Functional Retinal Physiology

Nilufa Akter B.Optom .2nd year

Institute of community ophthalmology

Functions of retinal layer

• RPE • -Plays important role in photoreceptor

and recycling of vit-A. • - plays phagocytic action • -provides mechanical support to the

processes of photoreceptors

Photoreceptor

• ROD • -provides peripheral of vision • -helps in scotopic vision • CON -Provides in central vision -helps in photopic vision and color

vision.

Inner neuclear layer

• BIPOLAR CELL -transmits signal from photoreceptors to

the ganglion cell. HORIZONTAL CELL -helps integrate and regulate the inpute

from multiple photoreceptors cell. -responsible for eyes to adjust to see well

both bright and dim light condition.

GANGLION CELL LAYER

has 2 types of cells 1.PERVO( midget) cell -have small receptive field -are sensitive to color . 2.MAGNO(parasol ) cell -large receptive field -sensitive to visual motion -participate very little in color perception.

MACULA

-Provides 15 degree of visual field -helps in photopic and color vision -provides central high resolution of vision ( main 2 function : -visual acuity -color vision ).

RETINAL PHYSIOLOGY

Vision ? Receptor ? Electrophysiology: Extracellular recording: a microelectode

placed in close proximity to a neuron records action potentials generated by the neuron.

Intracellula recording : microelectode pierces the membrane of a neuron and records the membrane potential. This technique technically more challanging than extracellular recoding

RETINAL NEURONS

3 TYPES OF NEURON

Cont..

• A direct 3 neuron chain • -from photoreceptor to bipolar to ganglion

cell- is the major route of information flow from the light source to the optic nerve.

• The horizontal and amacrine cells are primary responsible for lateral interactions.

Cont…

• Retinal receptive fields• The part of the retina that needs to be

stimulated to elicit action potentials from a ganglion cell is the receptive field.

• Retinal receptive fields are small and concentric and correspond to the part of the world that the cell can “SEE’.

Photoreceptors

• There are 2types of photoreceptor: rods and cones.

• These differences are the basis of the Duplex Theory of Vision.

Duplex theory of Vision

• Rods contain a single visual pigment (rhodopsin),sensitive to blue-green light.

• Cons contain visual pigment (iodopsin),sensitive to short ,medium or long wavelengths of light,permitting to color perception.

Duplex theory of vision

• Rods are very sensitive to light and are responsible for scotopic vision,where cones are much less sensitive to light and are responsible for photopic vision.

• Both photoreceptors are active during mesopic vision.

Cont..

Duplex theory of vision

• Rods and cones are unevenly distributed. • The density of rods exceeds that of cones

, except that of the fovea where the cone density is highest and it’s central region (Fovea) is rod-free.

Cont..

ROD SYSTEM CON SYSTEM

ACROMATIC

HIGH CONVERGENCE

HIGH LIGHT SENSITIVITY

LOW VISUAL ACUITY

CROMATIC

LOW CONVERGENCE

LOW LIGHT SENSITIVITY

HIGH VISUAL ACUITY

BIPOLAR CELL

Receptive field of 2 different types of bipolar cell.

1.ON- CENTER bipolar cell : -tends to form a specialized synapse

(TRIAD), at their synapse with photoreceptors. At this specialized synapse,a bipolar cell and 2 horizontal cells form an invagination within photoreceptor.

Cont..

2. OFF CENTER bipolar cell -bordered by an excitatory on-

surround. Synaptic arrangement associated with this off-center bipolar cell is a conventional flat synapse.

Cont..

Under dark conditions, photoreceptors continuously release neurotransmitter.

Under light condition, phoreceptors progressibely decrease neurotransmitter.

Cont..

That’s for on-center bipolar cells ,glutamate is inhibitory .causes a relative excitation (depolarization) of the bipolar cell.

For off-center bipolar cells ,same neurotransmitter has the opposite effect.

Ganglion cell

1.Light falling on the receptor field center excites the cell

- Increase frequency of action potential 2.Light falling on the receptive field

surroud inhibit the cell -produces reduce in the frequency of

action potentials .

cont..

• (other ganglion cells have the reverse arrangement).

.Light falling on the receptive field center –produces a decrease in the action potential .

- where as light falling on the surround –produces an increase in frequency.

References

1 . Visual perception 2.Anatomy and physiology of eye by A.K. Khurana 3.Internet

THANKS to all