RETINA:ANATOMY AND PHYSIOLOGY

It is the innermost tunica of the eyeball:

Thin, delicate, transperant and

highly developed.

Extends from the optic disc to the orra serrata.

Can be divided into three distinct regions:

1]optic disc

2]macula lutea

3]peripheral retina

Development:

EMBRYOLOGY OF RETINA

Embryologically the retina is part of the brain. It develops as a hollow out pushing of the neural tube, whose outer end becomes indented to form a stalked cup; the double-walled cup forms the two layers of the retina. 

• Retina appears purplish red due to:

1)visual purple color of the pigment epithelium.

2)vascular choroid.Orra serrata:it is the

serrated peripheral margin where retina ends and the cilliary body starts.

Optic disc:

Pale pink well defined circular area of about 1.5mm diameter.

At optic disc, all the retinal layers terminate

except the nerve fibres, which pass through the lamina cribrosa[sieve like sclera] to run into the optic disc.

Optic disc is white because: lamina cribrosa medullated white nerve fibers behind absence of vascular choroid.

Physiological cup: depression seen in the disc.The central retinal vessels emerge through the centre of the cup.

Importance of the cup: increase in the size of the cup is suspicious for glaucoma.

In the centre of the disc :Nerve fibers are thinnest,so it shines.

A dark area of 5.5 mm diameter, situated 2 disc diameter temporal from the edge of the optic disc.

Function:photopic and color vision.

Fovea centralis: central depressed part of macula, of 1.85mm diameter and 0.25 mm thickness.

Most sensitive part of retina, rich in cones. The central floor (of .35mm diameter) of fovea

is called foveola. Umbo:Tiny depression in the centre of

foveola,responsible for foveolar reflex in ophthalmoscopical examination. Loss of the reflex is an early sign of retinal damage.

1]retinal pigment epithelium2]layers of rod and cones3]external limiting membrane4]outer nuclear layer5]outer plexiform layer6]inner nuclear layer7]inner plexiform layer 8]ganglion cell layer9]nerve fibre layer10]internal limiting membrane.

These are the photoreceptors, serve as sensory nerve endings for visual sensations.

Light falling upon the retina is absorbed by the photosensitive pigments present in the rods and cones.

Rods :contains rhodopsin. responsible for peripheral vision and vision in low illumination.

Cones: responsible for central vision and colour vision.

Structure of rods and cones

• Highest density of cones: at fovea,avg.199000 cones/mm sq.

• Rods are maximum below optic disc, number reduces towards periphery, and absent in fovea in an area of 0.35 mm around it.

How can we see?

• Light on retinaabsorbed by the photosensitive pigments present in rods and conesinitiation of photochemical changestriggering of sequential eventsvisual perceptions.

The electrical changes produced and actively processed in retina are transmitted through the ganglion cells and along the fibers of the optic nerve via the visual pathway up to the visual cortex.

The mechanism is divided as:

1]initiation of vision(transduction)2]transmission of visual sensations3]visual perceptions.

Visual perceptions:

1.light sense

2.form sense

3.contrast sense

4.colour sense

Light sense:

• It is appreciation of light.

• Light minimum: minimum light required to evoke a sensation.

• Dark adaptation: the ability of the eye to adapt itself to decreasing illumination.

• Rods are more sensitive to low illumination than cones.• Duplicity theory of vision:

rods are used more in dim light(scotopic vision)

cones are more used in bright light(photopic vision)

Form sense: Ability to discriminate between the shapes of the objects.

Cones play a major role.

Form sense is more acute in fovea.Visual acuity recorded by Snellen’s test is a measure of form sense.

Colour sense: it is also function of cones.

Outer 4 layers :

From the choriocapillaris

Inner 6 layers: From the central

retinal artery.

Outer plexiform layer gets its blood supply from both.

Macula: small twigs from superior and inferior branches of central retinal artery. Sometimes, by a cilioretinal artery, when presents.

Fovea is an avascular area, mainly supplied by choriocapillaris.

Retinal arteries are end arteries.however,anastomosis occurs with cilliary vessels near lamina cribrosa.

The RPE is firmly attached to the underlying Bruchs’s membrane[basal lamina of choroid.]and loosely attached to the layer of rods and cones. There is a potential space between the sensory retina and pigment epithelium layer. This layer is called sub retinal space. The two layers can be readily separated and such an event is called retinal detachment.

Types—

1)Rhegmatogenous

2) Exudative, serous, or secondary

3) Tractional

• Rhegmatogenous retinal detachment – occurs due to a break in the retina that allows fluid to pass from the vitreous space into the subretinal space . Retinal breaks are divided into three types - holes, tears and dialyses.

• Exudative, serous, or secondary retinal detachment-occurs due to inflammation, injury or vascular abnormalities that results in fluid accumulating underneath the retina without the presence of a hole, tear, or break..

• Tractional retinal detachment – A tractional retinal detachment occurs when fibrous or fibrovascular tissue, caused by an injury, inflammation or neovascularization, pulls the sensory retina from the retinal pigment epithelium.