Nerve physiology


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  • Nerve physiology

    1. 1. The Nerve Physiology© 2008 Paul Billiet ODWS
    2. 2. HISTORY
    3. 3. Joseph Erlanger(1874-1965) andHerbert Spencer Gasser (1888-1963) 1944 Joseph Erlanger and Herbert Spencer Gasser "for their discoveries relating to the highly differentiated functions of single nerve fibres
    4. 4. Joseph Erlanger and Herbert Spencer Gasser Joseph Erlanger was an American physiologist Herbert Spencer Gasser was an American physiologist, and recipient of the Nobel Prize for Physiology or Medicine in 1944 for his work with action potentials in nerve fibers while on the faculty of Washington University. Erlanger and his student Gasser were interested in developing tools that could measure impulses fired through nerve cells, and they turned to the cathode-ray oscilloscope – an instrument that allows electrical currents to be visualized as a moving two- dimensional graph on a phosphorescent screen. After its invention by Ferdinand Braun, the oscilloscope soon became the most effective tool for detecting rapid changes in electrical voltage, but still it was not sensitive enough to measure the weak and rapid electrical impulses that are fired along nerve cells
    5. 5. Eccles, Hodgkin and HuxleyThe Nobel Prize in Physiology or Medicine 1963 was awarded jointly to Sir John CarewEccles, Alan Lloyd Hodgkin and Andrew Fielding Huxley "for their discoveries concerning theionic mechanisms involved in excitation and inhibition in the peripheral and central portions ofthe nerve cell membrane".
    6. 6. Eccles, Hodgkin and HuxleySir Alan Lloyd Hodgkin British physiologist and biophysicistSir John Carew Eccles , Australian neurophysiologistSir Andrew Fielding Huxley English physiologist and biophysicist, Huxley evidenced the existence of saltatory conduction in myelinated nerve fibres. By showing how these impulses are generated and transmitted, the three scientists who receivedan equal share of the 1963 Nobel Prize in Physiology or Medicine revealed the key triggers thatspark the nervous systems in-built electrical system into life.Seeking ways of measuring electrical currents inside nerves, Alan Hodgkin and his studentAndrew Huxley turned to giant nerve fibres in the squid, which are almost a thousand times thickerthan their human counterparts. Using tiny electrodes to record the electrical difference between the inside and outside of thesenerves, they were surprised to find that the polarity did not drop from negative to zero during thetransmission of an impulse as predicted, but in fact reversed, becoming electrically positive. By carrying out a series of measurements and using complex mathematical models to interpretthe findings, Hodgkin and Huxley formulated a theory to propose how impulses are formed.Changes in the permeability of the cell membrane allow charged atoms to flow in and out of anerve fibre, creating waves of electric charge that constitute the nerve impulse
    7. 7. Erwin Neher, Bert Sakmann The Nobel Prize in Physiology or Medicine 1991 was awarded jointly to Erwin Neher and Bert Sakmann "for their discoveries concerning the function of single ion channels in cells"
    8. 8. Erwin Neher, Bert Sakmann The two German cell physologisists Erwin Neher and Bert Sakmann have together developed a technique that allows the registration of the incredibly small electrical current (amounting to a picoampere- 10-12A) that passes through a single ion channel. The technique is unique in that it records how a single channel molecule alters its shape and in that way controls the flow of current within a time frame of a few millionths of a second. They have demonstrated what happens during the opening or closure of an ion channel with a diameter corresponding to that of a single sodium or chloride ion.
