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How the brain perceives the outside world Saturday Morning Physics December 4, 2004 Presenter: Rhonda Dzakpasu Brain blunders Kanizsa’s Triangle x “Peripheral drift” by Akiyoshi Kitaoka Taken from Nationalgeographic.com W.E. Hill Central Nervous System: Brain and spinal cord • The Brain – Three pound wrinkled mass – Fits into cranial cavity surrounded by cerebrospinal fluid cushioning – Consists of multiple areas – Localized regions are task specific Courtesy of Morphonix LLC, Sausalito, CA Central Nervous System: Brain and spinal cord • Spinal cord – Carries fibers that relay information from brain to rest of body and from the body to the brain – Encased in bone fortress known as the vertebrae – Surrounded by cerebrospinal fluid Courtesy of Morphonix LLC, Sausalito, CA Peripheral Nervous System: nerves to rest of body • Somatic nerves – Fibers that relay commands from brain to all muscle groups below the neck and controls somatosensory function such as touch, pain, temperature and joint position Courtesy of Morphonix LLC, Sausalito, CA Peripheral Nervous System: nerves to rest of body • Autonomic nerves – Fiber carrying information from brain to all organs in the body – And all this is done automatically • Cranial nerves – 12 nerves connecting the brain to the organs in the head; also have sensory and motor functions Courtesy of Morphonix LLC, Sausalito, CA The “Sense”ational Organs Receives “raw” data from around us Each organ handles single function Converts external stimuli to language of the brain Seeing is believing: The Eye Retina is initial visual processing center Has three basic layers of cells: Light sensitive photoreceptors Bipolar cells Ganglion cells Courtesy of Morphonix LLC, Sausalito, CA Seeing is believing: The Retina Color is coded by photoreceptor rods and cones Intensity is coded by frequency of firing of ganglion cells Courtesy of Morphonix LLC, Sausalito, CA Can you hear me now? The ear: Wide dynamic range: 20 Hz – 20000 Hz Three major components: Outer ear Middle ear Inner ear Courtesy of Morphonix LLC, Sausalito, CA Can you hear me now? Cochlea: neurosensory component for hearing Hair cells convert sound pressure to “brain language” Outermost end – high frequencies Innermost end – low frequencies Courtesy of Morphonix LLC, Sausalito, CA Olfaction and Taste Smell and taste are related (artificially separated?) Both are chemical senses Odors bind to receptors which send electrical signals to olfactory bulb Tastes are dissolved in mucous fluid in oral cavity to be carried to taste receptors Courtesy of Morphonix LLC, Sausalito, CA Like minded ordering Cells of similar type group according to their function Located in well defined layers Advantage: facilitates ease of communicaton Disadvantage: disease or injury can wipe out entire function Brain Structure whyfiles.org The Cerebral Cortex Purves et al., Life: The Science of Biology, 4th Edition Cortical Parallel Processing Visual system Auditory system Must decode shape,color, position and movement Must decode many aspects of sound, e.g., loudness, pitch, harmonics, timing and location of multiple sounds Olfactory System Odors are composed of different chemical groups that bind to different receptors The Father of Modern Neuroscience Santiago Ramon y Cajal, 1852-1934 Outlined fundamental architecture of CNS Demonstrated basic changes neurons undergo as they function in the CNS First to isolate neurons near surface of the brain Courtesy: The Nobel Foundation The Father of Modern Neuroscience Santiago Ramon y Cajal, 1852-1934 Courtesy: The Nobel Foundation Who are the main players? Neuron – Generate and propagate electrical activity – Responsible for our thinking, feeling, hearing, seeing, moving, loving and hating! Glial cells – Supporting cells The Neuron There are ~ 100 billion Small enough for 30000 to fit on head of a pin Connects with thousands of other neurons Generally do not reproduce Courtesy of Morphonix LLC, Sausalito, CA The Neuron Three major components: Dendrites: Receives information from other neurons Courtesy of Morphonix LLC, Sausalito, CA The Neuron Soma: Cell body containing the nucleus, the brain of the neuron Courtesy of Morphonix LLC, Sausalito, CA The Neuron Axon: Connects with other cells to transmit information Can be up to several feet long! Covered in myelin allowing for fast information transfer Courtesy of Morphonix LLC, Sausalito, CA The Neuron Always active! Constantly