Presentation on theme: "UNIT-I INTRODUCTION TO ARTIFICIAL NEURAL NETWORK"— Presentation transcript:
1UNIT-I INTRODUCTION TO ARTIFICIAL NEURAL NETWORK IT 0469 NEURAL NETWORKS
2Elementary Neuro- Physiology Neuron:A neuron nerve cell is an electricallyexcitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks.
3Parts of the Neuron Cell Body Dendrites Axons Contains the nucleusDendritesReceptive regions; transmit impulse to cell bodyShort, often highly branchedMay be modified to form receptorsAxonsTransmit impulses away from cell bodyAxon hillock; trigger zoneWhere action potentials first developPresynaptic terminals (terminal boutons)Contain neurotransmitter substance (NT)Release of NT stimulates impulse in next neuronBundles of axons form nerves
4Electrical SignalsNeurons produce electrical signals called action potentials ( = nerve impulse)Nerve impulses transfer information from one part of body to anothere.g., receptor to CNS or CNS to effectorElectrical properties result fromionic concentration differences across plasma membranepermeability of membrane
10Resting Membrane Potential (RMP) Nerve cell has an electrical potential, or voltage across its membrane of a –70 mV; (= to 1/20th that of a flashlight battery (1.5 v)The potential is generated by different concentrations of Na+, K+, Cl, and protein anions (A)But the ionic differences are the consequence of:Differential permeability of the axon membrane to these ionsOperation of a membrane pump called the sodium-potassium pump
11What Establishes the RMP? Diffusion of Na+ and K+ down their concentration gradientsNa+ diffuses into the cell and K+ diffuses out of the cellBUT, membrane is 75x’s more permeable to K+ than Na+Thus, more K+ diffuses out than Na+ diffuses inThis increases the number of positive charges on the outside of the membrane relative to the inside.BUT, the Na+-K+ pump carries 3 Na+ out for every 2 K+ in.This is strange in that MORE K+ exited the cell than Na+ entered!Pumping more + charges out than in also increases the number of + changes on the outside of the membrane relative to the inside.AND presence of anionic proteins (A-) in the cytosol adds to the negativity of the cytosolic side of the membraneTHEREFORE, the inside of the membrane is measured at a -70 mV (1 mv = one-thousandth of a volt)
12Resting Membrane Potential Number of charged molecules and ions inside and outside cell nearly equalConcentration of K+ higher inside than outside cell, Na+ higher outside than insidePotential difference: unequal distribution of charge exists between the immediate inside and immediate outside of the plasma membrane: -70 to - 90 mVThe resting membrane potential
13Sodium-Potassium Exchange Pump Insert Process Figure with verbiage; Insert Animation Sodium- Potassium Exchange Pump.exe
14Changes in the Membrane Potential Membrane potential is dynamicRises or falls in response to temporary changes in membrane permeabilityChanges in membrane permeability result from the opening or closing of membrane channelsTypes of channelsPassive or leak channels - always openGated channels - open or close in response to specific stimuli; 3 major typesLigand-gated channelsVoltage-gated channelsMechanically-gated channels
15Nongated (Leakage) channels Many more of these for K+ and Cl- than for Na+.So, at rest, more K+ and Cl- are moving than Na+.How are they moving?Protein repels Cl-, so Cl- moves out.K+ are in higher concentration on inside than out, they diffuse out.Always open and responsible for permeability when membrane is at rest.Specific for one type of ion although not absolute.How are they moving? Protein repels Cl, they move out.K are in higher  on inside then out, they move out.Gated ion channels open and close because of some sort of stimulus. When they open, they change the permeability of the cell membrane.Ligand-gated: (molecule that binds to a receptor) receptor: protein or glycoprotein to which a ligand can bind. E.g., acetylcholine binds to acetylcholine receptor on a Na channel. Channel opens, Na enters the cell.Voltage-gated: open and close in response to small voltage changes across the cell membrane. At rest, membrane is neg. on the inside relative to the outside. When cell is stimulated, that relative charge changes and voltage-gated ion channels either open or close. Most common voltage gated are Na and K. In cardiac and smooth muscle, Ca are important.Other than muscle/nerve, other cells that have voltage-gated channels, touch and temperature receptors.
