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Rabu, 03 Juli 2013

Outline: ch 35 Sensory Perception.


35.1   Overview of Sensory Receptors

  1. Each sensory system has three component parts: 
    1. Sensory receptors are the branched endings of sensory neurons or specialized cells adjacent to them that detect specific stimuli.

    1. Nerve pathways lead to the brain.

    1. Brain regions process the information into a sensation; later, perhaps, a perception (understanding) of the sensation will be made.

  1. All sensory receptors convert stimulus energy to local, graded potentials, which may result in an action potential if the stimulus is intense or repeated fast enough. 
    1. The brain assesses the nature of a given stimulus using three factors: 
      1. Genetically determined networks of neurons in the brain can interpret incoming action potentials only in specific ways; for example, receptors from eyes see only light.

      1. Strong stimulation of a receptor causes a greater frequency of action potentials.

      1. Strong stimulation causes a greater number of neurons to fire.

    1. In sensory adaptation the frequency of action potentials decreases or stops even when the stimulus is maintained, for example, clothing is no longer felt once it is put on for the day.

    1. Sense organs in more than one location in the body contribute to the somatic sensations; receptors restricted to special locations or organs are the special senses.

35.2   Somatic Sensations

  1. Each sensory pathway starts at receptors of sensory neurons that are sensitive to the same type of stimulus. 
    1. Sensory nerve pathways from different receptors lead to different parts of the somatosensory cortex of the brain.

    1. Cells in this region are laid out like a map, with different regions corresponding to the functional importance of the different body parts.

  1. Receptors Near the Body Surface 
    1. Free nerve endings are simply branched endings of sensory neurons in the skin that function as mechanoreceptors, thermoreceptors, and pain receptors.

    1. Encapsulated receptors are of several types: 
      1. Meissner corpuscles adapt slowly to vibrations of low frequencies.

      1. The bulb of Krause is a thermoreceptor that is sensitive to temperatures below 20 degrees C.

      1. Ruffini endings are sensitive to steady touching and pressure, and to temperatures above 45 degrees C.

      1. Pacinian corpuscles are located both in the dermis and near joints; they are able to detect rapid pressure changes associated with touch and vibrations.

  1. Muscle Sense 
    1. Mechanoreceptors in muscle joints, tendons, ligaments, and skin detect limb motions and the body's position in space.

    1. Examples include the stretch receptors of muscle spindles.

  1. What Is Pain? 
    1. Pain is the perception of injury to some region of the body.

    1. The messages originate in nociceptors, which include free nerve endings in the tissues. 
      1. Sensations of somatic pain come from receptors in the skin, skeletal muscles, joints, and tendons.

      1. Sensations of visceral pain, which is associated with internal organs, are related to excessive chemical stimulation, muscle spasms and fatigue, and inadequate blood flow.

    1. When cells are damaged, they release chemicals (bradykinins) that activate neighboring pain receptors.

    1. Signals from pain receptors reach interneurons in the spinal cord and induce them to release substance P which causes signals to either reach the thalamus and sensory cortex or to go to the brain stem and limbic system.

  1. Referred Pain 
    1. Much visceral pain is referred, that is, felt at some distance from the real stimulation point.

    1. Phantom pain is the sensation that amputees may feel from a limb that is no longer a part of the body.

35.3   Chemical Senses

  1. Olfactory receptors detect water-soluble or volatile substances. 
    1. Some receptors respond to molecules from food or predators.

    1. Others respond to pheromones, which are molecules released outside the body to elicit a social response in a member of the same species (example: bombykol in silkworm moths).

  1. Taste receptors enable animals to distinguish nutritious from noxious substances. 
    1. Receptors of some animals are located on antennae, legs, tentacles, or fins.

    1. In humans, taste receptors are often components of taste buds distributed mostly on the tongue.

35.4   Sense of Balance

  1. The sense of balance depends on the organs of equilibrium located in the inner ear.

  1. The vestibular apparatus is a closed system of fluid-filled sacs and canals inside the ear. 
    1. The otolith organs detect linear movements of the head (static equilibrium).

