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Sensory Processes Josée L. Jarry, Ph.D., C.Psych. Introduction to Psychology Department of Psychology University of Toronto May 28, 2003 Sensation & Perception • Sensation – Experience associated with stimuli – The initial steps by which the sense organs and neural pathways take in stimulus information. • Perception – Subsequent organizing of information – Meaningful interpretation of information The Process of Sensation physical stimulus physiological response sensory experience The Process of Sensation • Physical stimulus – Matter or energy impinging on the sense organs. • Physiological response – Pattern of electrical activity that occurs in the sense organs, the nerves, and the brain as a result of the stimulus. • Sensory experience – The subjective, psychological sensation - sound, taste, sight - that is experienced by the individual whose sense organs have been stimulated. Basic Anatomy of Human Senses • Receptors – Specialized structures that respond to physical stimulus by producing electrical changes that can initiate neural impulses • Sensory neurons – Carry neural impulses from the receptors to the central nervous system • Sensory areas – Specific areas of the cerebral cortex devoted to specific senses. Transduction Common to All Senses • Transduction – The process by which a receptor cell produces an electrical change in response to a physical stimulus • Receptor potential – In response to a stimulus, the membrane of the receptor cell depolarizes which leads to action potentials in the axons of sensory neurons. • Quantitative & Qualitative dimension Stimulus Quantity and Quality • Senses respond not only to a particular class of stimulus energy • The also respond to variations in that energy • All forms of energy vary along at least two dimensions • Quantitative dimension – Concerns the amount or intensity of energy present • Qualitative dimension – Concerns the precise kind of energy present Coding of Stimulus Quantity & Quality • Coding – Pattern of action potential sent to the brain that preserves the quantity and quality of a stimulus. • Coding of stimulus quantity – Stronger stimuli produce larger receptor potentials – Produce faster rates of action potentials in sensory neurons. • Coding of stimulus quality – Different receptors within any given sensory tissue are tuned to different forms of energy. Sensory Adaptation • Change in sensitivity that occurs when a sensory system is either stimulated or not stimulated for a length of time. • Absence of stimulation – Sensory system becomes temporarily more sensitive – Responds to weaker stimuli • Presence of stimulation – Sensory system becomes temporarily less sensitive – Requires stronger stimuli to produce a response. Pain • Other senses communicate information about the external world • The experience of pain comes from one’s own body • Pain is not only a sense but it is also a drive • People are motivate to avoid and/or reduce pain • Pain has survival value Neural Pathways of Pain • Anatomically related to the cutaneous senses • Free nerve endings – The sensitive terminals of pain neurons are not surrounded by special capsules or end organs as are the endings of touch and temperature receptors – Free nerve endings can be found in all body tissues from which pain is sensed, from the skin to the pulp of the teeth. Two Types of Peripheral Pain Neurons • A-delta fibers – Thick, myelinated, fast conducting neurons – Mediate the feeling of initial fast, sharp, highly localized pain. • C fibers – Very thin, unmyelinated, slow-conducting – Mediate slow, dull, more diffuse, often burning pain. Central Pain Pathways: Fast Pain • Fast pain and A-delta fibres – A-delta fibres synapse on cells in the spinal cord that lead to an area of the thalamus called the ventrobasal complex – ventrobasal complex also receives neurons that mediate touch – sends its output to the somatosensory cortex – allows us to localize where pain originates Central Pain Pathways: Slow Pain • Slow pain and C fibres – – – – C fibres synapse on cells in the spinal cord Relays to a midline nucleus in the thalamus and to the limbic system responsible for motivational and emotional aspects of pain – Those connections are important for the interpretation of pain. Neural and Chemical Inhibition of Pain • Gate control theory • Ronald Melzack and Patrick Wall (1965, 1982) – For pain to be experienced, input from peripheral pain neurons must pass through a gate located at the point where they enters the spinal cord and lower brain stem. Pain-inhibiting System • Periaqueductal gray (PAG) – PAG neurons have excitatory connections with inhibitory interneurons in the spinal cord – These inhibitory interneurons prevent ascending neurons to relay pain messages to the brain • Endorphins – The spinal cord inhibitory interneurons releases endorphins – Endorphins are inhibitory neurotransmiters