Focusing is accomplished by contraction and relaxation of ciliary muscle which

Focusing is accomplished by contraction and

This preview shows page 17 - 19 out of 24 pages.

Focusing is accomplished by contraction and relaxation of ciliary muscle, which adjusts the curvature of the lensVertebrate photoreceptors are rod cells and cone cells oRods Black and white vision when illumination is dimHave a rod shaped outer segment100 million rods in each retina No rods in the foveaoCones High visual acuity (sharpness) and color vision Have a cone shaped outer segment3 million cones in each retina Most cones are located in central region of the retina Fovea, where the eye forms its sharpest image oStructure of rods and cones Rods and cones have same basic cellular structure An inner segment rich in mitochondria has numerous vesicles filled with neurotransmitter molecules Connected by a narrow stalk to the outer segment which is packed with hundreds of flattened disks stacked on topof each other Light capturing molecules or photopigments are located on the membranes of these disks In rods Photopigmet is called rhodopsin oHas protein opsin bound to a molecule of cis-retinal which is produced by vitamin A Vitamin A is derived from carotene, a photosynthetic pigment in plants In cones Photopigment is photopsins oStructurally very similar to rhodopsin
Background image
oHumans have three kinds of cones, each of which has a photopsin consisting of cis-retinal bound to aprotein with a slightly different amino acid sequence FIGURE 44.18 for diagrams of rods and cones These differences shift the absorption maximum, the region of the electromagnetic spectrum that is best absorbed by the pigment Absorption max of the cis-retinal in rhodopsin is 500 nmAbsorption max of three kinds of cone photopsins are o420 nm (blue-absorbing)o530 nm (green-absorbing) o560 nm (red-absorbing)These differences in the light absorbing properties of the photopsins are responsible for different color sensitivitiesof three kinds of cones, which are referred to as simply blue, green and red conesoFocusing on the human eye In people with normal vision, the image remains focused on the retina in both near and far vision bc of the changes produced incurvature of lens Person with normal vision stands 20 feet or more from anobject, the lens is in its least convex form and the image is focused on retinaIn nearsighted people, the image comes to focus in front of the retina and the image appears blurred In farsighted people, the focus on the image would be behind the retina bc the distance from the lens to the retina is too short Corrective lenses adjust the angle of the light as it entersthe eye, focusing it on the retina FIGURE 44.17othe retinathe inside surface of the eye made up 3 layers of cells layer closest to external surface of eyeball has rods and cones next layer has bipolar cells layer closest to cavity of eye has ganglion cells light must pass through ganglion cells and bipolar cells in orderto reach the photorecepotrs rods and cones synapse with the bipolar cells and the bipolar cells synapse with the ganglion cells which transmit impulses to
Background image
Image of page 19

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture