RPE and will use up some Vitamin A
Cis-retinol then transported back to rod/ cone to reform “opsin”
