General aspects Sensory receptors ; External or internal environment. A stimulus is a change in the environmental condition which is detectable by a sensory receptor 1 Major structural layer of the wall of the eye is a thick layer of dense C.T.; that layer has two parts: relatively thin flat, stratified squamous epithelium covers the cornea and sclera continuous with (essentially a part of) the epidermis of the skin 2
At the interface between two media of different densities, a non-perpendicular light ray bends. Refraction. Once the light rays leave the posterior surface of the cornea they pass through the aqueous humor that fills the anterior chamber of the eye. 3 Iris and pupil Iris is flat, circular structure with a hole in its center called the pupil. Mostly loose C.T. Loose C.T. of the iris may have a scattered melanocytes; 4
Iris contains two smooth muscles: Contraction and relaxation by those two elements, controlled by the brain stem, control the diameter of the pupil 5 Lens and ciliary muscle Light rays passing through the pupil enter the lens.. The lens is attached to the outer wall of the eye all around its circumference, by 6
In mammals, the sclera usually exerts a tension on the edge of the lens, through the ciliary body and suspensory ligament Ciliary body is composed mainly of a circular band of smooth muscle tissue called the ciliary muscle 7 8
When the ciliary muscle contracts it pulls the sclera inward, all around; this increases the degree of light refraction in two ways: Greater light-refraction the eye can focus the light rays from a closer object. 9 Light rays leaving the lens pass through the vitreous humor of the posterior compartment of the eye, to the retina. 10
Retina Pigmented layer of the retina 11 Sensory layer of the retina (see retina.pdf) Non-foveal retina (structures listed in order of penetration by light rays) 12
Nerve fiber layer 13 Ganglion cell layer 14
Inner plexiform layer 15 Inner nuclear layer 16
Outer plexiform layer 17 Outer nuclear layer 18
Rod cells and cone cells General aspects Rod cells and cone cells are photoreceptor cells; they are photosensory cells Rod cells: structure and function 19 The rod-has two parts Inner segment Outer segment 20
Outer segment of rod: consists of a cylindrical stack of interconnected disc-shaped photosensitive lamellae. These lamellae are produced at the base of the rod by inward extensions of the cell membrane 21 These disc-shaped membranes are filled with the rhodopsin, aka visual purple. Opsins (or scotopsin) are a class of very similar proteins that are parts of the photopigment molecules of the sensory cells of the retina. 22
23 24
Cone cells: structure and function Cone cells are similar to rod cells in some ways 25 Outer segment of the cone is composed of lamellae which contain one of three kinds of iodopsin photopigment (or photopsins) molecules. Rao 2004 26
Thus the brain can receive an impulse rate ratio from the three cone cell types that provides information on the color of the object the perceived color depends, in addition, on additional steps of information-processing in the brain itself, including comparisons with the information received from adjacent parts of the image Rao 2004 27 Layer of rods and cones External limiting membrane Supportive-cells of the sensory layer of the retina 28
Light-scattering by layers inside the layer of rods and cones 29 Foveal retina 30
Fovea is cone cells only. 31 Choroid layer of the wall of the eye Choroid layer lies immediately adjacent to the retina, and thus between the retina and the sclera. 32
Optic disc ("blind spot"): the part of the back of the eye where the optic tract begins. 33 This results in a circular area where the other components of the sensory layer of the retina, as well as other layers of the wall of the eye, are absent 34