1. UNIT-I INTRODUCTION TO ARTIFICIAL NEURAL NETWORK
IT 0469 NEURAL NETWORKS

2. Elementary 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.

3. Parts of the Neuron Cell Body Dendrites Axons
Contains the nucleus
Dendrites
Receptive regions; transmit impulse to cell body
Short, often highly branched
May be modified to form receptors
Axons
Transmit impulses away from cell body
Axon hillock; trigger zone
Where action potentials first develop
Presynaptic terminals (terminal boutons)
Contain neurotransmitter substance (NT)
Release of NT stimulates impulse in next neuron
Bundles of axons form nerves

4. Electrical Signals
Neurons produce electrical signals called action potentials ( = nerve impulse)
Nerve impulses transfer information from one part of body to another
e.g., receptor to CNS or CNS to effector
Electrical properties result from
ionic concentration differences across plasma membrane
permeability of membrane

5. Single Neuron Physiology

6. Resting Potential

7. Inhibitory & Exitatory Action Potential

8. An Introduction to the Resting Membrane Potential

9. Electrochemical Gradients

10. Resting 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 ions
Operation of a membrane pump called the sodium-potassium pump

11. What Establishes the RMP?
Diffusion of Na+ and K+ down their concentration gradients
Na+ diffuses into the cell and K+ diffuses out of the cell
BUT, membrane is 75x’s more permeable to K+ than Na+
Thus, more K+ diffuses out than Na+ diffuses in
This 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 membrane
THEREFORE, the inside of the membrane is measured at a -70 mV (1 mv = one-thousandth of a volt)

12. Resting Membrane Potential
Number of charged molecules and ions inside and outside cell nearly equal
Concentration of K+ higher inside than outside cell, Na+ higher outside than inside
Potential difference: unequal distribution of charge exists between the immediate inside and immediate outside of the plasma membrane: -70 to - 90 mV
The resting membrane potential

13. Sodium-Potassium Exchange Pump
Insert Process Figure with verbiage; Insert Animation Sodium- Potassium Exchange Pump.exe

14. Changes in the Membrane Potential
Membrane potential is dynamic
Rises or falls in response to temporary changes in membrane permeability
Changes in membrane permeability result from the opening or closing of membrane channels
Types of channels
Passive or leak channels - always open
Gated channels - open or close in response to specific stimuli; 3 major types
Ligand-gated channels
Voltage-gated channels
Mechanically-gated channels

15. Nongated (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.

16. Gated Channels

17. Gated 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 occurs
Gated Ion Channels

18. Local Potentials/Graded Potentials
Graded: of varying intensity; NOT all the same intensity
Changes in membrane potential that cannot spread far from site of stimulation
Can result in depolarization or hyperpolarization
Depolarization
Opening Na+ channels allows more + charges to enter thereby making interior less negative (-70 mV -60mV); see next slide
RMP shifts toward O mV
Hyperpolarization
Opening of K+ channels allows more + charges to leave thereby making interior more negative (-70 mV  -80 mV); see next slide
RMP shifts away from O mV
Repolarization
Process 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 sarcolemma
Affect only a tiny area (maybe only 1 mm in diameter)
If so, how do neurons trigger release of neurotransmitter far from dendrites/cell body?

19. Changes in Resting Membrane Potential: Ca2+
Voltage-gated Na+ channels sensitive to changes in extracellular Ca2+ concentrations
If extracellular Ca2+ concentration decreases- Na+ gates open and membrane depolarizes.
If extracellular concentration of Ca2+ increases- gates close and membrane repolarizes or becomes hyperpolarized.

20. Depolarization and Hyperpolarization

21. Depolarization
Hyperpolarization

23. Graded Potentials
Graded potentials decrease in strength as they spread out from the point of origin

24. Action Potential: Resting State
Na+ and K+ channels are closed
Leakage accounts for small movements of Na+ and K+
Each Na+ channel has two voltage-regulated gates
Activation gates – closed in the resting state
Inactivation gates – open in the resting state

25. Action Potential: Depolarization Phase
Some stimulus opens Na+ gates and Na+ influx occurs
K+ gates are closed
Na+ influx causes a reversal of RMP
Interior 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-generating
I.e., depolarization of one segment leads to depolarization in the next
If threshold is not reached, no action potential develops

26. Action Potential: Repolarization Phase
Sodium inactivation gates close
Membrane permeability to Na+ declines to resting levels
As sodium gates close, voltage-sensitive K+ gates open
K+ exits the cell and internal negativity of the resting neuron is restored

