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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 Srini Narayanan EE141 Comp Sci 182 / Cog Sci 110 / Ling 109 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. 2 EE141 Intelligence and Neural Computation Embodied Intelligence 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. 3 EE141 Brains ~ Computers 1000 operations/sec 100,000,000,000 units 10,000 connections/ graded, stochastic embodied fault tolerant evolves, learns EE141 1,000,000,000 ops/sec 1-100 processors ~ 4 connections binary, deterministic abstract crashes designed, programmed 4 PET scan of blood flow for 4 word tasks EE141 5 Neurons structures 6 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 7 EE141 Neuron cells 8 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 9 EE141 Synapse axon of presynaptic neuron dendrite of postsynaptic neuron bipolar.about.com/library 10 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 11 EE141 Visual cortex of the rat 12 EE141 Somatotopy of Action Observation Foot Action Hand Action Mouth Action Buccino et al. Eur J Neurosci 14 2001 EE141 EE141 How does it all work? 15 Amoeba eating Artist’s rendition of a typical cell membrane 16 EE141 Neural Processing from Dr Rachel Swainson NEURAL COMMUNICATION 1: Transmission within a cell 17 EE141 Transmission of information Information must be transmitted within each neuron and between neurons 18 EE141 The Membrane The membrane surrounds the neuron. It is composed of lipid and protein. 19 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 20 EE141 EE141 Artist’s rendition of a typical cell membrane 21 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 (-) 22 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 23 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. 24 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]. +40 Membrane potential 0 (mV) [C] [B] [A] [D] excitation threshold -70 EE141 0 1 2 3 Time (msec) 25 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+ + 26 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 29 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 energyconsuming (Na+/K+ pump). A faster, more efficient mechanism has evolved: saltatory conduction. Myelination provides saltatory conduction. 30 EE141 Membrane potential EE141 31 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. 32 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 33 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. 34 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 35 EE141 Neurotransmitter release Ca2+ causes vesicle membrane to fuse with presynaptic membrane. Vesicle contents empty into cleft: exocytosis. Neurotransmitter diffuses across synaptic cleft. Ca2+ 37 EE141 Opening and closing of the channel in synaptic membrane EE141 38 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 39 EE141 40 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 (NB remember the action potential) Ca2+ . (Also activates structural intracellular changes -> learning.) Ca2+ Na+ outside + inside - 42 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+ (NB remember termination of the action potential) Cl- (if already depolarized) - EE141 K+ + Cl- outside inside 43 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 44 EE141 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. 45 EE141 Motor Control 46 EE141 Hierarchical Organization of Motor System Primary Motor Cortex and Premotor Areas 47 EE141 Primary motor cortex (M1) Hip Trunk Arm Hand Foot Face Tongue Larynx 48 EE141 Leg motor control 49 EE141 Two major descending pathways Pyramidal vs. extrapyramidal Motor cortex Pyramidal system • Pathway for voluntary movement • Most fibers originate in motor cortex (BA 4&6) • Most fibers cross to contralateral side at the medula Brain stem centers Lower motor neurons (brain stem and spinal cord) Striated muscles Extrapyramidal system • Pathways for postural control/certain reflex mov’t • Originates in brainstem • Fibers do not cross • Cortex can influence this system via inputs to brain stem 50 EE141 Cortical Motor System Pre-motor cortex Movement planning/sequencing • Many projections to M1 • But also many projections directly into pyramidal tract • Damage => more complex motor coordination deficits • Stimulation => more complex mov’t • Two distinct somatotopically organized subregions • SMA (dorso-medial) • May be more involved in internally generated movement • Lateral pre-motor • May be more involved in externally guided movement 52 EE141 Cortical Motor System Posterior parietal cortex (PPC) Sensory guidance of movement • Many projections to pre-motor cortex • But also many projections directly into pyramidal tract • Damage can cause deficits in visually guided reaching (Balint’s syndrom) and/or apraxia • Likely part of the dorsal visual stream 53 EE141 Vision 54 EE141 55 EE141 56 EE141 57 EE141 58 EE141 Vision and Action 59 EE141 Cortical Motor System Pre-motor cortex Movement planning/sequencing • Many projections to M1 • But also many projections directly into pyramidal tract • Damage => more complex motor coordination deficits • Stimulation => more complex mov’t • Two distinct somatotopically organized subregions • SMA (dorso-medial) • May be more involved in internally generated movement • Lateral pre-motor • May be more involved in externally guided movement 60 EE141 Somatotopy of Action Observation Foot Action Hand Action Mouth Action Buccino et al. Eur J Neurosci 61 2001 EE141 Fly Brain Basic Atlas of the Drosophila Brain http://flybrain.neurobio.arizona.edu/Flybrain/html/index.html 62 EE141 Antennal Lobe Stereopair This example shows the dendrites of two "uniglomerular" neurons, each invading a single olfactory glomerulus in the antennal lobe of the cockroach A stereopair taken with an "Edge microscope" (Edge Scientific Instrument Corporation, Santa Monica, CA 63 90404 EE141 The axons of projection neurons from the antennal lobes. EE141 64 Sagittal section of the antennal nerve and lobe, showing olfactory receptor axons one 65 EE141 of which is shown ending in a glomerulus that, is invaded by a local interneuron. Mechanosensory endings from the right antennal nerve revealing three classes of terminals: small diameter axons, that distribute lateral to and around the back of the antennal lobe (ant lob); intermediate diameter axons that reside between these 66 smaller fibers and a dense, bifurcating, tract of thicker varicose processes EE141 two neurons that invade the ellipsoid body from dendrites within the lateral deutocerebrum 67 EE141 Optic array of inputs from the medulla to the lobula showing the layer 68 EE141 relationship between the medulla afferents and lobula output dendrites Optic lobes invading the lobula plate with branches that also extend to the inner EE141 layer of the medulla and the outermost layer of the lobula. 69 Type 1 and Type 5 T-cell Horizontal section showing a Y-cell and a small-field Tm1 transmedullary cell in the medulla. The latter terminates in the outer layer of the lobula amongst dendrites of T5 neurons, several of which are shown here. Tm1 and T5 are 70 essential components in the elementary motion detecting circuit EE141 Drosphila optic lobes demonstrate the presence of orthogonally arranged dendrites of horizontal and vertical neurons in the lobula plate. Optical stacks show the vertical neurons and the three horizontal neurons. The 71 axons of columnar neurons elements converge onto the giant fibre . EE141 beneath the retina (ret) of the compound eye, the lamina neuropil is shown with the cross sections of the L1 and L2 monopolar cell axons denoting the mosaic of optic cartridges EE141 72 The lobula (lo) is shown connected to the medulla (me) via the second 73 optic chiasma (och 2) EE141 Lobula Complex The neural architecture of the lobula (lo) and the lobula plate (lo p), both connected to the inner layer of the medulla (me) by the second optic chiasma (och 2). The lobula plate shows three layers of tangential neurons (1-3). 74 They receive inputs from chromatic motion-sensitive interneurons EE141