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Motor cortex Somatosensory cortex Sensory associative cortex Pars opercularis Visual associative cortex Broca’s area Visual cortex Primary Auditory cortex Wernicke’s area Brain Structures [Adapted from Neural Basis of Thought and Language EE141 Feldman, Spring 2007, [email protected] Jerome 1 Intelligence Learning and Understanding • I hear and I forget • I see and I remember • I do and I understand attributed to Confucius 551-479 B.C. There is no erasing in the brain 2 EE141 Intelligence and Neural Computation What it means for the brain to compute and how that computation differs from the operation of a standard digital computer. How intelligence can be implemented in the structure of the neural circuitry of the brain. How is thought related to perception, motor control, and our other neural systems, including social cognition? How do the computational properties of neural systems and the specific neural structures of the human brain shape the nature of thought? What are the applications of neural computing? 3 EE141 Nervous System Divisions Central nervous system (CNS) brain spinal cord 4 EE141 Nervous System Divisions Peripheral nervous system (PNS) consists of: Cranial and spinal nerves Ganglia Sensory receptors Subdivided into: Somatic Autonomic – Motor component subdivided into: sympathetic parasympathetic Enteric 5 EE141 Brains ~ Computers 1000 operations/sec 100,000,000,000 units 10,000 connections/ graded, stochastic embodied fault tolerant evolves learns 1,000,000,000 ops/sec 1-100 processors ~ 4 connections binary, deterministic abstract crashes designed programmed 6 EE141 PET scan of blood flow for 4 word tasks EE141 7 Neurons structures 8 EE141 Neurons cell body dendrites (input structure) receive inputs from other neurons perform spatio-temporal integration of inputs relay them to the cell body axon (output structure) a fiber that carries messages (spikes) from the cell to dendrites of other neurons 9 EE141 Neuron cells unipolar bipolar multipolar 10 EE141 Synapse site of communication between two cells formed when an axon of a presynaptic cell “connects” with the dendrites of a postsynaptic cell science-education.nih.gov 11 EE141 Synapse axon of presynaptic neuron dendrite of postsynaptic neuron bipolar.about.com/library 12 EE141 Synapse • • • • a synapse can be excitatory or inhibitory arrival of activity at an excitatory synapse depolarizes the local membrane potential of the postsynaptic cell and makes the cell more prone to firing arrival of activity at an inhibitory synapse hyperpolarizes the local membrane potential of the postsynaptic cell and makes it less prone to firing the greater the synaptic strength, the greater the depolarization or hyperpolarization 13 EE141 Visual cortex of the rat 14 EE141 Somatotopy of Action Observation Foot Action Hand Action Mouth Action Buccino et al. Eur J Neurosci 16 2001 EE141 EE141 How does it all work? 17 Amoeba eating Artist’s rendition of a typical cell membrane 18 EE141 Neural Processing From lecture notes by Dr Rachel Swainson NEURAL COMMUNICATION 1: Transmission within a cell and from a lecture notes based on www.unisanet.unisa.edu.au/Information/12924info/Lecture Presentation - Nervous tissue.ppt 19 EE141 Transmission of information Information must be transmitted within each neuron and between neurons 20 EE141 The Membrane The membrane surrounds the neuron. It is composed of lipid and protein. 21 EE141 EE141 Artist’s rendition of a typical cell membrane 22 Cell Electrical Potential Every neuron is covered by a membrane The membrane is selectively permeable to the passage of chemical molecules (ions) The membrane maintains a separation of electrical charge across the cell membrane. The cell membrane has an electrical potential Electrical potentials Electrical charge of the membrane is related to charged ion that cross the membrane through lipids, ion channels and protein ion-transporters. Electrical currents (ionic flux) The flow of electrical charge between the cell’s interior and exterior cellular fluids 23 EE141 Forces determine flux of ions – Electrostatic forces • Particles with opposite charges attract, Identical charges repel – Concentration forces • Diffusion – molecules distribute themselves evenly – – Protein – ion channels • Selective Non – gated ion channels • Selective Voltage-dependent gated ion channels – Protein – ion transporters – K+ Na + pump • Cl - pump 24 EE141 The Resting Potential - - + - - + There is an electrical charge across the membrane. This is the membrane potential. The resting potential (when the cell is not firing) is a 70mV difference between the inside and the outside. + outside inside + + - Resting potential of neuron = -70mV 25 EE141 Ions and the Resting Potential Ions are electrically-charged molecules e.g. sodium (Na+), potassium (K+), chloride (Cl-). The resting potential exists because ions are concentrated on different sides of the membrane. Na+ and Cl- outside the cell. K+ and organic anions inside the cell. Na + Na Organic anions (-) K+ EE141 Cl- + Na+ Na+ K Organic anions (-) + Cl- outside inside Organic anions (-) 26 Maintaining the Resting Potential Na+ ions are actively transported (this uses energy) to maintain the resting potential. The sodium-potassium pump (a membrane protein) exchanges three Na+ ions for two K+ ions. Na Na+ + Na+ outside EE141 K+ K+ inside 27 Neuronal firing: the action potential The action potential is a rapid depolarization of the membrane. It starts at the axon hillock and passes quickly along the axon. The membrane is quickly repolarized to allow subsequent firing. 28 EE141 Course of the Action Potential The action potential begins with a partial depolarization (e.g. from firing of another neuron ) [A]. When the excitation threshold is reached there is a sudden large depolarization [B]. This is followed rapidly by repolarization [C] and a brief hyperpolarization [D]. 29 EE141 The Action Potential action potential is “all-or-none”. It is always the same size. Either it is not triggered at all - e.g. too little depolarization, or the membrane is “refractory”; Or it is triggered completely. The 30 EE141 Action potential 2 phases: Depolarisation – graded potentials move toward firing threshold – if reach threshold voltage regulated sodium channels open – reversal of membrane permeability Repolarisation – sodium channels close – potassium channels open 31 EE141 Before Depolarization 32 EE141 Action potentials: Rapid depolarization When partial depolarization reaches the activation threshold, voltage-gated sodium ion channels open. Sodium ions rush in. The membrane potential changes from -70mV to +40mV. Na+ + EE141 - Na + Na+ + 33 Depolarization 35 EE141 Action potentials: Repolarization Sodium ion channels close and become refractory. Depolarization triggers opening of voltage-gated potassium ion channels. K+ ions rush out of the cell, repolarizing and then hyperpolarizing the membrane. Na+ Na EE141 + K + Na+ K+ K + + - 36 Repolarization 37 EE141 Conduction of the action potential Passive conduction will ensure that adjacent membrane depolarizes, so the action potential “travels” down the axon. But transmission by continuous action potentials is relatively slow and energy-consuming (Na+/K+ pump). A faster, more efficient mechanism has evolved: saltatory conduction. Myelination provides saltatory conduction. 39 EE141 Action Potential 40 EE141 Propagation of the Action Potential • Action Potential spreads down the axon in a chain reaction • Unidirectional – it does not spread into the cell body and dendrite due to absence of voltage-gated channels there – Refraction prevents spread back across axon 41 EE141 Myelination Most mammalian axons are myelinated. The myelin sheath is provided by oligodendrocytes and Schwann cells. Myelin is insulating, preventing passage of ions over the membrane. 42 EE141 Saltatory Conduction Myelinated regions of axon are electrically insulated. Electrical charge moves along the axon rather than across the membrane. Action potentials occur only at unmyelinated regions: nodes of Ranvier. Myelin sheath EE141 Node of Ranvier 43 Summary of axonal conduction Unmyelinated fibres continuous conduction Myelinated fibres saltatory conduction – High density of voltage gated channels at Nodes of Ranvier Larger diameter axons propagate impulses faster Stimulus intensity encoded by: frequency of impulse generation number of sensory neurons activated EE141 44 Synaptic transmission Information is transmitted from the presynaptic neuron to the postsynaptic cell. Chemical neurotransmitters cross the synapse, from