Title: Announcements:
1 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
3Neural Signaling
A Simple Circuit
Between neurons
Within neurons
chemical electrical
electrical
4Bioelectric 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
5Measuring Membrane Potential
amplifier
microelectrode
Reference electrode
0 mV
cell
-80 mV
time
Bathing solution
6Electrophysiology techniques
Silver / Silver chloride wire electrode
Amplifier
output
Reference electrode
3M KCl solution
Glass micropipette
Very tiny hole (ltlt0.1?m)
7Resting 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 ?
9Equal 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
10Unequal 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
11Initial
New Equilibrium
CHEMICAL
CHEMICAL
ELECTRICAL
12Unequal 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
13Equilibrium 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
16Nernst 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
17Nernst 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
19Example 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
21What 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
23Goldman equation
Voltage
Ion concentration
Permeability
Because Cl is negative
24Goldman 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
25Goldman equation
- The resting membrane potential of most cells is
predicted by the Goldman equation
26Summary 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
27Summary 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
28So What???
- Everything the nervous system and muscles do
depends on the resting membrane potential
29Sample 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