integrating and generating information Hotbed of activity! Neurons don’t hibernate! Courtesy of Morphonix LLC, Sausalito, CA How is information transmitted?! The Resting Potential Primarily due to concentration of K+ions inside and outside cell membrane Typically between -80 mV to – 55 mV Action Potential: Generation Excitatory input from synapses to dendrites depolarizes cell Voltage activated Na+ channels open Inside of cell becomes positive, ~20mV Action Potential: Propagation Change in voltage creates current that depolarized adjacent region in axon Impulse travels down axon Lasts ~2 ms Neuronal Communications Neurons are like islands: no physical contacts Chemical signals are converted to electrical ones The supporting cells Glial cells Outnumber neurons by 10:1 margin Two types: Astrocytes Oligodendrites and Schwann cells The supporting cells Astrocytes: Provide physical support by forming a mesh around neurons Formation of blood-brain barrier Remove dead cells Transport of nutrients Destroy neurotransmitters in extra cellular space The supporting cells Oligodendrites (CNS) and Schwann cells (PNS): Prevents abnormal communication between neurons Contain myelin and wraps around neuronal axons to boost communications, up to 200 mph! Where’s the Physics?! The “big” questions? How do high-level psychological processes come about from basic neurophysiological effects? How does the complex organization of brain cells, give rise to behavior? Where’s the Physics?! Computational models of neuronal behavior How can neural activity be defined? What types of coding mechanisms are involved? Models from single ion channel and synaptic models to “black-box” models describing psychological phenomena The “Standard” Model • Hodgkin-Huxley (1950’s) – First model of action potential propagation e CV 0.1(V 67) 100m3 h(V 50) 80n 4 (V 100) I syn 0.32(54 V ) 0.28(V 27) (1 m) m, 1 exp( 0.25(V 54)) exp( 0.2(V 27)) 1 4 h 0.128 exp( (50 V ) / 18)(1 h) h, 1 exp( 0.2(V 27)) 0.032(V 52) n (1 n) 0.5 exp( (57 V ) / 40)n, 1 exp( 0.2(V 52)) m Hodgkin-Huxley Model • Simulates action potential from giant axon of squid Hodgkin, A. L. and Huxley, A. F. (1952) Journal of Physiology 117: 500-544 Where’s the Physics?! Study of spatio-temporal patterns What new types of behaviors occur in large-scale models? Synchronization Oscillations Are they important to brain function? Patterns of Emergence Large scale behavior of networks of neurons Makeig, S., et. al., Science, 295, 690, 2002 Patterns of Emergence Temporal and spatial changes in networks Seidemann, E., et. al., Science, 295, 862, 2002 Where’s the Physics?! Imaging techniques How can we “see” inside the brain? Structural methods Functional methods Imaging Techniques Clinical application – PET scans – CT scans – MRI Research applications – Fluorescence based imaging – Nuclear magnetic resonance (NMR) imaging Why should you care that we want to understand how the brain works??? What can we do when the brain fails? Hybrid Brain Machine Interfaces* Real-time direct interfaces between brain, electronics and mechanical devices Can be used to restore lost or impaired sensory and motor function * Nicolelis, M., Nature, 409, 403,2001 Prosthetic Ear: Cochlear Implants Converts acoustic signal to electrical stimuli Couples to array of implanted electrodes to auditory nerves Stimulates cochlea: High frequencies – basal Low frequencies – apical Mimics normal auditory processing Reprinted with permission from Rauschecker, J.P., and Shannon, R.V., Science, 295, 1025-1029, (2002) Copyright 2002, AAAS A Bionic Eye? Retinal Prostheses Reprinted with permission from Zrenner, E., Science, 295, 1022-1025, (2002) Copyright 2002, AAAS A Bionic Eye? Retinal Prostheses Type 1: subretinal implant Replaces photoreceptors with microphotodiodes and electrodes Type 2: epiretinal implant No light-sensitive areas Uses camera and processor outside of body Implanted electrodes stimulate axons of ganglion cells It’s all Monkey Business! Reprinted with permission, Nicolelis, M., Nature Reviews, 4, 417, 2003 It’s all Monkey Business! Monkeys learn to produce complex hand movements Implanted microwire arrays record activity of hundreds of neurons Linear and non-linear models are used to extract motor control signals Outputs used to control movement of the robot arm Reprinted with permission, Nicolelis, M., Nature Reviews, 4, 417, 2003 Stay tuned! Next week’s lecture • Imaging techniques – Clinical methods – Research methods