    9. 9. The neurone Nodes of RanvierDendrites Schwann cell Nucleus of Schwann cell Myelin sheath Axon Terminal dendrites
    10. 10. Experiments on the neurone of a giant squid Concentration /mmol kg-1 water Ion Axoplasm Blood Sea water (the cytoplasm plasma in an axon) K+ 400 20 10 Na+ 50 440 460 Cl- 120 560 540 Organic anions 360 - - (-ve ions)© 2008 Paul Billiet ODWS
    11. 11. Cells and membrane potentials All animal cells generate a small voltage across their membranes This is because there is a large amount of small organic molecules in the cytoplasm To balance this, animal cell pump Na+ out of the cells This regulates osmosis but it leaves a large number of organic molecules These are overall negatively changed (anions) in the cytoplasm Thus the cell has a potential difference (voltage) across its membrane© 2008 Paul Billiet ODWS
    12. 12. Passive movement of ions across a cell membrane The concentration gradient: causing the ions to diffuse down their concentration gradient The electrical potential: causing ions to be attracted to the opposite charge to the one they carry© 2008 Paul Billiet ODWS
    13. 13. Potassium & Sodium Ions The two important ions in a nerve cell (neurone or neuron) are K+ and Na+ Both are cations (positively charged ions) Na+ ions move more slowly across the membrane than K+ or Cl- ions This is because although the Na+ ion is smaller than the K+ ion Na+ has a larger coating of water molecules giving it a bigger diameter This makes the plasma membrane 25 times more permeable to K+ than Na+© 2008 Paul Billiet ODWS
    14. 14. Potassium & Sodium Ions In addition to this K+ ions leak out of K + ion pores when the nerve cell is at rest So to maintain the high concentration of K+ inside the cell, it has to be actively pumped inwards a bit when the cell is at rest The result is that the resting potential of the neurone is almost at the equilibrium for K+ ions K + leak out a bit and need pumping in Na + ions, however, are actively pumped out and kept out© 2008 Paul Billiet ODWS
    15. 15. A coupled Na + -K + pump plasma Cytoplasm membrane ECF K+ K+ coupled ion pump Na + Na +© 2008 Paul Billiet ODWS
    16. 16. Ionic basis of Em NaK- ATPase pumps 3Na+ out for 2 K+ pumped in. Some of the K+ leaks back out, making the interior of the cell
    17. 17. Gated channels: ligand-gated
    18. 18. Gated channels: voltage-gated
    19. 19. Characteristics of the NerveImpulse An electrochemical event that occurs in nerve cells following proper stimulation. An all-or-none process which is fast acting and quick to recover. An event that is described by a voltage curve that is called an action potential. The nerve impulse can be conducted the entire length of a nerve cell without diminishment (“domino effect”).
    20. 20. Characteristics of a NerveImpulse Continued: The nerve impulse serves as the primary information signal used by the nervous system to provide communication about stimuli, nerve cell activity, neurotransmitter release and to generate various output responses (motor action, glandular secretion, etc.). Typically initiated by graded or generator potentials from a stimulus.
    21. 21. Graded potentialA change in potential that decreases withdistance Localized depolarization or hyperpolarization
    22. 22. Graded Potentials
    23. 23. Graded Potentials
    24. 24. Graded potentialsvsAction Potential
    25. 25. PSPs vs APs Graded All-or-none Summation longer duration *10-100 msec short chemical-gated 1-2 msec passive spread instantaneous voltage-gated decremental propagated slow nondecremental
    26. 26. Resting Membrane Voltage(RMV)
    27. 27. Nernst EquationBy the end of the 19th century, it wasknown that the cytoplasm was high in K+and that [Na+] was very low--and that thisrelationship was reversed outside the cell.The assumption was made that the cellmembrane was permiable to K+ but not toNa+.
    28. 28. Goldman equationwas derived to solvefor transmembranepotential using all ionsinvolvedsimultaneously.
