- a one-celled flagellated organism. These organisms each have an eyespot, a structure that contains a group of photosensitive pigments embedded in the membrane. - The flatworm has photosensitive eye cups that relay information to the worm’s primitive nervous system.

- Arthropods have compound eyes. A compound eye is made up of many smaller units called ommatidia. A single ommatidium is made up of a lens and associated receptor proteins. - The ommatidium is made up of seven retinula cells. Retinula cells are elongated and surround the crystalline cone. The crystalline cone focuses light on the center of the ring of retinula cells. - The inner edges of the retinula cells are covered in microvilli. These microvilli contain rhodopsin. The microvilli from different cells overlap forming the rhabdom. The light signals are channeled to nerve axons at the base of the ommatidia and then to the arthropod’s brain.

- Light passes through the cornea and the lens. The lens, convex in shape, projects the image upside down. The image passes through the vitreous humor and is focused on the back of the eye, the retina (labeled retinal layer). The message goes from the retina to the brain via the optic nerve. - Embedded in the retina are retinal cells. Retinal cells are modified neurons (top left diagram). There are two types of retinal cells, rods and cones. - In the human, there are about 3 million cones in the center of the fovea. Cones function in perceiving color and providing sharpness of the image. Rods are found outside of the fovea and do not perceive color and are very light sensitive. - Light travels through the first four layers of the retina where it is absorbed by rods and cones. The sensory signal is then processed back from the rods and cones, through the four cell layers and after reaching the ganglion cells, it is finally sent to the optic nerve.

Which of the following statements regarding the human retina is false?

- The sensory signal is passed directly from the rod cells to the ganglion cells.

12.16.2 Photoreceptors and the Vertebrate Eye Flashcards by Irina Soloshenko

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12.16.2 Photoreceptors and the Vertebrate Eye Flashcards

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