The Special Senses (1433)Vision – 2

Photo-transduction

Professor A.M.A Abdel GaderMD, PhD, FRCP (London & Edinburgh)

Professor of Physiology, College of Medicine &King Khalid University Hospital

Riyadh, Saudi Arabia

The Physiology of Vision

Objectives: At the end of this lecture the student should be able to:• Understand the optical bases of image formation on the retina• Understand and explain the optical bases of common refractive errors• Understand the electrical bases of the photoreceptor function• Understand the nature and function visual pigments Understand color vision

The Physiology of Vision

Objectives: At the end of this lecture the student should be able to:• Understand the optical bases of image formation on the retina• Understand and explain the optical bases of common refractive errors• Understand the electrical bases of the photoreceptor function• Understand the nature and function visual pigments Understand color vision

Physiology of Vision - 1433

• Stimulus: Light• Receptor: Photoreceptors (Retina)

Light

Definition: Elctromagnetic radiation

that is capable of exciting the human eye (photoreceptors)

•Extremely fast

Which travels faster: light or sound?

Electromagnetic spectrum &The visible light spectrum

Visible light & Duplicity Theory of

vision Visible light Spectrum• Extends from 397 to 723nm• Eye functions under two 2

conditions of illumination: – Bright light (Photopic vision)…

Cones– Dim light (Scotopic vision) ..Rods

Duplicity theory of vision

Duplicity theory

• Photopic visibilty curve peaks at 505nm• Scotopic “” ” “ “ 550nm

Photoreceptors:Rods & Cones

Morphology & Distribution

Retina

Back of retina,

pigment epitheliu

m

(Choroid)

Light

Figure 17.13

Rods and Cones

Retina: distribution photoreceptors

Distribution of photoreceptors

Rec

epto

r de

nsity

(ce

lls x

103

/ mm

2 )

Normal Fundus

Optic disc

Photoreceptors are not distributed uniformly across the retina

Macula 5000um 650,000 cones

Fovea1500um

100,000 cones

Foveola350um

25,000 cones

Human foveal pit

ONL

INL

Foveola

Light

Convergence rod/cone cells

Low Convergence Cone-Fed Circuits

Retinalganglioncell

Bipolarcell

Cone

High Convergence Rod-Fed Circuits

Retinaganglion cell

Bipolar cell

Rod

Retina: photoreceptors

• 100,000,000 rods

• 5,000,000 cones

ConesRods

FoveaPeriphery

High light levelsLow light levels

ColorMonochromatic

Good acuityPoor acuity

Electrophysiology of Vision

Genesis of electrical responses

Retinal photoreceptors mechanism

Light

Absorption by photosensitive substances

Structural change in photosensitive substances

Phototransduction

Action potential in the optic nerve

Action Potential Propagated and obeys

“All-or-None”

Receptor Potential

Local & Graded

Ganglion cells send signal to the brain

Bipolar cells send signal to ganglion cells

Retina: Neural CircuitryLight hits

photoreceptors, sends

signal to the bipolar cells

In Darkness

Photoreception-cont.

Retina

Light

Electrophysiology of Vision

Electric recording in Retinal cells:•Rods & Cones: Hyperpolarization•Bipolar cells: Hyper- & Depolarization

•Horizental cells: Hyperpolarization

•Amacrine cells: Depolarizing potential

•Ganglion cells:Depolarizing potential

Intracellular disk

Disk membrane

Connectingcilium

outer segment outer segment

Disk membrane

Intracellularspace

Extracellularspace

Visualpigment

Visualpigment

Extracellularspace

Plasmamembrane

Intracellularspace

Connectingcilium

Rods and Cones

ROD CELL CONE CELL

Comparison Scotopic and Photopic systems

Light EnvironmentDim light - scotopicBright light - photopic

Spectral sensitivity1 pigment3 pigments

Color discriminationNoYes

Absolute sensitivityHighLow

Speed of responseSlowFast

Rate of dark adaptationFastSlow

Starlight Moonlight Indoor lighting Sunlight

Absolutethreshold

Conethreshold

Rod Saturation

begins

Best acuity

IndirectOphthalmoscope

DamagePossible

Scotopic Mesopic Photopic

No color visionPoor acuity

Good color visionBest acuity

Rods

Cones

Photoreceptor pigments

Photoreceptor pigments

•Composition:– Retinine1 (Aldehyde of vitamin A)

•Same in all pigments

– Opsin (protein) •Different amino acid sequence in different pigments

Rhodopsin (Rod pigment): Retinine +

scotopsin

Photoreceptor compounds-cont

Rhodopsin (visual purple, scotopsin):

Activation of rhodopsin:•In the dark: retinine1 in the 11-cis configuration

All-trans isomer

Metarhodopsin IIClosure of Na channels

Light

Visual cycle Rhodopsin

Prelumirhdopsin

Inermediates includingMetarhodopsin II

Vitamin A + Retinine & ScotopsinScotopsin

Light

Light

Change in photopigment

Metarhodopsin II

Activation of transducin

Activation of phophodiesterase

Decrease IC cyclic GMP

Closure of Na channels

Hyperpolarization of receptorDecrease release of synaptic tramitterAction potential in optic nerve fibres

From light reception to receptor potential

Ganglion cells send signal to the brain

Bipolar cells send signal to ganglion cells

Retina: Neural CircuitryLight hits photoreceptors, sends signal to the bipolar cells

Photoreception

Photoreception- cont.

Retina

•100,000,000 rods•5,000,000 cones•1,000,000 ganglion cells

Convergence

Convergence

Cones Rods

• Photoreceptors

• Ganglion cells

Figure 17.18

Convergence and Ganglion Cell Function

Dark adaptationDark adaptation

:Dark adaptationIncreased sensitivity of thephotoreceptors when vision shifts

from bright to dim light

Dark adaptation

• Reaches max in 20 minutes

• First 5 minutes …… threshold of cones

• 5 to 20 mins ……. Sensitvity of rods

Mechanism of dark adaptation: Regeneration of rhodopsin

Dark adaptation-cont.

In vitamin A deficiencyWhat happens to Dark

adaptation?

Night blindness(Nyctalopia)

THANK YOUSHUKRAN GAZEELAN