L20-2010 - BioNB2220 Cornell University Spring 2010 Carl D....

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BioNB2220 Spring 2010 Cornell University Carl D. Hopkins Lecture 20. Principles of Sensory Coding All of an animal’s knowledge of its physical world is obtained through the action of its sense organs which convert four principle attributes of a stimulus into a neural code. These attributes are modality, location, magnitude, and timing. The neural code of modality is determined by the particular type of sense organ which is activated as a labeled line. Location is encoded as the position of the activated receptor over the body surface which is represented as a topographic map of the sensory surface. Stimulus magnitude is encoded according by the timing or the frequency of nerve impulses or physiological rules that apply to all sensory modalities. The timing of the stimulus is encoded by receptors which differ in adaptation rates. Combined, the sense organs use neural codes to inform the brain about the what, where , how much , and when of the physical stimulus. Reading Assignment Purves et al., (2008) Neuroscience, 4th edition, Chapter 9. (Somatic Senses: Touch and Proprioception), pp. 207-229 Learning Objectives Be able to: Explain the meaning of each of these terms: Transduction, Encoding, Adaptation, Labeled Line, Integrate and Fire Model, Rate Coding, Lateral Inhibition, Receptive Field, Sensory Map. Explain how the nervous system is able to keep track of all these different parameters defining an external stimulus: what, where, when, and how much? Lecture Outline The focus today is on the somatosensory system and our sense of touch. In this lecture, we will consider the following key concepts, more or less in sequence: transduction; encoding; adaptation; labeled lines; receptive fields; and integration by convergence, map formation, and lateral inhibition. First, let us review the three natural ways that a neuron becomes activated: (1) by synaptic inputs, (2) by endogenous firing (pacemaker activity), or (3) by activating sensory receptors. In this lecture we will see how the sense organs tell us what has happened in the environment (the identification of the event), how much of a stimulus there was (determining the magnitude of the stimulus), where it was in space, and when it occurred. All sensory events are encoded in the universal language of the nervous system: all or none action potentials. In this lecture we review how these signals are encoded and how the brain interprets these sensory codes. We begin our discussion with the somatosensory system, and the mechanoreceptors that give us a sense of touch. Mechanoreceptors were some of the first sensory receptors to be explored physiologically (Adrian and Umrath, 1925). Fig. 1 shows a diagram of a Paccinian corpuscle illustrating the physiological processes of transduction, encoding, and adaptation. The Paccinian corpuscle consists of a
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This note was uploaded on 05/25/2010 for the course BIONB 2220 taught by Professor Hopkins,c.d. during the Spring '10 term at Cornell University (Engineering School).

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L20-2010 - BioNB2220 Cornell University Spring 2010 Carl D....

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