RETINA

ANATOMY OF RETINA AND PHOTOCHEMISTRY OF VISION

Introduction

Retina is a multilayered sensory tissue that lines the back of the eye.

It contains millions of light receptors that captures light rays and convert them into electrical impulses.

These impulses travel along the optic nerve to the brain where they are turned to images.

Structure of Fovea CentralisIn this area, there are no rods.

Cones are tightly packed and it is the most sensitive part of retina.

Its central part is called foveola.

All other retinal layers are absent in this region.

Histological Layers of the Retina

retinal layers of physiological significance

Photoreceptor LayerRods and Cones are the end organs of vision and are also known as photoreceptors.

Rods(120 million) contain a photosensitive substance rhodopsin(visual purple) and helps in peripheral vision and vision of low illumination (scotopic vision)

Cones (6.5 million) also contain a photosensitive substance and helps in highly discriminatory central vision(photopic vision) and colour vision.

8Visual Pigments Rhodopsin Light-sensitive pigment in rods Decomposes in presence of light Triggers a complex series of reactions that initiate nerve impulses Impulses travel along optic nerve Pigments on cones Each set contains different light-sensitive pigment Each set is sensitive to different wavelengths Color perceived depends on which sets of cones are stimulated Erythrolabe responds to red Chlorolabe responds to green Cyanolabe responds to blue

Rods and Cones

conesrods

RodsConesNumber

Distribution~ 120 millions in each retinaMore in peripheryNon in Fovea~ 6 millions in each retinaMore in centrePresentPhotosensitive pigmentRhodopsin3 types (iodopsin)ConnectionConvergence(300:1 connection)No convergence (1:1; direct private line)Functionlight sensitivity visual acuity- colour visionNight vision light sensitivity visual acuity+ colour vision Day vision

Why Fovea is the most sensitive spot in retina?

Why Fovea is the most sensitive spot in retina?All layers are shifted aside leaving outer segments of photosensors to be hit directly by lightHigh density of small diameter Cones with long outer segments 1:1 convergence (cone-BC-GC)Wide presentation in occipital primary visual area

1)- Retinal Pigmented EpitheliumAbsorption of light (due to presence of black pigment melanin) reduction of glare (Albinos!!)Production of extracellular matrix keeping outer segments of photoreceptors straight Storage of vitamin A (precursor of 11-cis retinal) for regeneration of photosensitive pigmentsPhagocytosis of the tips of outer segments after their shedding off by the photoreceptors continual renewal of outer segments

Photochemistry of Vision

What Happens in Retina

The light rays are focused directly onto the retina, the light sensitive tissue lining the back of the eye.

Light energy is converted into neural signal

Through visual pathway, these signals reach brain.

Action potentialsexcitation of rods causeshyperpolarisation rather than depolarisationincreased negativity of the membranethis is due to decreased permeability to Na

inner segment pumps Na outouter segment is very leaky to Nanormally membrane is -40mV (inside)

when excited outer segment prevents Na influxinner segment continually pumps Na outincreased negativity inside -> hyperpolarisationinside becomes -80mV

Physiology of vision The main mechanisms are [A]Initiation of vision (Phototransduction)

[B]Processing and transmission of visual sensations

[C]Visual perception

Phototransduction(Initiation of Vision)

The whole phenomenon of conversion of light energy into nerve impulse is known as phototransduction.

Light falling upon retina cause photochemical changes(1) which trigger a cascade of biochemical reactions that result in generation of electrical changes(2).

(1)Photochemical Changes

RHODOPSIN BLEACHINGRhodopsin refers to the visual pigment present in the rods-the receptors for night(scotopic) vision.

Its maximum absorption spectrum is around 500 nm.

Rhodopsin consists of a colourless protein called opsin coupled with a carotenoid called retinine(Vit A or 11-cis-retinal)

Bleaching and Regeneration of Visual Pigments

Visual Cycle

Photoreception - In More Detail

Photoreception

Effect of Light on Retinal Neurons

VISUAL CYCLE

In the retina of living animals , under constant lght stimulation, a steady state must exist under which the rate at which the photochemicals are bleached is equal to the rate at which they are regenerated.

This equilibrium between the photo-decomposition and regeneration of visual pigments is referred to as visual cycle.

(2)Electrical Changes

Activated rhodopsin cascade of biochemical reactions generation of receptor potential

Thus light energy is converted to electrical energy

Light Adaptationretinal sensitivity depends on the amount of chemical pigmentif a person is in bright light for some time, large amount of photochemical is reduced to retinal and opsinretinal converted to vitamin Athis reduces the sensitivity of the retinathis is known as light adaptation

now if the person goes into a dark roomhe cannot see any objectreason: severe reduction in retinal sensitivity

Dark AdaptationAbility of the eye to adapt to decreasing illumination.

When one goes from bright sunshine into a dimly-lit room, one cannot perceive the objects in the room until some time has elapsed.

This is called dark adaptation time.

It is the time taken for regeneration of rhodopsin pigment which was bleached by the bright light.

Night blindnessvitamin A deficiencynot enough quantities of retinal to reform rhodopsinbut in daytime cones can still be excited

Autonomic Nervous SystemUOB EXAM QUESTIONSWhat is Autonomic nervous system? What are its parts and their significance?Compare and contrast between the sympathetic and parasympathetic nervous system. Compare and contrast between the somatic and autonomic nervous system. Differentiate between the adrenergic and cholinergic fibers. What is the location and significance of the various receptors of the parasympathetic nervous system.