27. Action 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

28. Phases of the Action Potential
1 – RESTING STATE
RMP = -70 mV
2 – DEPOLARIZATION
Increased Na+ influx
MP becomes less negative
If threshold is reached, depolarization continues
Peak reached at +30 mV
Total amplitude = 100 mV
3 – REPOLARIZATION
Decreased Na+ influx
Increased K+ efflux
MP becomes more negative
4 – HYPERPOLARIZATION
Excess K+ efflux
Blue line = membrane potential
Yellow line = permeability of membrane to sodium
Green line = permeability of membrane to potassium

29. The Generation of an Action Potential

30. Propagation of an Action Potential along an Un myelinated Axon

31. Action Potential Propagation
Illustration shows continuous propagation of a nerve impulse
on 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

32. Saltatory 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….

33. Action Potential: Role of the Sodium-Potassium Pump
Repolarization
Restores the resting electrical conditions of the neuron
Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is restored by the sodium-potassium pump

34. All-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

36. Refractory Period Parts
Sensitivity of area of the membrane to further stimulation decreases for a time
Parts
Absolute
Complete insensitivity exists to another stimulus
From 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 muscle
Relative
A stronger-than-threshold stimulus can initiate another action potential

37. Speed of Impulse Conduction
Faster in myelinated than in non-myelinated
In myelinated axons, lipids act as insulation (the myelin sheath) forcing local currents to jump from node to node
In myelinated neurons, speed is affected by:
Thickness of myelin sheath
Diameter of axons
Large-diameter conduct more rapidly than small-diameter. Large diameter axons have greater surface area and more voltage-gated Na+ channels

38. Nerve Fiber Types
Type 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 touch
Type B: medium-diameter (2-4 µm), lightly myelinated. Conduct at 3-15 m/s.
Sensory neurons carrying info. about temperature, pain, general touch, pressure sensations
Type 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

39. Coding 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 knob
The CNS determines stimulus intensity by the frequency of impulse transmission

40. Frequency of Action Potentials

41. Trigger Zone: Cell Integration and Initiation of AP

42. Trigger Zone: Cell Integration and Initiation of AP

43. Trigger Zone: Cell Integration and Initiation of AP
Excitatory signal:
Opening of Na+ channels
Depolarizes membrane (-70 mV -60 mV)
Brings membrane closer to threshold
More likely to give rise to an action potential
Inhibitory signal
Opening of K+ channels
Hyperpolarizes the membrane (-70 mV -80 mV)
Takes membrane further from threshold
Less likely to give rise to an action potential

44. Postsynaptic Potentials
Excitatory postsynaptic potential (EPSP)
Depolarization occurs and response stimulatory
Depolarization might reach threshold producing an action potential and cell response
Inhibitory postsynaptic potential (IPSP)
Hyperpolarization and response inhibitory
Decrease action potentials by moving membrane potential farther from threshold

45. SUMMATION Individual EPSPs can combine through summation
Individual EPSP has a small effect on membrane potential
Produce a depolarization of about 0.5 mV
Could never result in an AP
Individual EPSPs can combine through summation
Integrates the effects of all the graded potentials
GPs may be EPSPs, IPSPs, or both
Two types of summation
Temporal summation
Spatial summation

46. Summation Fig. A illustrates spatial summation
Fig. B illustrates temporal summation
Fig. C shows both EPSPs and IPSPs
affecting the membrane

47. Neuronal Pathways and Circuits
Organization of neurons in CNS varies in complexity
Convergent 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.

48. Oscillating 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

49. Mc-Pitts Model of Neural Networks

50. How does neuron learn ?

51. Structure of Processing Element

52. Signal processing Techniques- Frequency Modulation of signals

53. Radio Transmission-Frequency domain

54. Echo suppression in Telephone networks

55. Frequency response characteristics of different Filters

56. Neural network architectures
Several NN have been proposed & investigated in recent years
Supervised versus unsupervised
Architectures (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

57. Sample Feed forward Network (No loops)
Weights
Weights
Weights
Wji
Vik
F(S wji xj

58. Lms learning rule 1. Apply input to Adaline input
2. Find the square error of current input
Errsq(k) = (d(k) - W x(k))**2
3. Approximate Grad(ErrorSquare) by
differentiating Errsq
approximating 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.

59. Structure of ADALINE

60. Structure of ALC(Adaptive Linear Combiner)

61. ALC as a transversal Filter

62. Use of ADALINE in solving XOR problems

63. MDALINE Architecture

64. BPN Architecture

65. Image to ASCII Conversion using Neural Network

66. Image to ASCII Conversion using Neural Network

67. Image to ASCII Conversion using Neural Network (Cont.d)

68. Review questions
What is Processing Element. How would you relate the PEs with real neurons
Define 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 potential
State Hebbs Learning Rule. Draw a sample memory mapping diagram by your own.
How would you factor out the weight vector from the exception value terms
What is the use of signal processing techniques in neural networks

69. Review questions (contd..)

70. References
J. 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.htm
faculty.washington.edu/chudler/chnt1.html