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Resting membrane Potential.ppt

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					RESTING MEMBRANE
    POTENTIAL
       D
Faith is like electricity.
 You can not see it….
     But
 You can see the light!

                             (Anonymous)
John Carew Eccles   Alan Lloyd Hodgkin   Andrew F. Huxley
                   Outline

• Introduction
  - Cell Membrane
  - Basic Principles of Electricity

• Membrane Potentials
  - Equilibrium Potentials for Na+ & K+
  - Resting Membrane Potentials
     (Mechanism & Ionic Movement)
    Cell Membrane




Cell Membrane- A Selective Barrier
              Basic Principles of Electricity

      Membrane potential is a BATTERY providing power to drive currents
                          when the cell is activated




•    Opposite charges attract & like charges repel

•    Work/Energy required to separate the opposite charges when
    they have come together

•    When opposite particles are separated, the electrical force of
    attraction can be harnessed to perform work.

•   Correlation to the ionic composition of ICF & ECF & Cell
    Membrane Physiology
Certain Facts in the Life of a Cell
• All cells have a membrane potential

• Cells of excitable tissues (nerve/muscle) produce
  transient changes in their membrane potentials when
  excited.

• Electric charges primarily responsible for membrane
  potentials are Na+, K+ and Anions (A-)

• Anions are non diffusible

• K+ has more Leaky Channels than N+
• Proteins and phosphates are negatively charged at
  normal cellular pH.

• Concentration gradient for K+ will always be outward
  whereas that for Na+ will be inwards

• Na+/ K+-ATPase pumps 3 Na+ out for 2 K+ in.

• All contribute to unequal charge across the
  membrane.
Basis for Membrane Potential

For Potassium
 More inside than outside
For Sodium
More outside than inside
Potential difference of – 90 mV, if K+ were the only diffusible ion.
     Resting Membrane Potentials
Voltage difference across the cell membrane
  resulting from differential concentrations of Na+
  & K+ on either side of cell membrane

            OR
Difference of cations & anions

            OR
Separation of unequal charges across the
  membrane
Equilibrium Potentials (E ion)

 • Theoretical voltage produced across the
   membrane if only 1 ion could diffuse
   through the membrane.

 • Potential difference

 • Magnitude of difference in charge on
   the two sides of the membrane.
Deduction of Equilibrium Potential … Nernst Law
  • E = 61 log Co/Ci

    E= Equilibrium potential for ion (mV)
    61: A constant that incorporates the universal gas
    constant (R), absolute temperature (T), ions valence
    (z), & electrical component of Faraday (F).
    Co= Concentration of ion outside cell
    Ci= Concentration of ion inside cell

  ENa+ = +60 mV                               EK+ = -90 mV
                           THE NERNST EQUATION
          The cytoplasmic and extracellular concentrations of an ion
             determine the chemical driving force for that ion and
          the equilibrium membrane potential if this is the ONLY ion
                   that is permeable through the membrane



                      58 mV log [X]out                        Nernst Equation
                  EX = z
                                           [X]in
                 Where EX is the chemical potential and z is the charge of ion X




For potassium:      [K+]in = 130 mM              [K+]out = 5 mM                    z = +1

                             58 mV log 5
                        EK+ = 1                            = - 82 mV
                                       130
                      THE GOLDMAN EQUATION

                           PK EK + PNa ENa + PCl ECl
from before
                      Vm =       PK + PNa + PCl


                                     58 mV log [X]out
Nernst equatiion            EX =       z            [X]in


Goldman equation      Vm = 58 mV log10       (   PK[K+]o + PNa[Na+]o + PCl[Cl-]i
                                                 PK[K+]i + PNa[Na+]i + PCl[Cl-]o   )
              The greater an ion’s concentration and permeability, the more
                    it contributes to the resting membrane potential
RELATIVE PERMEABILITY & THE RESTING POTENTIAL

            PK EK + PNa ENa + PCl ECl
       Vm =       PK + PNa + PCl

        [K+]o = 5 mM     [Na+]o = 145 mM   [Cl-]o = 100 mM

        [K+]i = 130 mM [Na+]i = 5 mM       [Cl-]i = 8 mM

      EK = - 82.1 mV    ENa = 84.8 mV      ECl = - 63.6 mV

         PK = 1           PNa = 0.05        PCl = 0.2


                       Vm = - 72.4 mV
    Resting Membrane Potential
  OUTSIDE                                          Na+                                          Cl-
                             K+

Electrostatic Force                                                                                     Force of Diffusion

            +++++++++++++++++++++++++++++++++++++++++++


                          open                 Close 3Na/2K no                                        open
                         channel                  d      pump chann                                channel
            - - - - - - - - - - - - - - - - - -chann - - - - - - - - - - - - - - el - - - - - - - - - - - - -- - - - - - -
                                                ------                           --

    Force of Diffusion                            el                                                         Electrostatic Force

                                                      Na
INSIDE                         K+                          +                                      Cl-

                                                                            Pr-
         - 65 mV
                                                                                                                   23
  K+ = Potassium; Na+ = Sodium; Cl- = Chloride; Pr- = proteins
Summary
•    All cells have a membrane potential which is the
    separation of opposite charges across the membrane.

• Na+-K+ pump makes small direct contribution to the
  membrane potential by transporting three N+ ions out
  of cell and two K+ ions inside cell.

• Being more permeable to K+, membrane is influenced
  more by K+ movement.

• At resting membrane potential, no net movement of
  ions takes place because any further leaking of the ions
  is counter balanced by Na-K pump.
               Target Ahead…




                2.              threshold
1.                            3.            4.
                  Excitator
Resting                       Action        Inhibitory
                  y Post-
   Potential                  Potential     Post-
                  synaptic
(just                                          synaptic
                  potential
   describe                                    potential
ACTION POTENTIAL
Action Potential


Changes in Ion Permeability allows inward
 Na flux and triggers an increased outward
 K flux through voltage gated ion channels
Causes transient change in Membrane
 Potential
The change in ion permeability is triggered
 by transient depolarization of the
 membrane
             Ionic Basis for AP
Two Types of Channels
  Voltage-gated Na+
  Voltage-gated K+
 Stages of Action Potential
1. Resting Stage
2. Depolarization stage
3. Repolarization stage
4. Hyper-polarization stage
                     Refractory Periods
• Absolute refractory
  period:
   – Axon membrane is
     incapable of producing
     another AP.
• Relative refractory
  period:
   – VG ion channel shape
     alters at the molecular
     level.
   – VG K+ channels are
     open.
   – Axon membrane can
     produce another action
     potential, but requires
     stronger stimulus.
           Conduction in Myelinated Axon
• Myelin prevents movement
  of Na+ and K+ through the
  membrane.
• Interruption in myelin
  (Nodes of Ranvier) contain
  VG Na+ and K+ channels.
• AP occurs only at the
  nodes.
    – AP at 1 node
      depolarizes membrane
      to reach threshold at
      next node.
• Saltatory conduction
  (leaps).
    – Fast rate of conduction.
Thank You

				
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