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  1. 1. Membrane Potential (Vm): - charge difference across the membrane - K+ Na+ K+ Na+ inside outside …how can passive diffusion of potassium and sodium lead to development of negative membrane potential?
  2. 2. Simplest Case If a membrane were permeable to only K+ then… inside outside K+ K+ K+ would diffuse down its concentration gradient until the electrical potential across the membrane countered diffusion.
  3. 3. Simplest Case K+ K+ If a membrane were permeable to only K+ then… The electrical potential that counters net diffusion of K+ is called the K+ equilibrium potential (EK). inside outside
  4. 4. The Potassium Nernst Potential Example: If Ko = 5 mM and Ki = 140 mM EK = -61 log(140/4) EK = -61 log(35) EK = -94 mV EK = 61 log Ki Ko So, if the membrane were permeable only to K+, Vm would be -94 mV …also called the equilibrium potential
  5. 5. Simplest Case Na+Na+ If a membrane were permeable to only Na+ then… The electrical potential that counters net diffusion of Na+ is called the Na+ equilibrium potential (ENa). inside outside Na+ would diffuse down its concentration gradient until potential across the membrane countered diffusion.
  6. 6. The Sodium Nernst Potential Example: If Nao = 142 mM and Nai = 14 mM EK = -61 log(14/142) EK = -61 log(0.1) EK = +61 mV EK = 61 log Nai Nao So, if the membrane were permeable only to Na+, Vm would be +61 mV
  7. 7. Resting Membrane Potential 0 mV EK -94 ENa +61 Vm -90 to -70 Why is Vm so close to EK? Ans. The membrane is far more permeable to K than Na..
  8. 8. The Goldman-Hodgkin-Katz Equation (also called the Goldman Field Equation) Calculates Vm when more than one ion is involved. oCliNaiK iCloNaoK m ClpNapKp ClpNapKp V ][']['][' ][']['][' log. -++ -++ ++ ++ = 61 NOTE: P’ = permeability iCloNaoK oCliNaiK m ClpNapKp ClpNapKp V ][']['][' ][']['][' log. -++ -++ ++ ++ = -61 or
  9. 9. The Goldman-Hodgkin-Katz Equation The resting membrane potential is closest to the equilibrium potential for the ion with the highest permeability!
  10. 10. ATP 3 Na+ 2 K+ ADP ActiveTransport K+ Na+ Na+ K+ inside outside Remember: sodium is pumped out of the cell, potassium is pumped in...
  11. 11. Resting Membrane Potential Summary
  12. 12.  Cells:  contain high a K+ concentration  have membranes that are essentially permeable to K+ at rest  Membrane electrical potential difference (membrane potential) is generated by diffusion of K+ ions and charge separation  measured in mV (=1/1000th of 1V)  typically resting membrane potentials in neurons are -70 to 90 mV Resting and action potentials + + + + + + + + + + + – – – – – – – – –– – Voltmeter – +0 mV-80 mV +
  13. 13. Nerve Action Potential  Nerve signals are transmitted by action potentials, which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane  Action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and then ends with an almost equally rapid change back to the negative potential
  14. 14.  Changes that occur at the membrane during the action potential  Transfer positive charges to the interior of the fiber at the onset and return positive charges to the exterior at its end Nerve Action Potential
  15. 15. Stages of action potential  Resting Stage.This is the resting membrane potential before the action potential begins. The membrane is said to be "polarized" during this stage because of the -90 millivolts negative membrane potential that is present
  16. 16. Depolarization Stage  The membrane suddenly becomes very permeable to sodium ions, allowing tremendous numbers of positively charged sodium ions to diffuse to the interior of the axon.  The normal "polarized" state is neutralized by the inflowing positively charged sodium ions, with the potential rising rapidly in the positive direction  This is called depolarization
  17. 17. Repolarization Stage  After the membrane becomes highly permeable to sodium ions, the sodium channels begin to close and the potassium channels open more than normal  Rapid diffusion of potassium ions to the exterior re-establishes the normal negative resting membrane potential  This is called repolarization of the membrane
  18. 18. The AP - membrane permeability • During the upstroke of an action potential:  Na permeability increases  due to opening of Na+ channels  memb. potential approaches ENa Na+ channels  K permeability increases  due to opening of K+ channels  mem. potential approaches EK • After hyperpolarization of membrane following an action potential: Membrane hyperpolarized resting potential K+ channels  There is increased K+ conductance  due to delayed closure of K+ channels • During the downstroke of an action potential:  Na permeability decreases  due to inactivation of Na+ channels 1 ms +61 0 (mV) -90 ENa EK
  19. 19. Properties of action potentials  Action potentials:  are all-or-none events  threshold voltage (sudden increase in the membrane potential) threshold -70 +60 mV 0 non- myelinated 0 800400  have constant conduction velocity  Fibers with large diameter conduct faster than small fibers Fiber diameter (mm) 0 3 6 9 Myelinated 12 75 15 50 25 0  are initiated by depolarization  action potentials can be induced in nerve and muscle by extrinsic stimulation
  20. 20. Propagation: Rest Opening of Na+ channels generates local current circuit that depolarizes adjacent membrane, opening more Na+ channels… Stimulated (local depolarization) Propagation (current spread)
  21. 21. Signal Transmission: Myelination • Schwann cells surround the nerve axon forming a myelin sheath • Sphingomyelin decreases membrane capacitance and ion flow 5,000-fold • Sheath is interrupted every 1-3 mm : node of Ranvier
  22. 22. Saltatory Conduction • AP’s only occur at the nodes (Na channels concentrated here!) • increased velocity • energy conservation
  23. 23. Conduction velocity non-myelinated myelinated - non-myelinated vs myelinated -