Physiology of Vision Retina Dr. Sumera Gul

Learning Objectives At the end of the lecture the students should be able to: Explain the physiological anatomy of retina Explain the importance of each layer of retina Discuss the fovea and its importance Explain the importance of melanin in the pigmented layer

Review of Previous Lecture

Reduced Eye In the reduced eye, a single refractive surface is considered to exist, Its central point 17 millimeters in front of the retina total refractive power of 59 diopters when the lens is accommodated for distant vision. About two thirds of the 59 diopters of refractive power of the eye is provided by the anterior surface of the cornea

This ability to determine distance is called depth perception

A person normally perceives distance by three main means: (1) the sizes of the images of known objects on the retina (2) the phenomenon of moving parallax, (3) the phenomenon of stereopsis.

Out flow of Aqueous Humor through the pupil into the anterior chamber of the eye into the angle between the cornea and the iris through a meshwork of trabeculae canal of Schlemm extraocular veins

Vitreous Humor Vitreous Humor is the clear gelatinous fluid in the posterior chamber (back 2/3) of the eye. The outer surface of the Vitreous humor is attached to the retina. Vitreous body, is a gelatinous mass held together by a fine fibrillar network composed primarily of greatly elongated proteoglycan molecules. Both water and dissolved substances can diffuse slowly in the vitreous humor, but there is little flow of fluid.

Retina

Retina Light sensitive Contains rods and cones

Layers of Retina From the outside to the inside as follows: Pigmented layer, Layer of rods and cones projecting to the pigment Outer nuclear layer containing the cell bodies of the rods and cones, Outer plexiform layer Inner nuclear layer, Inner plexiform layer, Ganglionic layer, Layer of optic nerve fibers, Inner limiting membrane.

Fovea

Fovea 1 square mm area in center of retina Central fovea is 0.3 mm Central fovea is composed almost entirely of cones with a diameter of only 1.5 micrometers. Long and slender cones in central fovea All the layers are displaced to the sides

Rods and Cones 100 million rods and 3 million cones in each retina In general, the rods are narrower and longer than the cones In the peripheral portions of the retina, the rods are 2 to 5 micrometers in diameter, whereas the cones are 5 to 8 micrometers in diameter

Functional Segment of Rod and Cones (1) the outer segment, (2) the inner segment, (3) the nucleus, (4) the synaptic body.

The light-sensitive photochemical is found in the outer segment. In the case of the rods, this photochemical is rhodopsin In the cones, it is one of three “color” photochemicals, usually called simply color pigments, conjugated proteins.

In the outer segments of the rods and cones are large numbers of discs. Each disc is actually an infolded shelf of cell membrane. There are as many as 1000 discs in each rod or cone.

Functions of Inner segment Nucleus Synaptic Body

Pigmented Layer Pigmented layer has melanin Melanin in the pigment layer prevents light reflection throughout the globe of the eyeball Makes the image clear Stores large quantities of Vitamin A

Absence of melanin in Albinos When they enter a bright room Light is reflected in many directions after impinging retina Visual acuity is usually 20/100 to 20/200

Questions?