The Special Senses Vision 2 Professor A M

The Special Senses Vision - 2 Professor A. M. A Abdel Gader MD, Ph. D, FRCP (London & Edinburgh), FRSH (London) 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 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 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 Light Receptor: Retina (Photoreceptors) • Stimulus: •

Light • Definition: ‘elctromagnetic’ radiation that is capable of exciting the human eye’ • Extremely fast

Which travels faster: light or sound?

Electromagnetic spectrum & The visible light spectrum

The Electromagnetic Spectrum

Visible light & Duplicity Theory of vision Visible light Spectrum • Extends from 397 to 723 nm • 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 505 nm • Scotopic “” ” “ “ 550 nm

Photoreceptors Rods & Cones Morphology & Distribution

Retina Back of retina, pigment epithelium (Choroid) Light

Rods and Cones Figure 17. 13

Photoreceptors Figure 16. 11

Retina: distribution photoreceptors

Receptor density (cells x 103 / mm 2) Distribution of photoreceptors

Normal Fundus Photoreceptors are not distributed uniformly across the retina Optic disc Macula 5000 um 650, 000 cones Fovea 1500 um 100, 000 cones Foveola 350 um 25, 000 cones

Human foveal pit INL Light ONL Foveola

Low Convergence Cone-Fed Circuits Retinal ganglion cell Bipolar cell Cone High Convergence Rod-Fed Circuits Retina ganglion cell Bipolar cell Rod Convergence rod/cone cells

Retina: photoreceptors • 100, 000 rods • 5, 000 cones Cones Fovea High light levels Color Good acuity Rods Periphery Low light levels Monochromatic Poor 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 “All-or-None” Receptor Potential Local & Graded

Retina: Neural Circuitry Light hits photoreceptors, sends signal to the bipolar cells Bipolar cells send signal to ganglion cells Ganglion cells send signal to the brain

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

outer segment Disk membrane Intracellular disk Intracellular space Disk membrane Extracellular space Visual pigment Extracellular space Intracellular space Visual pigment Plasma membrane Connecting cilium ROD CELL CONE CELL Rods and Cones

Rods Light Environment Dim light - scotopic Bright light - photopic Spectral sensitivity 1 pigment 3 pigments Color discrimination No Yes Absolute sensitivity High Low Speed of response Slow Fast Rate of dark adaptation Fast Slow Starlight Moonlight No color vision Poor acuity Scotopic Absolute threshold Cones Indoor lighting Good color vision Best acuity Mesopic Cone threshold Sunlight Photopic Rod Saturation begins Best acuity Indirect Ophthalmoscope Damage Possible Comparison Scotopic and Photopic systems

Photoreceptor pigments

Photoreceptor pigments • Composition: – Retinine 1 (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: retinine 1 in the 11 -cis configuration Light All-trans isomer Metarhodopsin II Closure of Na channels

Visual cycle Rhodopsin Light Prelumirhdopsin Inermediates including Metarhodopsin II Vitamin A + Scotopsin Retinine & Scotopsin

Light Change in photopigment Metarhodopsin II Activation of transducin Activation of phophodiesterase Decrease IC cyclic GMP Closure of Na channels Hyperpolarization of receptor Decrease release of synaptic tramitter Action potential in optic nerve fibres

From light reception to receptor potential

Retina: Neural Circuitry Light hits photoreceptor s, sends signal to the bipolar cells Bipolar cells send signal to ganglion cells Ganglion cells send signal to the brain

Photoreception

Photoreception- cont.

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

Convergence Cones • Photoreceptors • Ganglion cells Rods

Convergence and Ganglion Cell Function Figure 17. 18

Dark adaptation

Dark adaptation: Increased sensitivity of the photoreceptors 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 deficiency What happens to Dark adaptation? Night blindness (Nyctalopia)