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Chapter 22; Fundamentals of Sensory Systems 11/21/07 Each system responds with some specificity to a stimulus and each employs specialized cells—peripheral receptors—to translate the stimulus into a signal that all neurons can use Labor is divided among neurons such that not only different stimulus energies but also different stimulus qualities are analyzed by separate groups of neurons Sensory Systems (SS) provide the CNS w/ a representation of the external world Sensory Systems perform common types of operations 1. Ability of each system to compare events that occur simultaneously at different receptors a. Brings about the greatest response where the difference in stimulus strength is greatest 2. Systems make comparisons w/ past events and w/ sensations received by other sensory systems Sensory systems provide the CNS w/ a constantly updated representation of the external world Requires close interaction between ascending pathways and descending mechanisms o Ascending (Bottom-Up) = stimulus-driven; peripheral receptors o Descending Mechanisms; sort out from the large amount of sensory input events that need immediate attention, alter ascending inputs in ways that optimize perception Perception of a sensory experience can change even though the input remains the same Detection/ Recognition of a stimulus is referred to as __________ Interpretation and appreciation of that event constitutes ____________. Psychophysics Psychophysical experiment determines the quantitative relationship b/w a stimulus and a sensation; used to establish the limits of sensory performance ________________________________________________________________________ Receptors; specific for a narrow range of input Receptors; specialized cells which convert external stimulus energy into a signal that produces a neural response Each of the fundamental types of stimuli has a separate population of receptors selective for the particular form of energy Within a single sensory system are classes of receptors that are particularly sensitive to one stimulus and not another (i.e. hot or cold) These different functions individual receptors serve is directly associated to receptor structure and chemistry Receptors transduce energy into a change in membrane voltage Transmit voltage change to ganglion cells that send their axons into CNS Receptor types vary across sensory systems Correlation between the number of receptor types within a system and the types of stimuli that system is able to detect o Somatosensory; large number of receptor types and ability to detect many different types of stimuli o Auditory; two classes of receptor—inner hair cells and out hair cells of the cochlea—transduce mechanical energy of the basilar membrane o Visual system; transduction performed by rods and cones Receptors have a characteristic pattern of position and density Orderly arrangement of receptors exists along skin, basilar membrane, and retina Differences in peripheral innervation density are tightly correlated w/ spatial acuity Receptors; sites of convergence and divergence A single ganglion cell receives input from several receptors and in many cases a single receptor sends information to two or more ganglion cells. Convergence and divergence go hand-in-hand for the Somatosensory system o An individual receptor is often innervated by axons of several ganglion cells while at the same time branching its projections to become part of several receptor organs If spatial resolution is a requirement; convergence of receptor inputs is low For detection of weak signals; convergence of receptor inputs is high If a receptors input is used for a complex function; divergence of input from receptor onto many ganglion cells occurs Receptors vary in embryonic origin Auditory and Somatosensory system receptors and ganglion cells are part of PNS and therefore originate from neural crests Retina is generated from embryonic diencephalons; part of the CNS and therefore originates from neural tube Sensory information is transmitted along labeled lines Labeled Line Principle; when a particular population of neurons is active the conscious perception is of a specific stimulus Receptors are selective not only in what drives them but also in the postsynaptic targets with which they communicate Orderly relay from receptor to ganglion cell to CNS makes up labeled line Modality: all sensory information arising from a single class of receptors Labeled lines means that neurons in sensory systems carry specific modalities Topographic Pattern; neurons positioned side by side in one region communicate w/ neurons so positioned in the next region Neural signaling is by a combination of rate and temporal codes The firing rate or frequency of A.P.s signals the strength of the sensory input Temporal code in some systems Lateral Mechanisms enhance sensitivity to contrast Lateral Inhibition; sensory neuron displays a receptive field with an excitatory center and an inhibitory surround o Enhances contrast, sharpens the selectivity of a neuron ________________________________________________________________________ Thalamus Axons from each sensory system cross the midline on their way to the thalamus Decussate-tendency for axons to cross the midline on way to thalamus Serve the broad function of bringing the axons together carrying sensory information into alignment with motor output Specific Thalamic Nuclei Exist for each sensory system Information from each sensory system must be relayed through the thalamus on its way to the cerebral cortex Neurons performing one function are segregated from those performing another For each thalamic nucleus; Population of large and small neurons o Population of large neurons; carry the most rapidly transmitted signals from the periphery to the cortex Multiple Maps and parallel pathways Nuclei in the central pathways often contain multiple maps LGN receives direct input from the retina Inputs from ganglion cells vary in location, structure, and function Sensory cortex includes Primary and Association areas Primary sensory areas; parts of cortex that receive axons of sensory relay Association areas; neighboring areas with which the primary areas communicate directly Response mappings and plasticity Each area of sensory cortex shares w/ its subcortical components a map of at least part of the sensory periphery Mappings of sensory cortex are plastic Neurons in areas of the sensory cortex are organized into six layers Middle Layers (III and IV) are main site of termination of axons from thalamus Columnar Organization Columns (neurons stacked upon each other) are similar but differ from neurons on either side o I.e. ocular dominance columns, orientation columns, direction selective columns