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BME 301: Biomedical Sensors

BME 301: Biomedical Sensors. Lecture Note 3: Bioelectric Potentials and Biopotential Electrodes. Bioelectric potentials. RESTING POTENTIAL-BASIC CONCEPT.

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BME 301: Biomedical Sensors

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  1. BME 301: Biomedical Sensors LectureNote 3: BioelectricPotentialsandBiopotentialElectrodes BME 301 Lecture Note 3 - Ali Işın 2013

  2. Bioelectric potentials BME 301 Lecture Note 3 - Ali Işın 2013

  3. RESTING POTENTIAL-BASIC CONCEPT • Cell membranes are typically permeable to only a subset of ionic species like pottasium(K+),Chloride(Cl-) & effectively blocks the entry of sodium(Na+) ions. • The various ions seeks a balance between inside & outside the cell according to concentration & electric charge. • Two effects result from inability of Na+ ions to penetrate membrane- • Concentration of Na+ ions inside cell is much lower than outside. Hence,outside of cell becomes more positive than inside. • In an attempt to to balance electric charge,additional K+ ions enters the cell,causing higher concentration of K+ ion inside the cell. • Charge balance can never be reached. • Equilibrium is reached with a potential difference across the membrane, negative on inside and positive on outside called Resting Potential. Polarized Cell during RP BME 301 Lecture Note 3 - Ali Işın 2013

  4. RESTING POTENTIAL IN NERVE CELL • A nerve cell has an electrical potential, or voltage, across its cell membrane of approximately 70 millivolts (mV). This means that this tiny cell produces a voltage roughly equal to 1/20th that of a flashlight battery (1.5 volts). • The potential is produced by the actions of a cell membrane pump, powered by the energy of ATP. • As shown in Figure, this membrane protein forces sodium ions (Na+) out of the cell, and pumps potassium ions (K+) in. As a result of this active transport, the cytoplasm of the neuron contains more K+ ions and fewer Na+ ions than the surrounding medium. However, the neuron cell membrane is much leakier to K+ than it is to Na+. As a result, K+ ions leak out of the cell to produce a negative charge on the inside of the membrane. • This charge difference is known as the Resting Potentialof the neuron. The neuron is not actually "resting" because it must produce a constant supply of ATP to fuel active transport. BME 301 Lecture Note 3 - Ali Işın 2013

  5. Na+ Na+ K+ Cl- Force of Diffusion Electrostatic Force Force of Diffusion Electrostatic Force Pr- K+ 3Na/2K pump Cl- RESTING POTENTIAL PROPOGATION OUTSIDE + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + open channel open channel Closed channel no channel - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - INSIDE - 65 mV K+ = Potassium; Na+ = Sodium; Cl- = Chloride; Pr- = proteins BME 301 Lecture Note 3 - Ali Işın 2013

  6. ACTION POTENTIAL-BASIC CONCEPT • When section of cell membrane is excited by some form of externally applied energy, membrane characteristics changes & begins to allow some sodium ions to enter. • This movement of Na+ ions constitutes an ionic current that further reduces the barrier of the membrane to Na+ ions. • Result-Avalanche effect, Na+ ions rush into the cell to balance with the ions outside . • At the same time K+ ions which were in higher concentration inside the cell during resting state, try to leave the cell but are unable to move as rapidly as Na+ ions. • As a result the cell has slightly positive potential on inside due to imbalance of K+ ions. • This potential is called as Action Potential . Depolarized cell during AP BME 301 Lecture Note 3 - Ali Işın 2013

  7. WAVEFORM SHOWING DEPOLARIZATION & REPOLARIZATION IN ACTION POTENTIAL • The cell that displays an action Potential is said to be depolarized; • The process of changing from resting state to action potential is called Depolarization. • Once the rush of Na+ ions through the cell membrane has stopped, the membrane reverts back to its original condition wherein the passage of Na+ ions from outside to inside is blocked • This process is called Repolarization. BME 301 Lecture Note 3 - Ali Işın 2013

  8. ACTION POTENTIAL PROPOGATION • It “travels” down the axon • Actually, it does not move. Rather the potential change resulting from Na+ influx disperses to the next voltage-gated channel, triggering another action potential there. BME 301 Lecture Note 3 - Ali Işın 2013

  9. PROPOGATION OF POTENTIALS IN NERVE IMPULSE • The Moving Impulse An impulse begins when a neuron is stimulated by another neuron or by the environment. Once it begins, the impulse travels rapidly down the axon away from the cell body and towards the axon terminals. • As Figure shows, an impulse is a sudden reversal of the membrane potential. What causes the reversal? The neuron membrane contains thousands of protein channels or gates, that allow ions to pass through. Generally, these gates are closed. At the leading edge of an impulse, however, sodium gates open, allowing positively charged Na+ ions to flow inside. The inside of the membrane temporarily becomes more positive than the outside, reversing the resting potential. This reversal of charges is called an Action Potential. As the action potential, potassium gates open, allowing positively charged K+ ions to flow out. This restores the Resting Potential so that the neuron is once again negatively charged on the inside of the cell membrane and positively charged on the outside. BME 301 Lecture Note 3 - Ali Işın 2013