the terminal to the dendrite or soma. The synapse is very narrow, so transmission is fast. 45 EE141 Structure of a synapse An action potential causes neurotransmitter release from the presynaptic membrane. Neurotransmitters diffuse across the synaptic cleft. They bind to receptors within the postsynaptic membrane, altering the membrane potential. terminal extracellular fluid synaptic cleft presynaptic membrane postsynaptic membrane dendritic spine 46 EE141 Neurotransmitter release Synaptic vesicles, containing neurotransmitter, congregate at the presynaptic membrane. The action potential causes voltage-gated calcium (Ca2+) channels to open; Ca2+ ions flood in. vesicles Ca2+ Ca2+ Ca2+ Ca2+ 2+ Ca Ca2+ Ca2+ Ca2+ 47 EE141 Neurotransmitter release Ca2+ causes vesicle membrane to fuse with presynaptic membrane. Vesicle contents empty into cleft: exocytosis. Neurotransmitter diffuses across synaptic cleft. Ca2+ 48 EE141 49 EE141 Opening and closing of the channel in synaptic membrane EE141 50 Ionotropic receptors Synaptic activity at ionotropic receptors is fast and brief (milliseconds). Acetyl choline (Ach) works in this way at nicotinic receptors. Neurotransmitter binding changes the receptor’s shape to open an ion channel directly. ACh ACh 51 EE141 Ionotropic Receptors 4 nm 52 EE141 Metabotropic Receptors (G-Protein) 53 EE141 Postsynaptic Ion motion 54 EE141 Excitatory postsynaptic potentials (EPSPs) Opening of ion channels which leads to depolarization makes an action potential more likely, hence “excitatory PSPs”: EPSPs. Inside of post-synaptic cell becomes less negative. Na+ channels (remember the action potential) Ca2+ . (Also activates structural intracellular changes -> learning.) Ca2+ + Na+ outside inside - 56 EE141 Inhibitory postsynaptic potentials (IPSPs) Opening of ion channels which leads to hyperpolarization makes an action potential less likely, hence “inhibitory PSPs”: IPSPs. Inside of post-synaptic cell becomes more negative. K+ (remember termination of the action potential) Cl- (if already depolarized) - K+ + Cl- outside inside 57 EE141 Integration of information PSPs are small. An individual EPSP will not produce enough depolarization to trigger an action potential. IPSPs will counteract the effect of EPSPs at the same neuron. Summation means the effect of many coincident IPSPs and EPSPs at one neuron. If there is sufficient depolarization at the axon hillock, an action potential will be triggered. axon hillock 58 EE141 Requirements at the synapse For the synapse to work properly, six basic events need to happen: 1. 2. 3. 4. 5. 6. Production of the Neurotransmitters Storage of Neurotransmitters Release of Neurotransmitters Binding of Neurotransmitters Generation of a New Action Potential Removal of Neurotransmitters from the Synapse 59 EE141 Three Nobel Prize Winners on Synaptic Transmission Arvid Carlsson discovered dopamine is a neurotransmitter. Carlsson also found lack of dopamine in the brain of Parkinson patients. Paul Greengard studied in detail how neurotransmitters carry out their work in the neurons. Dopamine activated a certain protein (DARPP-32), which could change the function of many other proteins. Eric Kandel proved that learning and memory processes involve a change of form and function of the synapse, increasing its efficiency. This research was on a certain kind of snail, the Sea Slug (Aplysia) that has relatively low number of neurons (20,000 ). 60 EE141 Neural circuits Divergence Single presynaptic neuron synapses with several postsynaptic neurons – Example: sensory signals spread in diverging circuits to several regions of the brain Convergence Several presynaptic neurons synpase with single postsynaptic neuron – Example: single motor neuron synapsing with skeletal muscle fibre receives input from several pathways originating in different brain regions 61 EE141 Neural circuits Pulsing circuit Once presynaptic cell stimulated causes postsynaptic cell to transmit a series of impulses – Example: coordinated muscular activity Parallel after-discharge circuit Single presynaptic neuron synapses with multiple neurons which synapse with single postsynaptic cell – results in final neuron exhibiting multiple postsynaptic potentials Example: may be involved in precise activities (eg mathematical calculations) 62 EE141 63 EE141