Figure 12.33

Figure 12.34

Figure 12.35

Posterior Cavity Space enclosed by lens, ciliary body, retina Filled with vitreous humor –jelly-like fluid –supports internal structures –maintains shape

Light Refraction Bending of light Occurs when light passes at an oblique angle from a medium of one optical density into a medium of a different density Convex surfaces cause light to converge Concave surfaces cause light to diverge Figure 12.36

Figure 12A

Figure 12B

Figure 12.38

Visual Receptors Stimulated when light reaches the receptors Rods –sensitive to light, provide vision in dim light –produce black and white outline images Figure 12.39

Visual Receptors Cones –provide color vision and sharp images –fovea centralis of the macula lutea has cones and lacks rods

Visual Pigments Rods and cones contain light-sensitive pigments Rods contain rhodopsin embedded in membranous disks in the photoreceptors Rhodopsin or visual purple –opsin: colorless protein –retinal: vitamin A

Figure 12.40

Response to Light In darkness, sodium channels are kept open by cGMP. When rhodopsin absorbs light, they release opsin which activates transducin. Transducin activates phosphodiesterase which breaks down cGMP. Sodium channels close resulting in hyperpolarization of the receptor.

Response to Light In bright light, almost all the rhodopsin decomposes, reducing the rod sensitivity. Cones function in bright light resulting in color vision. In dim light, rhodopsin is regenerated from opsin and retinal so rods can function. Cones are not stimulated in dim light.

Cones Cones contain iodopsins –retinal and cone-specific protein Three cone pigment –erythrolabe: responds to red light –cyanolabe: responds to blue light –chlorolabe: responds to green light

Visual Nerve Pathways Ganglion cell axons leave the eye to form the optic (II) nerve. The nerve fibers cross anterior to pituitary gland, the optic chiasma. Nerve fibers travel to the thalamus and to reflex centers. Optic radiations are nerve pathways that lead to the visual cortex in the occipital lobe.

Figure 12.42