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    Biology II Essentials
    Biology II Essentials
    Biology II Essentials
    Ebook175 pages1 hour

    Biology II Essentials

    By Editors of REA and Lauren Gross

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    About this ebook

    REA’s Essentials provide quick and easy access to critical information in a variety of different fields, ranging from the most basic to the most advanced. As its name implies, these concise, comprehensive study guides summarize the essentials of the field covered. Essentials are helpful when preparing for exams, doing homework and will remain a lasting reference source for students, teachers, and professionals. Biology II includes stimulus receptors in animals, nervous systems, locomotion, elements of behavior, reproduction of cells and organisms, genetic inheritance, embryonic development, evolution, and ecology.
    LanguageEnglish
    PublisherResearch & Education Association
    Release dateJan 1, 2013
    ISBN9780738671451
    Biology II Essentials

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      Biology II Essentials - Editors of REA

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      CHAPTER 9

      STIMULUS RECEPTORS IN ANIMALS

      9.1 COMPONENTS OF NERVOUS COORDINATION

      Receptor - This is a structure that must have the ability to detect a change in the environment and initiate a signal in the nerve cell.

      Conductors - These are the nerves which conduct impulses.

      Effectors - These are the structures that respond to the stimuli, which is information received by receptors.

      9.2 PHOTORECEPTORS

      The photoreceptive system includes the sense of sight. The eye is a delicate receptor which is sensitive to light. Tears secreted by tear glands help in keeping the surface of the eyeball moist and dust-free. The eyelashes help to keep dust particles out of the eye while the eyelids help spread tears over the eyeball surface.

      Fig. 9.1 The human eye

      Sclera - This is the outer layer of the eyeball. It is white in color except for the cornea which is transparent and admits light into the eye interior.

      Choroid - The choroid is the middle layer of the eye. Its purpose is to stop the reflection of scattered light within the eye. The choroid coat forms the iris of the eye which has a central opening known as the pupil. The pupil permits light to enter the eye. The lens of the eye is located behind the pupil.

      Retina - The retina is the innermost layer of the eye. It contains two types of receptor nerve cells known as the rods and cones which are the actual visual receptors.

      Rods - These are sensitive to light and are basically used for vision in dim light.

      Cones - These function as bright-light color receptors. Both rods and cones are connected to a network of nerve cells which forms the optic nerve. The optic nerve is connected to the brain.

      9.3 VISION DEFECTS

      Nearsightedness (Myopia) - In most cases of nearsightedness, the eyeball is too long, thus, the retina is too far from the lens. The light rays converge in front of the retina and diverge when they reach it. This results in a blurred image. Eyeglasses with concave lenses, correct this defect.

      Farsightedness ( Hypermetropia) - In most cases of farsightedness, the eyeball is too short so the retina is too close to the lens. Light rays will strike the retina before they converge which results in a blurred image. Eyeglasses with convex lenses correct farsightedness.

      Astigmatism - In astigmatism, there is an irregularity in the curvature of the cornea or the lens. This causes a blurred image. Astigmatism can be corrected with lenses that correct the irregular curvature of the cornea or the lens.

      Fig. 9.2 Illustrations of normal eye and common eye defects with corrective lenses.

      A. NORMAL EYE

      B. FAR-SIGHTED EYE

      C. ASTIGMATIC EYE a. Sharp image b. Blurred image c. Correction

      D. NEAR-SIGHTED EYE

      9.4 CHEMORECEPTORS

      The senses of taste and smell comprise the chemoreceptive system. These are the receptors of chemicals in the external environment.

      Smell - The nose is the organ in humans which detects odors by means of receptor cells in two olfactory epithelia. An individual receptor consists of a cell with tiny hairs at one end and a nerve cell fiber at the other end. Present theories suggest that active sites on the receptor cells join with specific odor molecules. This combination forms a complex that generates a signal in the receptor cell. The signal then passes through a nerve fiber which is part of the olfactory nerve, to the brain for interpretation.

      Fig. 9.3 Receptors in the nose

      Taste - The taste buds, which are located on the tongue, are the principal receptors of chemical stimuli in the external environment. There are four different types of taste buds which, when stimulated, initiate one primary taste sensation such as sweet, sour, salty, or bitter. Each type of taste receptor cell has its own specific active site which combines with the specific food molecule.

      Fig. 9.4 Distribution of the taste buds sensitive to salty, bitter, sweet, and sour tastes.

      Fig. 9.5 Taste bud cells

      9.5 MECHANORECEPTORS

      The senses involving touch and hearing comprise the category of mechanoreceptors. These receptors are sensitive to mechanical stimuli such as pressure or compression.

      Touch - In man, touch is detected by receptors near the surface of the skin next to a hair follicle. The Pacinian corpuscle is a receptor found in the skin and in some internal organs. Each Pacinian corpuscle is connected to a sensory neuron. It is a pressure receptor; therefore, any application of pressure will deform the corpuscle.

      Proprioreceptors are sense receptors distributed throughout skeletal muscle and tendons. Any muscle contraction or stretching will trigger the receptors to initiate nerve impulses.

      Hearing - The organs of hearing and equilibrium are found in the ear. The ear is especially sensitive to sounds of varying frequencies and intensities. The human ear consists of an outer ear, a middle ear, and an inner ear.

      Outer ear - The outer ear consists of the pinna, which is an ear flap and the auditory canal which provides the passageway for waves to the middle ear. The eardrum separates the outer ear from the middle ear.

      Middle ear - The middle ear contains three tiny bones known as the hammer, anvil, and stirrup. These bones transmit sound waves across the middle ear cavity. The Eustachian tube connects the middle ear to the pharynx and equalizes the pressure between the outer ear and middle ear.

      Inner ear - The inner ear consists of the cochlea and three semicircular canals. Lying within the inner chamber of the cochlea is the organ of Corti, which contains the vibration receptors. Nerve impulses, once initiated, travel along the auditory canal to the brain for interpretation as sound.

      Fig. 9.6 The structure of the ear

      9.6 RECEPTORS IN OTHER ORGANISMS

      Protozoans - The plasma membrane in these organisms is the receptor for external stimuli.

      Hydra - The hydra possesses nerve endings in the ectoderm and endoderm layers that are sensitive to light, touch, and chemicals.

      Grasshopper

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