Transduction

rods and cones

contain synaptic terminal-axon-nucleus forming outer nuclear layer of retina

inner segment containing cell organelles

outer segment containing stacks of lamellae which have photoreceptors 

each lamellae is made up of…

cell membrane - contains visual pigment rhodopsin in rods and cones

is S, M, L in rods or cones

just cones

what do the visual pigments differ in

spectral sensitivity

what vision does rods support

under low light conditions - scotopic vision

what vision does cones support

daylight vision - photopic vision

differences between rods and cones

cones less sensitive than rods

light response of cones is faster

cones terminated more rapidly than that of rods

opsins

integral transmembrane helical proteins (contains vitamin A compound) which has the configuration of 11-cis retinal, when light falls on 11-cis retinal it isomerises to all-trans retinal

why does rhodopsin split

because all-trans retinal cannot fit into opsin so rhodopsin splits which results in bleaching of the visual purple

phototransduction

conversion of light energy to an electrochemical response by the photoreceptors rods and cones which will activate optic nerve cells (generate an action potential) 

phototransduction cascade summary

an electromagentic wave of light caused a change in the vitamin A which induces a cascade of reactions within the rod and cone which hyperpolarises this cell and ultimately transmits this hyperpolarisation until it reaches the cell of the optic nerve where it becomes an action potential

visual pigment regeneration

in pigment epithelial cell:

all-trans retinol → 11-cis retinol →11 cis-retinal which can then go back into your photoreceptor, join with the opsin and reform your visual pigment or rhodopsin 

what happens to some pigments at each stage in pigment epithelial cell

 become retinyl esters, difficult to convert back into 11-cis retinal, degraded and lost to body

what is required in order to have enough 11-cis retinal

constant supply of vitamin A which we get through our diet and stored in liver 

what is vitamin A responsible for

visual pigment regeneration

vitamin A deficiency

= night blindness as rods deteriorate which usually work at darker light

what else is vitamin A essential for

healthy epithelium so conjuctivae and corneal epithelium 

to not be malnourished

to not have malabsorption syndromes e.g. coeliac disease and sprue

Bitot’s spot in conjunctivae

cornal ulceration, conreal melting leading to future opacification of the cornea 

cone rod dystrophy

eye disorder that affect the light sensitive cells of the retina called the cones and rods.

experience vision loss over time as the cones and rods deteriorate

symptoms:

-decreased sharpness of vision (visual acuity

-abnormal sensitivity to light (photophobia)

-blind spots in central field of vision (scotomas)

-loss of colour perception

-loss of peripheral vision

most individuals legally blind by mid adulthood

red-green colour deficiency

colour blindness

X-linked resulting from anomalies in the red (L cones) or green (M cones) cone opsin gene

difficulty distinguishing between shades of red, green, brown, orange, and purple

non progressive, doesn’t affect visual acuity

ahcromatopsia

severe congenital condition where the cone cells do not function properly

symptoms:

Complete color blindness (only seeing black, white, and grey shades)

very poor visual acuity (usually 20/200 or less)

nystagmus (involuntary eye movements)

severe photophobia. 

blue cone monochromatism

X-linked congenital disorder where only the blue cone system (S-cones) and rods are functional. 

Symptoms:

Severely impaired colour vision

blurred vision

light sensitivity

nystagmus from birth. 

green cone issues are called

deuteranomaly/deuteranopia

red cone issues are called

protanomaly/protanopia