  10. A nerve impulse is self-propagating. That is, an impulse at any point on the membrane causes an impulse at the next point along the membrane. We might compare the flow of an impulse to the fall of a row of dominoes. As each domino falls, it causes its neighbour to fall. Then, as the impulse passes, the dominoes set themselves up again, ready for another Action Potential. BME 301 Lecture Note 3 - Ali Işın 2013

  11. Resting and action potentials • The resting potential is the result of an unequal distribution of ions across the membrane. • The resting potential is sensitive to ions in proportion to their ability to permeate the membrane. BME 301 Lecture Note 3 - Ali Işın 2013

  12. Resting potentials • Forget the membrane and consider what factors determine the movement of ions in solution. • Aqueous diffusion and • Electrophoretic movement BME 301 Lecture Note 3 - Ali Işın 2013

  13. Resting potentials 0 mV BME 301 Lecture Note 3 - Ali Işın 2013

  14. Resting potentials 0 mV BME 301 Lecture Note 3 - Ali Işın 2013

  15. Resting potentials -80 mV BME 301 Lecture Note 3 - Ali Işın 2013

  16. Resting potentials + + + + + - + - - + - -80 mV - BME 301 Lecture Note 3 - Ali Işın 2013 -

  17. Resting potentials [K+] = 2.5 [Na+] = 125 [Cl-] = 130 A- + + + + + - + - - + [K+] = 135 [Na+] = 7 [Cl-] = 11 A- - -80 mV - BME 301 Lecture Note 3 - Ali Işın 2013 -

  18. Resting potentials Resting membrane potential is independent of external Na+ concentration BME 301 Lecture Note 3 - Ali Işın 2013

  19. Resting potentials Resting membrane potential strongly depends upon the external K+ concentration BME 301 Lecture Note 3 - Ali Işın 2013

  20. Summary • The membrane conducts ions very poorly and allows the separation of ionic species. • This results in a potential difference between the outside and the inside of the membrane. • The magnitude of the resting potential is determined by the selective permeability of the membrane to ionic species. • We can quantify the magnitude of the resting potential by considering both the diffusive and electrophoretic properties. • In order to understand the time dependence and individual contributions of ionic species to the membrane potential it is convenient to use an electrical equivalent circuit. BME 301 Lecture Note 3 - Ali Işın 2013

  21. Cl- Na+ + + + + + + + + + + + + + + K+ + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - K+ Cl- Na+ Resting Membrane Potential outside Membrane A- inside BME 301 Lecture Note 3 - Ali Işın 2013

  22. Membrane is polarized • more negative particles in than out • Bioelectric Potential • like a battery • Potential for ion movement • current ~ BME 301 Lecture Note 3 - Ali Işın 2013

  23. POS NEG Bioelectric Potential OUTSIDE INSIDE BME 301 Lecture Note 3 - Ali Işın 2013

  24. Biopotentials • ECG • electrocardiogrphy • EEG • electroencephalography • EMG • electromyography • ERG • electroretinograpy • EOG… • electrooculography BME 301 Lecture Note 3 - Ali Işın 2013

  25. Frequencies of Biopotentials BME 301 Lecture Note 3 - Ali Işın 2013

  26. Electrocardiogram (ECG) BME 301 Lecture Note 3 - Ali Işın 2013

  27. Recording System EEG • EEG recording is done using a standard lead system called 10-20 system • Recall dipole concept to identify source of brain activity BME 301 Lecture Note 3 - Ali Işın 2013

  28. Electromyogram (EMG) • Measures muscle activity • Recordintramuscularly through needle electrodes • Record surface EMG using electrodes on biceps,triceps… • Use in muscular disorders,muscle based prosthesis –prosthetic arm, leg BME 301 Lecture Note 3 - Ali Işın 2013

  29. Electroretinogram (ERG) • Biopotential of the eye (retina) • Indicator of retinal diseases such as retinal degenration, macular degernation • Invasive recording BME 301 Lecture Note 3 - Ali Işın 2013

  30. Electrodes KINDS OF ELECTRODES BME 301 Lecture Note 3 - Ali Işın 2013

  31. Eectrodes BME 301 Lecture Note 3 - Ali Işın 2013

  32. Eectrodes Figure A disposable surface electrode. A typical surface electrode used for ECG recording is made of Ag/AgCl. The electrodes are attached to the patients’ skin and can be easily removed. BME 301 Lecture Note 3 - Ali Işın 2013