    29. 29. Restingmembrane potential(MP) resting potential(RP)•nerve, muscle negative, constant, cell typee.g.: warm-blooded animal: -55 to -100 mV; smooth muscle: -30 mV
    30. 30. Resting PotentialOutside of cell Sodium/Potassium pump continuously and actively pumps (3) Na+ out of the cell and (2) K+ into the cell. Na+ channels are closed so Na+ are not able to move into the cell. K+ channels are open so K+ can diffuse out of the cell. This generates a separation of charges so that the inside of the cell is relatively – and the outside is relatively +. The cell will remain in this state (at rest) until it is stimulated.Inside of cell
    31. 31. Depolarisation Depolarising membranes may be achieved by: a stimulus arriving at a receptor cell (e.g. vibration of a hair cell in the ear) a chemical fitting into a receptor site (e.g. a neurotransmitter) a nerve impulse travelling down a neurone© 2008 Paul Billiet ODWS
    32. 32. Action PotentialAppears when region of excitablemembrane depolarizes to thresholdSteps involved Membrane depolarization and sodium channel activation Sodium channel inactivation Potassium channel activation Return to normal permeability
    33. 33. The Action Potential Key Properties of the Action Potential  Threshold  Rising phase  Overshoot  Falling phase  Undershoot  Absolute refractory period  Relative refractory period
    34. 34. Introduction Action Potential in the Nervous System  Conveys information over distances  Action potential  Spike  Nerve impulse  Discharge
    35. 35. AP Characteristics Voltage-gated channels All or none Slow Non-decremental Self Propagated
    36. 36. The action potential The action potential is the state of the neurone membrane when a nerve impulse passes by A small change in the membrane voltage will depolarise the membrane enough to flip open Na+ channels These are called voltage-gated Na + channels As Na+ moves into the cell more and more Na+ channels open A small change in the membrane permeability to Na+ results in a big change in membrane potential© 2008 Paul Billiet ODWS
    37. 37. Properties of the ActionPotential The Ups and Downs of an Action Potential  Oscilloscope to visualize an AP  Rising phase, overshoot, falling phase, and undershoot
    38. 38. Properties of the ActionPotential The Generation of an Action Potential  “All-or-none”: Cross threshold value for action potential  Chain reaction  Opens Na+-permeable channels Na+ influx depolarized membrane reaches threshold action potential
    39. 39. Properties of the ActionPotential  Firing frequency reflects the magnitude of the depolarizing current
    40. 40. The Generation of an ActionPotential Figure 2.16.1
    41. 41. Characteristics of actionpotentials Generation of action potential follows all-or-none principle Refractory period lasts from time action potential begins until normal resting potential returns Continuous propagation  spread of action potential across entire membrane in series of small steps Saltatory propagation  action potential spreads from node to node, skipping internodal membrane
    42. 42. The Generation of an Action Potential
    43. 43. Induction of an action potential
    44. 44. Induction of an action potential
    45. 45. Action potential propagation When the V-G Na+ channels open, they cause a depolarization of the neighboring membrane. This causes the Na+ and K+ channels in that piece of membrane to be activated
    46. 46. AP propagation cont. The V_G chanels in the neighboring membrane then open, causing that membrane to depolarize. That depolarizes the next piece of membrane, etc. It takes a while for the Na+ channels to return to their voltage-sensitive state. Until then, they won’t respond to a second depolarization.
    47. 47. Propagation of an Action Potentialalong an Unmyelinated Axon
    48. 48. Saltatory Propagation along aMyelinated Axon
    49. 49. Saltatory Propagation along aMyelinated Axon
    50. 50. Action Potential Conduction Propagation of the action potential  Down axon to the axon terminal  Orthodromic: Action potential travels in one direction  Antidromic (experimental): Backward propagation  Typical conduction velocity: 10 m/sec  Length of action potential: 2 msec
    51. 51. Action Potential Conduction Factors Influencing Conduction Velocity  Spread of action potential along membrane  Dependent upon axon structure  Path of the positive charge  Inside of the axon (faster)  Across the axonal membrane (slower)  Axonal excitability  Axonal diameter (bigger = faster)  Number of voltage-gated channels
    52. 52. Action Potential Conduction Factors Influencing Conduction Velocity  Myelin: Facilitates current flow  Layers of myelin sheath  Myelinating cells  Schwann cells in the PNS  Oligodendroglia in CNS
    53. 53. Action Potential Conduction Factors Influencing Conduction Velocity  Saltatory conduction
    54. 54. +35 0 More Na+ channels open mV Na+ floods into neurone Na+ voltage- gated channels open -55 Threshold -70 Time Resting potential Action potential© 2008 Paul Billiet ODWS
    55. 55. All-or-nothing As Na+ moves in the cell will become more positive with respect to the outside The ion pumps resist the change in the membrane potential but it only has to rise by 15mV and the pumps cannot restore the equilibrium Na+ floods in Nerve impulses all look the same , there are not big ones and little ones This is the all-or-nothing law© 2008 Paul Billiet ODWS
    56. 56. All-or-None PrincipleThroughout depolarisation, the Na+ continues to rush inside until the action potential reaches its peak and the sodium gates close.If the depolarisation is not great enough to reach threshold, then an action potential and hence an impulse are not produced.This is called the All-or-None Principle.
