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BIO203 - Chapter 6 Summary

BIO203 - Chapter 6 Summary - CHAPTER 6 SUMMARY Introduction...

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CHAPTER 6 SUMMARY Introduction Sensory cells have ion channels that are opened in response to external stimuli. Some versions of these stimulus-sensitive channels are found in all types of life including micro- organisms. •In animals, interoreceptors respond to factors in the internal environment, exteroreceptors to those in the external environment, and proprioceptors to body and limb position and motion. Receptor Physiology •Receptors are specialized peripheral endings of afferent neurons; they respond to particular stimuli, translating the energy forms of the stimuli into electrical signals, the language of the nervous system. •In vertebrates, there are discrete labeled-line pathways from the receptors to the CNS so that information about the type and location of the stimuli can be deciphered by the CNS, even though all the information arrives in the form of action potentials. What the brain perceives from its input, however, is an abstraction and not reality. The only stimuli that can be detected are those for which receptors are present. Furthermore, as sensory signals ascend through progressively more complex processing, some of the information may be suppressed, whereas other parts of it may be enhanced. •Stimulation of a receptor produces a graded receptor potential. The strength and rate of change of the stimulus are reflected in the magnitude of the receptor potential, which in turn determines the frequency of action potentials generated in the afferent neuron. •The magnitude of the receptor potential is also influenced by the extent of receptor adaptation, which refers to a reduction in receptor potential in spite of sustained stimulation. Tonic receptors adapt slowly or not at all and thus provide continuous information about the stimuli they monitor.
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Phasic receptors adapt rapidly and frequently exhibit off responses, thereby providing information about changes in the energy form they monitor. Photoreception: Eyes and Vision •The vertebrate eye is a camera-type eye, specialized structure housing the light-sensitive receptors essential for vision perception—namely, the rods and cones found in its retinal layer. The iris controls the size of the pupil, thereby adjusting the amount of light permitted to enter the eye. The cornea and lens are the primary refractive structures that bend the incoming light rays to focus the image on the retina. The cornea contributes most to the total refractive ability of the eye. •Rods and cones are activated when the photopigments they contain differentially absorb various wavelengths of light. Light absorption causes a biochemical change in the photopigment that is ultimately converted into a change in the rate of action potential propagation in the visual pathway leaving the retina. The visual message is transmitted to the visual cortex in the brain for perceptual processing. •Cones display high acuity but can be used only for day vision because of their low sensitivity to
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BIO203 - Chapter 6 Summary - CHAPTER 6 SUMMARY Introduction...

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