  33. Biopotential Electrodes BME 301 Lecture Note 3 - Ali Işın 2013

  34. Electrode–electrolyte interface fig_05_01 The current crosses it from left to right. The electrode consists of metallic atoms C. The electrolyte is an aqueous solution containing cations of the electrode metal C+ and anions A–. where n is the valence of C and m is valence of A BME 301 Lecture Note 3 - Ali Işın 2013

  35. table_05_01 BME 301 Lecture Note 3 - Ali Işın 2013

  36. Vp = total patential, or polarization potential, of the electrode E0 = half-cell potential Vr = ohmicoverpotential Vc = concentration overpotential Va = activation overpotential BME 301 Lecture Note 3 - Ali Işın 2013

  37. BME 301 Lecture Note 3 - Ali Işın 2013

  38. A silver/silver chloride electrode, shown in cross section fig_05_02 BME 301 Lecture Note 3 - Ali Işın 2013

  39. 1.73  10–10  142.3 = 2.46  10–8 g BME 301 Lecture Note 3 - Ali Işın 2013

  40. Sintered Ag/AgCI electrode fig_05_03 BME 301 Lecture Note 3 - Ali Işın 2013

  41. Equivalent circuit for a biopotential electrode in contact with an electrolyte fig_05_04 Ehc is the half-cell potential, Rd and Cd make up the impedance associated with the electrode-electrolyte interface and polarization effects, Rs is the series resistance associated with interface effects and due to resistance in the electrolyte. BME 301 Lecture Note 3 - Ali Işın 2013

  42. Impedance as a function of frequency for Ag electrodes coated with an electrolytically deposited AgCl layer fig_05_05 The electrode area is 0.25 cm2. Numbers attached to curves indicate number of mAs for each deposit. (From L. A. Geddes, L. E. Baker, and A. G. Moore, “Optimum Electrolytic Chloriding of Silver Electrodes,” Medical and Biological Engineering, 1969, 7, pp. 49–56.) BME 301 Lecture Note 3 - Ali Işın 2013

  43. Experimentally determined magnitude of impedance as a function of frequency for electrodes fig_05_06 BME 301 Lecture Note 3 - Ali Işın 2013

  44. Example We want to develop an electrical model for a specific biopotential electrode studies in the laboratory. The electrode is characterized by placing it in a physiological saline bath in the laboratory, along with an Ag/AgCl electrode having a much greater surface area and a known half-cell potential of 0.233 V. The dc voltage between the two electrodes is measured with a very-high-impedance voltmeter and found to be 0.572 V with the test electrode negative The magnitude of the impedance between two electrodes is measured as a function of frequency at very low currents; it is found to be that given in Figure in slide 12. From these data, determine a circuit model for the electrode. BME 301 Lecture Note 3 - Ali Işın 2013

  45. Solution = 29.5 kΩ Half cell potential of the test electrode Ehc = 0.223 V – 0.572 = -0339 V At frequencies greater than 20 kHz Cd is short circuit. Thus Rs = 500 Ω = 0.5 kΩ, At frequencies less than 50 Hz Cd is open circuit. Thus Rs+ Rd = 30 kΩ.Thus Rd = 30 kΩ - Rs = 29.5 kΩ Corner frequency is 100 Hz. Thus Cd = 1/(2πf Rd) = 1/(2π100×29500) = 5.3×10-8 F = 0.53×10-9 F = 0.53 nF = 500 Ω = -0339 V = 0.53nF BME 301 Lecture Note 3 - Ali Işın 2013

  46. Magnified section of skin, showing the various layers fig_05_07 BME 301 Lecture Note 3 - Ali Işın 2013

  47. A body-surface electrode is placed against skin, showing the total electrical equivalent circuit obtained in this situation fig_05_08 Each circuit element on the right is at approximately the same level at which the physical process that it represents would be in the left-hand diagram. BME 301 Lecture Note 3 - Ali Işın 2013

  48. Body-surface biopotential electrodes fig_05_09 • Metal-plate electrode used for application to limbs, • Metal-disk electrode applied with surgical tape, • Disposable foam-pad electrodes, often used with electrocardiographic monitoring apparatus. BME 301 Lecture Note 3 - Ali Işın 2013

  49. A metallic suction electrode fig_05_10 A metallic suction electrode is often used as a precordial electrode on clinical electrocardiographs. BME 301 Lecture Note 3 - Ali Işın 2013 fig_05_10

  50. Examples of floating metal body-surface electrodes fig_05_11 • Recessed electrode with top-hat structure, • Cross-sectional view of the electrode in (a), • Cross-sectional view of a disposable recessed electrode of the same general structure shown in figure (c) in slide 17. The recess in this electrode is formed from an open foam disk, saturated with electrolyte gel and placed over the metal electrode. BME 301 Lecture Note 3 - Ali Işın 2013

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