17Gated ion channels. Gated ion channels open and close because of some sort of stimulus. When they open, they change the permeability of the cell membrane.Ligand-gated: open or close in response to ligand (a chemical) such as ACh binding to receptor protein.Acetylcholine (ACh) binds to acetylcholine receptor on a Na+ channel. Channel opens, Na+ enters the cell.Ligand-gated channels most abun- dant on dendrites and cell body; areas where most synaptic commu- nication occursGated Ion Channels
18Local Potentials/Graded Potentials Graded: of varying intensity; NOT all the same intensityChanges in membrane potential that cannot spread far from site of stimulationCan result in depolarization or hyperpolarizationDepolarizationOpening Na+ channels allows more + charges to enter thereby making interior less negative (-70 mV -60mV); see next slideRMP shifts toward O mVHyperpolarizationOpening of K+ channels allows more + charges to leave thereby making interior more negative (-70 mV -80 mV); see next slideRMP shifts away from O mVRepolarizationProcess of restoring membrane potential back to normal (RMP)Degree of depolarization decreases with distance from stimulation site; called decremental spread (see next slide)Graded potentials occur on dendrites and cell bodies of neurons but also on gland cells, sensory receptors, and muscle cell sarcolemmaAffect only a tiny area (maybe only 1 mm in diameter)If so, how do neurons trigger release of neurotransmitter far from dendrites/cell body?
19Changes in Resting Membrane Potential: Ca2+ Voltage-gated Na+ channels sensitive to changes in extracellular Ca2+ concentrationsIf extracellular Ca2+ concentration decreases- Na+ gates open and membrane depolarizes.If extracellular concentration of Ca2+ increases- gates close and membrane repolarizes or becomes hyperpolarized.
23Graded PotentialsGraded potentials decrease in strength as they spread out from the point of origin
24Action Potential: Resting State Na+ and K+ channels are closedLeakage accounts for small movements of Na+ and K+Each Na+ channel has two voltage-regulated gatesActivation gates – closed in the resting stateInactivation gates – open in the resting state
25Action Potential: Depolarization Phase Some stimulus opens Na+ gates and Na+ influx occursK+ gates are closedNa+ influx causes a reversal of RMPInterior of membrane now less negative (from -70 mV -55 mV)Threshold – a critical level of depolarization (-55 to -50 mV)At threshold, depolarization becomes self-generatingI.e., depolarization of one segment leads to depolarization in the nextIf threshold is not reached, no action potential develops
26Action Potential: Repolarization Phase Sodium inactivation gates closeMembrane permeability to Na+ declines to resting levelsAs sodium gates close, voltage-sensitive K+ gates openK+ exits the cell and internal negativity of the resting neuron is restored
27Action Potential: Hyperpolarization Potassium gates remain open, causing an excessive efflux of K+This efflux causes hyperpolarization of the membrane (undershoot)The neuron is insensitive to stimulus and depolarization during this time
28Phases of the Action Potential 1 – RESTING STATERMP = -70 mV2 – DEPOLARIZATIONIncreased Na+ influxMP becomes less negativeIf threshold is reached, depolarization continuesPeak reached at +30 mVTotal amplitude = 100 mV3 – REPOLARIZATIONDecreased Na+ influxIncreased K+ effluxMP becomes more negative4 – HYPERPOLARIZATIONExcess K+ effluxBlue line = membrane potentialYellow line = permeability of membrane to sodiumGreen line = permeability of membrane to potassium
30Propagation of an Action Potential along an Un myelinated Axon
31Action Potential Propagation Illustration shows continuous propagation of a nerve impulseon an unmyelinated axon.Action potentials occur over the entire surface of the axon membrane.Insert Process Fig with verbiage; Insert Animation Action Potential Propagation in an Unmyelinated Axon.exe
32Saltatory Conduction Impulse Conduction in Myelinated Neurons Most Na+ channels concentrated at nodes. No myelin present.Leakage of ions from one node to another destabilize the second leading to another action potential in the second node. And so on….
