The retina

functions of the retina (2 main)

• converts light into neural signals (electrochemical signals)

• sends neural signals (via optic nerve) to the brain (visual cortex) for visual recognition

brain cant interpret light - needs another form of communication - retina.

what three structures makes up the uveal tunic (middle layer of eye)

choroid

ciliary body

iris

where does the retina reside

innermost layer of the eye (neural layer) or tunic of the eye

state the 2 parts of the retina

retinal pigment epithelium (RPE) - pigmented epithelial cells - this is continuous with the epithelial of the ciliary body too while neural part ends abruptly and is not continuous (NON LIGHT SENSITIVE PART)

neural layer of retina / neural retina - find all neural parts and cells - for example photoreceptors - contains photopigment responsible for detecting light and initiating the photo transduction cascade (light to neural) (LIGHT SENSITIVE PART)

explain the position of the retina in the eye

• begins from the optic disk

• comes out from optic disk and is anterior up until the ORA serrata

• vitreous humor is anterior to the retina

• choroid is posterior to the retina

• therefore the retina is sandwiched between the choroid and vitreous

• we can say choroidal retina and vitreous retina when were referring to it location wise

• RPE adheres retina to choroid

• vitreous humor keeps retina in place - pressure

• close to the fovea / macula - highest VA - cone cells no rods - color perception/fine detail

what is the ORA serrata

boundary between the retina and ciliary body - retina transitions into ciliary body

marks the anterior boundary of the retina separating the light sensitive part of the retina from the non light sensitive area that extends to the ciliary body

light sensitive part of retina (neural part) ends abruptly at the ORA serrata

why is the optic disk referred to as a blind spot

optic disk - contains no neural retina / the point where optic nerve exits the eye so no photoreceptors

so light cannot be perceived by the brain or converted from light to neural signals and at ORA serrata (? ask)

explain the landmarks of the ocular fundus (optic disk and central retina) (2:3)

optic disk:

• pale yellow color - due to lost elements like lack of RPE and neural retinal parts

• site at which branching blood vessels extend from (veins and arteries)

• lies medially (nasal side)

central retina:

• consists of macula and fovea centralis

• fovea is at the center of the macula and is the site of best VA

• macula is darker - only contains cone cells - contain a special pigment making it dark

how would you know if you are looking at the ocular fundus of the right eye / left eye

optic disk lies medially

optic disk is on the left in the left eye

optic disk is on the right in the right eye

state the ten retinal layers

outer retina:

• 1 RPE - (above this is choroid)

• 2 photo receptors

• 3 external limiting membrane

• 4 outer nuclear layer (contain cell bodies of neural cells - purple - dense, rich area)

• 5 outer plexiform layer

inner retina:

• 6 inner nuclear layer (cell bodies of neural cells - but less than outer layer so less cells)

• 7 inner plexiform layer (axons and dendrites of neural cells)

• 8 ganglion cell layer (few cell bodies)

• 9 nerve fiber layer

• 10 internal limiting membrane (below this is the vitreous humor)

state the cells in the 10 layered retina (7) and describe their location in relation to each other

• 1 - RPE (retinal pigment epithelial cells)

• 2 - photoreceptors - rods and cones / attached to RPE / and synapse with bipolar cells

• 3 - bipolar cells / synapse with ganglion cells

• 4 - horizontal cells / connect bipolar cells to photoreceptors

• 5 - amacrine cells / synapse with ganglion cells

• 6 - interplexiform neuron / synapse with amacrine and bipolar cells

• 7 - ganglion cells - axons of ganglion cells converge and join to make up the optic nerve/tract that leaves the eye

1 is the RPE

2-7 all make up the neural retina - involved in transmission of light energy to neural in optic nerve

what are neurological cells (2)

not actively involved in transfer of neural signals

provide structure and support and have a role in fighting neural tissue infection

name the neurological cells in the retina (3)

muller cells

microglia

astrocytes

explain muller cells generally (structure)

• extend throughout the retina

• large neuroglial cells

• apex of cell in photoreceptor layer

• basal aspect at the inner retinal surface

explain functions of muller cells (4)

• structure - cellular processes fill in empty retinal space / no gaps

• support - cellular processes enclose dendritic processes within synaptic layers / particularly to areas prone to damage like plexiform layers

• separates retina from vitreous - the end foot (expanded muller cell process), along the basal aspect contributes to membrane separating retina from vitreous

• maintain pH in retina - absorb waste products and neurotransmitters in retina and regulate ion concentration

explain microglia, location and what they do

• wandering phagocytic cells

• found anywhere in the retina / can migrate

• become active when infection is present / injury has occurred

explain astrocytes, location and what they do

• star-shaped fibrous cells

• found in inner retina - nerve fiber and ganglion cell layers

• provide support to nerve fibers and retinal capillaries in inner retina

explain how light reaches the photo receptors (2)

• target of light is photoreceptors - phototransduction begins here

• light must pass through all the neural layers / inner retinal layers before reaching the photoreceptors

explain the direction of information flow in the retina (3)

• 1 - vertical flow of information - from outer to inner retina:

rods and cones (photoreceptors) — bipolar cells — ganglion cells — optic nerve — leaves eye and into visual cortex of brain

• 2 - interlayer flow of information - lateral flow

horizontal and amacrine cells (and interplexiform) modify the neural information from rods and cones before it reaches and leaves the optic nerve - edit it

