retina revision week 3

from year 1 content

layers of eye till retina

sclera(outer white coating) →choroid(middle layer) - retina(inner layer responsible for vision).

retina extend to ?

the ora errata which is most anterior point of retina - and where ciliary body begins

what is the retina in direct contact with

the vitreous

what is the vitreous

 substance in anterior chamber ( appears dark on OCT scan)

what is function of retina

detects light and tells brain about aspects of light in relation to objects

where do we receive signals from in retina

Photoreceptors

what does retina do with signals

doesn’t allow it to be processed individually- packages information and sorts it- without overloading brain with repetitive or redundant facts

what is a spatial temporal change

associated with movement- due to variations in light intensity

how are colours detected

wavelength of light reflected off object and into eye

layers of retina in order

-internal limiting membrane

-nerve fibre layer

-ganglion cell layer

inner plexiform layer

-inner nuclear layer

-outer plexiform layer

-outer nuclear layer

-external limiting membrane

-photoreceptor layer

-retinal pigment layer

image of layers

Photoreceptor

rods- 95%

cones-5%

distribution of photoreceptors

centre of fovea aren’t any rods only cones (but smaller cones than in the rest of retina- so more densely packed)

no rods or cones in blind spot- where optic nerve located

away from fovea there is high number of rods - peak is just water fovea and slowly decreases as it moves away

where are they placed

at the back of the retina- placed side by side- light reaches other structures of retina first

how is fovea adapted

all cones at fovea-densely packed for highest acuity

outer layers of retina above photoreceptors not present so more light can be directly supplied to photoreceptors

cones

at centre they are long and thin- more into periphery become shorter and wider diameter

all photoreceptors have

pigment, which absorb energy from the photons oflight-

pigment lies in outer segment next to epithelium

inner segment is filled with mitochondria and merges with nucleus of cell

what happens to photopigment

when it absorbs light they decrease the rate of neuro transmitter and this is how vision occurs

channels close

when in the dark

photoreceptors are most polarised and release glutamate as ion channels are open

photo pigments

4 types

rhodopsin- rods

Erytholabe (long), Chlorolabe(medium) , Cyanolabe(short) wavelength sensitive - cones

what pigment made of

retinal and opsin ( large portion) make up all pigments - there are four

a small protein (retinal )Is the first to be affected by light absorption- this is the active part 

outer segemts of photo receptors contain

series of discs - which contain pigment

how does it work

if only one pigment was used different wavelengths would get confused- but brain uses multiple pigments to understand which wavelengths used- so can detect change in intensity of light or wavelength

what happens in dark (scotopic)

rods are more sensitive can detect single photons

cones are weaker- so can’t distinguish colour as much

in photopic conditions

cones work much better

rods work too but not as well as cones

rhodopsin light activation

(11-cis-retinal + opsin) opsin protein loops in and out of membrane forms cage that encloses retinal protein

photon absorption raises energy and breaks bonds so retinal changes shape

into its activated form (all-trans-retinal + opsin)

formula of activated rhodopsin

R to R*

retinal becomes retinol (bleaching of rhodopsin) -removal of opsin from retinal

to regenerate enzymes required for to retinol to become retinal

cycle of photopigment

renewal of photoreceptors

regeneration continuous cycle ( most neurons don’t degenerate ) - 2 week process

-amino acids from outside cell make new proteins

• diffuse through cell inner segments and photopigments

• join as a band

• band moves up to outer segment

• leaves photo receptor for the pigment epithelial cells (engulfed)

• new discs formed and old discs got ride of

retina vertical communication pathway

dendritic end- receives from synaptic connection with photoreceptors

cell body

then axons and terminal end - which communicates with retinal ganglion cell

dendrites of retinal ganglion cell communicate with bipolar cells

essentially (photoreceptors - bipolar cells- retinal ganglion cells)

how does the communication work

can be individual so one photoreceptor to one bipolar to one ganglion ( provides finest spatial detail-  mainly at fovea)

can be one photoreceptor to more than one bipolar cell and more than one ganglion (divergent wiring)

can be multiple photoreceptors to less bipolar to one ganglion (convergent wiring ) - good for small amounts of light , poor in good light conditions)

horizontal cells

neurons that receive inputs from photoreceptors and pass onto neighbouring photoreceptors

effect depends on sign / polarity 

so if positive it makes resolution less sharp as its same sign as direct photoreceptor  input

if negative its opposite so transforms vertical pathway to signalling contrast in illumination instead of amount of light - helps spatial resolution

more on lateral pathway (horizontal)

amacrine cells- they receive input from other amacrine or bipolar cells

and output to amacrine or bipolar or ganglion cells

amacrine cells responses

 if burst of activity when :

light intensity increases= On response

light intensity decreases= Off response

some cells have both responses= ON/OFF

interplexiform neurons

receive inputs from bipolar and amacrine from inner retina

output to bipolar cells and horizontal cells in outer retina

opposite direction of flow

involved in adjustment of retinal sensitivity