33Action Potential: Role of the Sodium-Potassium Pump RepolarizationRestores the resting electrical conditions of the neuronDoes not restore the resting ionic conditionsIonic redistribution back to resting conditions is restored by the sodium-potassium pump
34All-or-none principle All-or-none principle. No matter how strong the stimulus, as long as it is greater than threshold, then an action potential will occur.The amplitude of the de- polarization wave will be the same for all action potentials generated.Action Potentials
36Refractory Period Parts Sensitivity of area of the membrane to further stimulation decreases for a timePartsAbsoluteComplete insensitivity exists to another stimulusFrom beginning of action potential until near end of repolarization.No matter how large the stimulus, a second action potential cannot be produced.Has consequences for function of muscleRelativeA stronger-than-threshold stimulus can initiate another action potential
37Speed of Impulse Conduction Faster in myelinated than in non-myelinatedIn myelinated axons, lipids act as insulation (the myelin sheath) forcing local currents to jump from node to nodeIn myelinated neurons, speed is affected by:Thickness of myelin sheathDiameter of axonsLarge-diameter conduct more rapidly than small-diameter. Large diameter axons have greater surface area and more voltage-gated Na+ channels
38Nerve Fiber TypesType A: large-diameter (4-20 µm), heavily myelinated. Conduct at m/s (= 300 mph).Motor neurons supplying skeletal muscles and most sensory neurons carrying info. about position, balance, delicate touchType B: medium-diameter (2-4 µm), lightly myelinated. Conduct at 3-15 m/s.Sensory neurons carrying info. about temperature, pain, general touch, pressure sensationsType C: small-diameter (0.5-2 µm), unmyelinated. Conduct at 2 m/s or less.Many sensory neurons and most ANS motor neurons to smooth muscle, cardiac muscle, glands
39Coding for Stimulus Intensity All action potentials are alike (of the same amplitude) and are independent of stimulus intensity.The amplitude of the action potential is the same for a weak stimulus as it is for a strong stimulus.So how does one stimulus feel stronger than another?Strong stimuli generate more action potentials than weaker stimuli.More action potentials stimulate the release of more neurotransmitter from the synaptic knobThe CNS determines stimulus intensity by the frequency of impulse transmission
41Trigger Zone: Cell Integration and Initiation of AP
42Trigger Zone: Cell Integration and Initiation of AP
43Trigger Zone: Cell Integration and Initiation of AP Excitatory signal:Opening of Na+ channelsDepolarizes membrane (-70 mV -60 mV)Brings membrane closer to thresholdMore likely to give rise to an action potentialInhibitory signalOpening of K+ channelsHyperpolarizes the membrane (-70 mV -80 mV)Takes membrane further from thresholdLess likely to give rise to an action potential
44Postsynaptic Potentials Excitatory postsynaptic potential (EPSP)Depolarization occurs and response stimulatoryDepolarization might reach threshold producing an action potential and cell responseInhibitory postsynaptic potential (IPSP)Hyperpolarization and response inhibitoryDecrease action potentials by moving membrane potential farther from threshold
45SUMMATION Individual EPSPs can combine through summation Individual EPSP has a small effect on membrane potentialProduce a depolarization of about 0.5 mVCould never result in an APIndividual EPSPs can combine through summationIntegrates the effects of all the graded potentialsGPs may be EPSPs, IPSPs, or bothTwo types of summationTemporal summationSpatial summation
46Summation Fig. A illustrates spatial summation Fig. B illustrates temporal summationFig. C shows both EPSPs and IPSPsaffecting the membrane
47Neuronal Pathways and Circuits Organization of neurons in CNS varies in complexityConvergent pathways: several neurons converge on a single postsynaptic neuron. E.g., synthesis of data in brain.Divergent pathways: the spread of information from one neuron to several neurons. E.g., important information can be transmitted to many parts of the brain.
48Oscillating circuits: Arranged in circular fashion to allow action potentials to cause a neuron in a farther along circuit to produce an action potential more than once. Can be a single neuron or a group of neurons that are self stimulating. Continue until neurons are fatigued or until inhibited by other neurons. Respiration? Wake/sleep?Oscillating Circuits
55Frequency response characteristics of different Filters
56Neural network architectures Several NN have been proposed & investigated in recent yearsSupervised versus unsupervisedArchitectures (feedforward vs. recurrent)Implementation (software vs. hardware)Operations (biologically inspired vs. psychologically inspired)In this chapter, we will focus on modeling problems with desired input-output data set, so the resulting networks must have adjustable parameters that are updated by a supervised learning rule
57Sample Feed forward Network (No loops) WeightsWeightsWeightsWjiVikF(S wji xj
58Lms learning rule 1. Apply input to Adaline input 2. Find the square error of current inputErrsq(k) = (d(k) - W x(k))**23. Approximate Grad(ErrorSquare) bydifferentiating Errsqapproximating average Errsq by Errsq(k)obtain -2Errsq(k)x(k)Update W: W(new) = W(old) + 2mErrsq(k)X(k)Repeat steps 1 to 4.
67Image to ASCII Conversion using Neural Network (Cont.d)
68Review questionsWhat is Processing Element. How would you relate the PEs with real neuronsDefine Resting Potential. What is the average refractory period of a neuron. Is it limited to a particular value. If Yes mention How?Differentiate Resting potential and action potentialState Hebbs Learning Rule. Draw a sample memory mapping diagram by your own.How would you factor out the weight vector from the exception value termsWhat is the use of signal processing techniques in neural networks
70ReferencesJ. A. Freeman and D. M. Skapura, Neural Networks- Algorithms, Applications and Programming Techniques, Pearson Education( singapore) Pvt. Ltd., 1991.(Chapters 1 &2)psychology.about.com/od/biopsychology/f/neuron01.htmfaculty.washington.edu/chudler/chnt1.html