Youtor
Photorecptors in invertebratesInvertebrate Sensory Receptorschapter24
>> YOUR LINK HERE: ___ http://youtube.com/watch?v=xu7IP9XlCnI
#millerandharleyzoology #invertebratesensorysystem #sirpervaizkhan #GhazianZoologist #photoreceptors #stigma #ocelli #compoundeye #ommatidium • Photoreceptors are sensitive • to light. All photoreceptors possess light-sensitive pigments • (e.g., carotenoids and rhodopsin). These pigments absorb photons of light energy and then produce a generator potential. Beyond this basic commonality, the complexity and arrangement of photoreceptors within various animals vary incredibly. • Certain flagellated protozoa (Euglena) that contain chlorophyll possess a mass of bright red photoreceptor granules • called the stigma. The granules are carotenoid pigments. The actual photoreceptor is the swelling at the base of the flagellum. The stigma probably serves as a shield, which is essential if the Photoreceptors is to detect light coming from certain directions but not • from others. Thus, the photoreceptor plus the stigma enable Euglena to orient itself so that its photoreceptor is exposed • to light. This helps the protozoan maintain itself in the region of the water column where sufficient light is available for photosynthesis. • Some animals, such as the earthworm Lumbricus, have simple unicellular photoreceptor cells scattered over the epidermis or concentrated in particular areas of the body. Others possess multicellular photoreceptors that can be classified into three basic types: ocelli, compound eyes, and complex eyes. • An ocellus (L. dim. of oculus, eye) (pl., ocelli) is simply a small cup lined with light-sensitive receptors and backed by • light-absorbing pigment. • The light-sensitive cells are called retinular cells and contain a photosensitive pigment. Stimulation by light causes a chemical change in the pigment, leading to a generator potential, which causes an action potential that sensory neurons carry • for interpretation elsewhere in the animal’s body. This type of visual system gives an animal information about light direction • and intensity, but not image formation. Ocelli are common in many phyla (e.g., Annelida, Mollusca, and Arthropoda). • Compound eyes consist of a few to many distinct units called ommatidia. Although compound eyes occur in some annelids and bivalve molluscs, they are best developed and understood in arthropods. A compound eye may contain thousands of ommatidia, each oriented in a slightly • different direction from the others as a result of the eye’s overall convex shape. The visual field of a compound eye is very wide, as anyone who has tried to catch a • fly knows. Each ommatidium has its own nerve tract leading to a large optic nerve. The visual fields of adjacent ommatidia • overlap to some degree. Thus, if an object within the total visual field shifts position, the level of stimulation of several • ommatidia changes. As a result of this physiology, as well as a sufficiently sophisticated central nervous system, compound eyes are very effective in detecting movements and are probably capable of forming an image. In addition, most compound eyes can adapt to changes in light intensities, and some provide • for color vision. Color vision is particularly important in active, day-flying, nectar-drinking insects, such as honeybees. Honeybees learn to recognize particular flowers by color, scent, and shape. • The complex camera eyes of squids and octopuses are the best image-forming eyes among the invertebrates. In fact, the giant squid’s eye is the largest of any animal’s, exceeding 38 cm in diameter. Cephalopod eyes are often compare to those of vertebrates because they contain a thin, transparent cornea and a lens that focuses light on the retina and is suspended by, and controlled by, ciliary muscle. However, the complex eyes of squids are different from the vertebrate eye in that the receptor sites on the retinal layer face in the direction of light entering the • eye. In the vertebrate eye, the retinal layer is inverted, and the receptors are the deepest cells in the retina. Both eyes are • focusing and image-forming, although the process differs in detail. In terrestrial vertebrates, muscles that alter the shape • (thickness) of the lens focus light. In fishes and cephalopods, light is focused by muscles that move the lens toward or • away from the retina (like moving a magnifying glass back and forth to achieve proper focus), and by altering the shape • of the eyeball.
#############################
