Presentation on theme: "IT 0469 NEURAL NETWORKS. Neuron: A neuron nerve cell is an electricallyexcitable cell that processes and transmits information by electrical and chemical."— Presentation transcript:
IT 0469 NEURAL NETWORKS
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.
Cell Body 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
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
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
Diffusion of Na + and K + down their concentration gradients Na + diffuses into the cell and K + diffuses out of the cell BUT, membrane is 75xs 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)
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
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
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.
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
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?
Voltage-gated Na + channels sensitive to changes in extracellular Ca 2+ concentrations If extracellular Ca 2+ concentration decreases- Na + gates open and membrane depolarizes. If extracellular concentration of Ca 2+ increases- gates close and membrane repolarizes or becomes hyperpolarized.
Graded potentials decrease in strength as they spread out from the point of origin
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
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
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
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
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
Illustration shows continuous propagation of a nerve impulse on an unmyelinated axon. Action potentials occur over the entire surface of the axon membrane.
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….
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
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.
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
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
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
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
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
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
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
Fig. A illustrates spatial summation Fig. B illustrates temporal summation Fig. C shows both EPSPs and IPSPs affecting the membrane
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.
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?
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
Weights Wji Vik F( wji xj
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.
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
J. A. Freeman and D. M. Skapura, Neural Networks- Algorithms, Applications and Programming Techniques, Pearson Education( singapore) Pvt. Ltd., (Chapters 1 &2) psychology.about.com/od/biopsychology/f/neuron01.htm faculty.washington.edu/chudler/chnt1.html