Review of anatomy physiology of the Retina Dr

Review of anatomy & physiology of the Retina Dr. Ayesha Abdullah 17. 12. 2015

Learning outcomes By the end of this lecture the students would be able to; • Correlate the structural organization of the retina with its functions and development. • Identify structural landmarks on retinal photographs. • Name the investigations commonly employed for the assessment of various categories of retinal disorders.

Camera and the eye

• Unlike the image from a camera the resolution of the retinal image is not uniform. • Why? • What is the result? • There about 100 times more photoreceptors than the Ganglion cells. • Retina compresses images as unlike the camera.

Anatomical landmarks of the retina

Normal dimensions Anatomical characteristics Clinically Observable characteristics Optic disc Place where the optic nerve fibers leave the retina. It is devoid of rods and cones hence the blind spot. Contains the central retinal artery and vein It’s a pale disc like structure with vessels emerging out of its center called the cup. Its about 1. 5 mm in size. Macula It is the area where the ganglion cells are two layered. Contains the xanthophyl pigment giving it the pigmented look. It is about 5. 5 mm in diameter (3. 5 disc diameter/ 180 of visual angle). Roughly the area between the arterial arcades. Fovea A depression in the inner retinal surface. It contains cones only. The inner nuclear layer and the ganglion cell layer is absent. A concave central retinal depression about the same size as the disc (1. 5 mm) The thickest part of the retina Area surrounding the fovea Foveola Parafovea

Histological structure of the retina

Development of the retina

Functions of the retina Light perception Brightness appreciation Contrast sensitivity Two point discrimination and appreciation of details • Colour perception • Light and dark adaptation • Circadian rhythms & hormonal balance • •

Some important facts 1. There about 150 million receptors and only 1 million optic nerve fibers, there must be convergence and thus mixing of signals 2. The horizontal action of the horizontal and amacrine cells can allow one area of the retina to control another (e. g. , one stimulus inhibiting another). This inhibition is key to the sum of messages sent to the higher centers of the brain. 3. The response of cones to various wavelengths of light is called their spectral sensitivity 4. There are blue, green, and red cones but more accurately short, medium, and long wavelength sensitive cone subgroups- trichromatic vision

Some important facts 5. The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron 6. The receptive field of a Ganglion cell in the retina of the eye is composed of input from all of the photoreceptors which synapse with it, and a group of ganglion cells in turn forms the receptive field for a cell in the brain. This process is called convergence.

vitreous RPE

Rods & Cones • Rod System – Achromatic – High convergence – High light sensitivity – Low visual acuity • Cone System – Chromatic – Low convergence – Low light sensitivity – High visual acuity

Direction of visual impulse Direction of light

Investigations for retinal structural and functional assessment • Clinical assessment- Ophthalmoscopy

Ophthalmic investigations • • • Ultrasound –B & A scans Ocular coherence tomography (OCT) Angiography Elctroretinogram Elctro-oculogram

OCT

Angiography

Electroretinogram

Homework • What is the blood supply of the inner and outer retinal layers? • What makes the inner and outer retinal blood barriers and what is their significance? • Why light has to travel through all the layers before generating the sensation of vision?