• 3 - interlayer flow of information - from inner to outer retina (feedback)

interplexiform cells communicate in between plexiform layers - convey information between them / and feeds back information from inner plexiform layer to outer plexiform layer

explain how the horizontal cells modify the neural information (3)

• synapse with photoreceptors, ganglion cells and other horizontal cells

• inhibitory feedback (to cone cells) or feed-forward (to bipolar cells)

• not thought to influence rod cells

explain how amacrine cells modify neural information (2)

• synapse with bipolar, ganglion, interplexiform and other amacrine cells

• modulates feed-forward information (info that reaches ganglion cells)

explain general photoreceptor structure (6)

• outer segment

• inner segment

• connecting cilium - joins outer and inner segments

• nucleus in cell body

• axon

• synaptic terminal - from where the photoreceptor synapses with bipolar cells

explain the outer segment structure of the rods and cones (and differences between rod and cone disks) (6) AND its function (1)

• flattened membranous sac

• contains photopigment, rhodopsin in rods and photopsin/iodopsin in cones - which is stacked in layers and embedded in disks

• rod disks (600-1000) - separate to outer membrane

• cone disks - continuous with outer membrane

• resides in photoreceptor layer of retina

• in direct contact with RPE

function:

• contains photopigment and initiates the phototransduction cascade

explain inner segment structure (2) and function (1)

• contains mitochondria (production of ATP)

• resides in photoreceptor layer of retina

  function:

• serves and metabolic center (and site of protein synthesis)

summarize the differences in the anatomical differences between rods and cones (8)

• rods contain rhodopsin whereas cones contain photopsin/iodopsin (blue,red,green) - photopigments

• rods involved in scotopic vision whereas cones are involved in photopic vision - vision

• rods features: dim light, dark shades of grey whereas cones: bright light, color vision, detail and VA - features

• rods ~100 million whereas cones ~6 million - cell number in human retina

• rods have a higher concentration in the periphery whereas cones have a higher concentration in the fovea/macula

• rods are long, thin, rod-shaped whereas cones are shorter, cone-shaped and larger width (except in fovea) - anatomy

• rod disks are separate to the outer membrane whereas cone disks are continuous with outer membrane

• rods are elongated so come in direct contact with the RPE layer whereas cones are close to the RPE but their outer layer does not come into direct contact with it, so the RPE will wrap around cone (sn)

summarize the differences between rods and cones (2)

• mechanism of photopigment stimulation by light is exactly the same in both rods and cones

• both come into contact with the RPE layer

explain what the process of phototransduction is (3)

• transformation of light energy into a communicable form of electrochemical energy

• brain cannot process/perceive light so needs to be transformed to then reach the visual cortex of brain

• this process begins in the photoreceptors

what are the 2 parts of a photopigment

opsin (a protein) - does not/cannot absorb light

11-cis-retinal (the chromophore which is a Vit A derivative)

define chromophore

part of a molecule that can absorb light/responsible for molecules ability to detect and interact with light

what makes rhodopsin different to photopsin (3)

• both contain the same 11-cis-retinal

• differ in opsin: molecular structure of opsin varies in both (amino acid sequence)

• results in chromophore responding to different wavelengths of light

explain isomerization of photopigment (5)

• when light hits the disks in the outer segment of photoreceptors, 11-cis-retinal absorbs this light

• 11-cis-retinal undergoes isomerization (transforms into another molecule with same molecular formula, but different structural formula) = trans-retinal

• light breaks a double bond in 11-cis-retinal, forming the isomer trans-retinal

• trans-retinal changes shape and dissociates from opsin, is said to be bleached

• opsin is free - initiates phototransduction cascade

why is the photopigment said to be bleached

trans-retinal cannot absorb light

define depolarization

occurs when inside of cell becomes less negative (more positive ions coming into the cell than leaving)

becomes more positive than the resting membrane potential

define hyperpolarization

occurs when inside of cell becomes more negative (more positive ions leaving the cell than entering)

hyperpolarizes beyond the resting membrane potential

explain phototransduction - in the DARK (7)

• photopigment not activated

• cGMP continually being produced

• cGMP binds to ligand gated Na+ channels : channels then open

• Na+ floods in: cell becomes more positive and membrane is depolarized

• depolarization triggers influx of Ca2+ in synaptic terminal

• intracellular Ca2+ triggers release of glutamate into synaptic cleft

• photopigment, transducin and phosphodiesterase : not doing anything in dark conditions

explain phototransduction in the light (9)

• light is absorbed by the photopigment (specifically in chromophore)

• 11-cis-retinal undergoes isomerization and is converted to trans-retinal

• opsin is free to move and activates transducin

• transducin activates PDE (phosphodiesterase)

• activated PDE catalyzes the breakown of cGMP

• ligand-gated Na+ channles remain closed: Na+ cannot eneter

• so cell becomes more negative (less positive) and is therefore hyperpolarized

• Ca2+ channels remain closed

• less glutamate released - no exocytosis of glutamate

explain the phototransduction CASCADE (other visual neurons) - light conditions (4)

photoreceptors : light absorbed - hyperpolarized - less glutamate released

bipolar: stimulate bipolar cell to become depolarized - more glutamate released

ganglion: stimulates ganglion cells to become depolarized - triggers action potential which travels down optic nerve

when cells are hyperpolarized = less glutamate released / wen cells are depolarized = more glutamate released