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Title: Announcements:


1
  • Announcements

2
  • Last lecture
  • Organization of the nervous system
  • Introduction to the neuron
  • Today electrical potential
  • Generating membrane potential
  • Nernst equation
  • Goldman equation
  • Maintaining ionic distributions

3
Neural Signaling
A Simple Circuit
Between neurons
Within neurons
chemical electrical
electrical
4
Bioelectric Potentials
  • Neurons have an electrical potential (voltage)
    across the cell membrane
  • The inside of the cell is more negative than the
    outside
  • called the Resting Membrane Potential

5
Measuring Membrane Potential
amplifier
microelectrode
Reference electrode
0 mV
cell
-80 mV
time
Bathing solution
6
Electrophysiology techniques
Silver / Silver chloride wire electrode
Amplifier
output
Reference electrode
3M KCl solution
Glass micropipette
Very tiny hole (ltlt0.1?m)
7
Resting Membrane Potential
  • How is it generated?
  • differential distribution of ions inside and
    outside the cell
  • Selective Permeability of the membrane to some
    ions

8
  • How does unequal concentration of ions give rise
    to membrane potential ?

9
Equal concentrations of ions
0 volts
Artificial ion selective membrane (only K, not
Cl-)
voltmeter
K
I
II
K
0.01 M KCL
0.01 M KCL
K
K
No net movement
10
Unequal concentrations of ions
volts

-
Ion selective membrane (only K, not Cl-)
K
I
II
K
K
K
0.1 M KCL
0.01 M KCL
K
K
K
K concentration gradient
11
Initial
New Equilibrium






CHEMICAL
CHEMICAL
ELECTRICAL
12
Unequal concentrations of ions
  • Initial diffusion of K down concentration
    gradient from I to II
  • This causes charge to accumulate in II because
    and - charges are separated
  • Remember that Cl- cant cross the membrane !
  • Therefore II becomes positive relative to I

13
Equilibrium Potential
  • As II becomes , movement of K is repelled
  • Every K near the membrane has two opposing
    forces acting on it
  • Chemical gradient
  • Electrical gradient
  • These two forces exactly balance each other
  • Called the electrochemical equilibrium

14
  • The electrical potential that develops is called
    the equilibrium potential for the ion.
  • Electrical potential at which there is no net
    movement of the ion
  • Note
  • only a very small number of ions actually
    contribute to the electrical potential
  • the overall concentrations of K and Cl in
    solution do not change.

15
  • To calculate the equilibrium potential of any ion
    (eg. K, Na, Ca,) at any concentration
  • we use the Nernst Equation

16
Nernst Equation
Ion Concentration I
Temp (?K)
Gas Constant
Equilibrium Potential of X ion (eg. K) in Volts
Ion Concentration II
Valence of ion (-1, 1, 2)
Faraday constant
17
Nernst Equation
  • At 18?C, for a monovalent ion, and converting to
    log10 ,the equation simplifies to

18
  • By convention electrical potential inside of
    cells is expressed relative to the outside of the
    cell

19
Example K
-0.040 Volts - 40 mV
20
  • Therefore,
  • initial movement of K down concentration
    gradient
  • When electrical potential of -40 mV develops,
    there will be no net movement of K
  • Thus K is in electrochemical equilibrium

21
What if there is more than one permeable ion?
Permeable to K and Na, but not Cl-
22
  • To calculate the overall potential of multiple
    ions
  • use the Goldman Equation
  • Considers the permeability of ions and their
    concentrations

23
Goldman equation
Voltage
Ion concentration
Permeability
Because Cl is negative
24
Goldman equation
  • Example, typical mammalian cell
  • Assume permeability for Na is 1/100 of
    permeability for K, and permeability of Cl is 0
  • Assume Kin 140, Kout5
  • Nain 10, Naout120

25
Goldman equation
  • The resting membrane potential of most cells is
    predicted by the Goldman equation

26
Summary Key Concepts
  • Unequal distributions of an ion across a
    selective membrane
  • causes an electrochemical potential called the
    equilibrium potential
  • Two opposing forces act on ions at the membrane
  • A chemical force down the concentration gradient
  • An opposing electrical force

27
Summary Key Concepts
  • The equilibrium potential for an ion is described
    by the Nernst equation
  • Cell membranes are permeable to more than one ion
  • the membrane electrical potential is described by
    the Goldman equation

28
So What???
  • Everything the nervous system and muscles do
    depends on the resting membrane potential

29
Sample question
  • If two concentrations of KCl solution across a
    membrane give an equilibrium potential for K of
    -60 mV, what will the equilibrium potential be if
    the concentrations on each side are reversed
  • -120 mV
  • 0
  • 60 mV
  • -30 mV
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