    1. Cristae located in the semicircular canals detect changing movements (dynamic equilibrium) when fluid bends hair cells attached to sensory neurons.

    1. Overstimulation of the hair cells of the vestibular apparatus can result in motion sickness.

35.5   Sense of Hearing

  1. Properties of Sound 
    1. Hearing is the perception of sounds, which are traveling vibrations of mechanical energy.

    1. These wavelike forms of mechanical energy show amplitude (loudness) and frequency (pitch).

  1. Evolution of the Vertebrate Ear 
    1. The middle ear contains small bones with amplify the sounds before transmittal to the inner ear.

    1. The external ear in mammals has a pinna for collecting the sounds.

    1. In the cochlea of the inner ear, acoustical receptors in the form of hair cells respond to pressure waves transmitted through the surrounding fluid. 
      1. Impulses are sent along the auditory nerve to the brain for interpretation.

      1. The hair cells of the human ear can be permanently damaged by prolonged exposure to intense sounds.

35.6   Sense of Vision

  1. What Are the Requirements for Vision? 
    1. Vision requires a complex system of photoreceptors and neural program in the brain that can interpret the patterns of action potentials.

    1. All photoreceptors incorporate pigment molecules that can absorb photon energy, which can be converted into excitation energy in sensory neurons.

  1. A Sampling of Invertebrate Eyes 
    1. Simple Eyes 
      1. The simplest eye is an ocellus, a patch of photoreceptors in the integument.

      1. These eyespots of invertebrates are photosensitive, but form no images.

    1. Complex Eyes 
      1. Some mollusks have image-forming eyes each with a lens and cornea.

      1. Insects and crustaceans have compound eyes with numerous photosensitive units (ommatidia) each capable of sampling a portion of the visual field to assemble a visual mosaic.

      1. Cephalopods also possess an iris for adjusting light intensity, a pupil through which the light passes, and a retina for photoreception.

35.7   Structure and Function of Vertebrate Eyes

  1. The Eye's Structure 
    1. The outer layer consists of a sclera ("white" of the eye) which covers most of the eye; the cornea covers the front.

    1. The middle layer consists of a dark-pigmented choroid and a lens with a pupil opening, as well as jellylike substances (aqueous humor on the lens, and vitreous body behind the lens).

    1. The pigmented iris controls the amount of light entering through the pupil.

  1. Visual Accommodation 
    1. Because of the bending of the light rays by the cornea, accommodation must be made by the lens so that the image is in focus on the retina.

    1. In fish and reptiles, the lens is moved forward and back (like a camera lens) to focus.

    1. In birds and mammals, the ciliary muscle changes the shape of the lens to focus.

35.8   Case Study--From Signaling to Visual Perception

  1. How Is the Retina Organized? 
    1. Photoreceptors, linked to neurons, are located in the retina. 
      1. Rods are sensitive to dim light and detect changes in light intensity.

      1. Cones respond to high-intensity light, contribute to sharp daytime vision, and detect colors.

    1. The sense of vision is the result of processing the information through levels of synapsing neurons. 
      1. Stimulation begins in the rods and cones, then moves to bipolar sensory neurons, then to ganglion cells whose axons form the optic nerves that lead to the brain's visual cortex.

      1. Before leaving the retina, signals flow among horizontal cells and amacrine cells, which dampen or enhance the signals.

  1. Neuronal Responses to Light 
    1. Each rod cell contains molecules of rhodopsin that can be altered by light, resulting in voltage changes in membranes.

    1. Cone cells carry each carry a different pigment for red, green, and blue colors; cone cells at the fovea (center of retina) provide the greatest visual acuity.

    1. Ganglion cells form restricted areas of the retinal surface called "receptive fields" which respond best to small spots of light.

    1. Axons of the two optic nerves end in the lateral geniculate nucleus of the brain, where the positions of the receptive fields correspond to those of the retina; final interpretation of sight occurs in the visual cortex.

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