Resting (membrane) Potential

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Resting (membrane) Potential
DENT/OBHS 131Neuroscience
2009
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Electrical signaling in neurons

• dendritic synaptic inputs
• transfer to the soma
• generate APs
• axonal propagation
• ionic basis of RMP
• AP initiation propagation

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Learning Objectives

1. Explain how the concentration gradient of
   potassium ions across the membrane gives rise to
   the resting membrane potential
2. Compute the equilibrium potential of an ion using
   the Nernst equation
3. Predict the effect of changing the concentration
   of an ion (or its relative permeability) on the
   membrane potential

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How familiar are you with resting and active
properties of membranes?

1. Not at all
2. Somewhat
3. Very
4. Intimately

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The major ion involved in setting the resting
membrane potential is...

1. Sodium
2. Calcium
3. Chloride
4. Potassium
5. Bicarbonate
6. Hydrogen

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Learning Objective 1

• Explain how the concentration gradient of
  potassium ions across the membrane gives rise to
  the resting membrane potential

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The RMP
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The membrane acts to

• separate and maintain (pumps) gradients of
  solutions with different concentrations of
  charged ions
• selectively allow certain ionic species
  (K) to cross the membrane

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• initial conditions
• different distribution of a K-salt
• membrane is only permeable to K
• there is no potential difference across the
  membrane

• at equilibrium
• K ions diffuse down concentration gradient
• anions are left behind net negativity develops
  inside the cell
• further movement of ions is opposed by the
  potential difference

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Electrical difference.. IN vs. OUT

-
0 mV
DS Weiss
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Electrical difference.. IN vs. OUT

-
0 mV
-70 mV
DS Weiss
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Can we calculate the potential?

• The Nernst equation determines the voltage at
  which the electrical and chemical forces for an
  ion (X) are balanced there is NO net movement of
  ions.

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Learning Objective 2

• Compute the equilibrium potential of an ion using
  the Nernst equation

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The Nernst potential for K

• if K is 10-fold higher on the inside
• in excitable cells the RMP is primarily
  determined by K ions

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If we lowered the KOUT 10-fold to 1 mM, the
RMP would..

1. Not change
2. Hyperpolarize
3. Depolarize

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The Nernst potential for K

• What about hyperkalemia?

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Learning Objective 3

• Predict the effect of changing the concentration
  of an ion (or its relative permeability) on the
  membrane potential

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Other ions affect RMP

• different ions have different distributions
• cell membrane is not uniformly permeable
  (leaky) to all ions
• relative permeability of an ion determines its
  contribution to the RMP
• a small permeability to Na and Cl offsets some of
  the potential set up by K
• in reality the cell membrane is lt negative than
  EK

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Concentrations of other ions..
ion Xin Xout Eq. (mV)

K 155 4 -98
Na 12 145 67
Cl 4.2 123 -90
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General rule(s)

• relationship between
• membrane potential
• ion equilibrium potentials
• if the membrane becomes more permeable to one ion
  over other ions then the membrane potential will
  move towards the equilibrium potential for that
  ion (basis of AP) - DRIVING FORCE
• artificial manipulation of MP - reverse direction
  of current flow (hence reversal or equilibrium
  potential)

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ion flux explanation

• driving force on an ion X will vary with MP
• (Em - Ex)
• Ohms law
• V IR I/g, or transformed I gV
• Ix gx (Em - Ex)
• there will be no current if
• no channels for ion X are open (no conductance,
  g)
• no driving force (MP is at Ex)