    57. 57. The threshold –55mV represents the threshold potential Beyond this we get a full action potential The membrane potential rises to +35mV this is the peak of the action potential The cells are almost at the equilibrium for Na+ ions© 2008 Paul Billiet ODWS
    58. 58. +35 Na+ channels close and K+ channels open, K+ floods out of neurone 0 mV -55 Threshold -70 Time Resting potential Action potential Resting potential© 2008 Paul Billiet ODWS
    59. 59. Potassium takes over After Na+ moves in passively until the Na+ channels start to close At the same time K+ permeability increases as voltage-gated K + channels open – they are a bit slower to respond to the depolarisation than the Na+ channels The K+ ions move out This makes the cell negative inside with respect to outside again The membrane potential falls© 2008 Paul Billiet ODWS
    60. 60. Hyperpolarisation The membrane potential falls below the resting potential of –70mV It is said to be hyperpolarised Gradually active pumping of the ions (K + in and Na+ out) restores the resting potential During this period no impulses can pass along that part of the membrane This is called the refractory period© 2008 Paul Billiet ODWS
    61. 61. +35 Hyperpolarisation 0 of the membrane mV Active pumping of Na+ out and K+ in during the refractory period -55 Threshold -70 Time Resting potential Action potential Resting© 2008 Paul Billiet ODWS potential
    62. 62. Repolarization1. The sodium/potassiumpumps return the cell to aresting state by activelypumping (3) Na+ out ofthe cell and (2) K+ intothe cell.2. The K+ continues todiffuse out of the cell.
    63. 63. Refractory Period after AP  won’t fire again  relative & absolute Relative  during after hyperpolarization  requires greater depolarization ~
    64. 64. Refractory PeriodThere are two types of refractory period:Absolute Refractory Period – Na+ channels are inactivated and no matter what stimulus is applied they will not re- open to allow Na+ in & depolarise the membrane to the threshold of an action potential.Relative Refractory Period - Some of the Na+ channels have re-opened but thethreshold is higher than normal making it more difficult for the activated Na+channels to raise the membrane potential to the threshold of excitation.
    65. 65. Absolute refractory period Na+ channels deactivate  will not trigger AP  must reset
    67. 67. Cellular ElectrophysiologyMembrane PotentialThe Na+ / K+ Pump
    68. 68. SEIZURE
    69. 69. Anti-seizure Medications Seizures caused by hyperactive brain areas Multiple chemical classes of drugs  All have same approach  Decrease propagation of action potentials  ⇓ Na+, Ca++ influx (delay depolarization/prolong repolarization)  ⇑ Cl- influx (hyperpolarize membrane)
    70. 70. Anti-Seizure MedicationsBenzodiazepines Ion Channel Inhibitorsdiazepam (Valium®) carbamazepinelorazepam (Ativan®) (Tegretol®)Barbiturates phenytoinphenobarbital (Dilantin®) (Luminal®) Misc. Agents valproic acid (Depakote®)
    71. 71. Clinical Correlation It is the rate of action potential propagation that determines neurologic function.  Determined by frequency of action potentials. What is a seizure? What is a seizure? What would be the What would be the effect on the membrane effect on the membrane of ⇑ Cl- -influx of ⇑ Cl influx during a seizure? during a seizure? Hyperpolarization & … ⇓ seizure activity!
    72. 72. Cl - -Cl Gamma Amino Butyric Acid Receptors GABA GABA Receptor Receptor Exterior Hyperpolarized! Hyperpolarized! Interior
    73. 73. Cl - - Cl GABA+Bz Complex Bz Bz GABA GABAReceptorReceptor Receptor Receptor Profoundly Profoundly Exterior Hyperpolarized! Hyperpolarized! Interior
    74. 74. THANK YOU