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  • 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. 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. 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. 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. 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. 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. 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. 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. The Goldman-Hodgkin-Katz Equation The resting membrane potential is closest to the equilibrium potential for the ion with the highest permeability!
  • 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. Resting Membrane Potential Summary
  • 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. 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.  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. 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. 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. 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. 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. 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. 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. 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. Saltatory Conduction • AP’s only occur at the nodes (Na channels concentrated here!) • increased velocity • energy conservation
  • 23. Conduction velocity non-myelinated myelinated - non-myelinated vs myelinated -