Physiology of Vision By Dr Abdel Aziz M

Physiology of Vision By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology

The Retina • The retina is the innermost photosensitive layer of the eyeball. • It contains about 55 types of cells and 70% of total sensory receptors • Histologically, it is formed of 10 layers • Physiologically, 4 layers have special importance

The Retina

Functional Layers Pigmented Epithelium Photoreceptors Bipolar cells Ganglion cells

Pigmented Epithelium

Pigmented Epithelium

Pigmented Epithelium It contains large amounts of melanin pigments which absorb light produces a sticky extracellular matrix material that keeps the outer segments of rods and cones straight and aligned participates in the process of the breakdown and resynthesis of the photopigments helps the continual renewal of the outer segment by ingesting the old outermost tips of the rods and cones. stores large amount of vit. A needed for photopigments recycling

Photoreceptor Layer

Structure of Rod and cone Outer segment Inner segment

Structure of Rod and cone

Number of Rods and Cones Rod 120 millions Rod One pigment (rhodopsin) Cones 7 millions Cond 3 pigments (iodopsin)

Distribution of Rods and Cones cones are central Rods are peripheral

Convergence of Rods and Cones

Functions of Rods and Cones Rod Cone more sensitive to light less sensitive to light Less accurate more accurate Cannot perceive colours Can perceive colours Night vision Day vision

Ganglion and Bipolar cells

Ganglion and Bipolar cells 1 st order neuron Ganglion cells 2 nd order neuron

Blood Supply of the Retina

Blood Supply to the Retina Central retinal artery Supply inner layer of retina Choroidal blood vessels Supply pigmented epithelium and photoreceptors

Blood Supply to the Retinal detachment (occurs between the pigmented epithelium and photoreceptors) In this case the retina can resist degeneration for days due to its own blood supply (from central retinal artery) and diffusion of fluids across the detachment gap. If the retina is not replaced rapidly within few days degeneration of the retina occurs.

Special Areas of the Retina

Special Areas of the Retina • Fundus oculi is the part of the retina which can be seen by ophthalmoscope. Fovea centralis Optic disc

Fovea Centralis Fovea centralis

Phototransduction

Phototransduction 1. Decomposition of photosensitive pigment in rods & cones 2. Excitation of the photoreceptors by activated rhodopsin and generation of photoreceptor potential 3. Termination of excitation 4. Regeneration of photopigments

Decomposition of Pigment

Photoreceptor Potential In Dark

Photoreceptor Potential In Light

Termination of Excitation • Within a sec the activated rhodopsin is inactivated by the enzyme rhodopsin kinase

Regeneration of Pigment

Summary of Generation of Photoreceptor Potential

THANKS