Transcript Document

LECTURE 3: ION CHANNELS & THE RESTING MEMBRANE POTENTIAL
REQUIRED READING: Kandel text, Chapters 7, pgs 105-139
+++++++++++++++++++++++++++++
Vm = Vin - Vout
-------------------------In resting neuron:
--------------------------
Vm ~ - 60 to - 75 mV
+++++++++++++++++++++++++++++
Membrane potential is a BATTERY providing power to drive currents
when the cell is activated
This lecture discusses how membrane potential is established
and maintained
MEASURING THE RESTING MEMBRANE POTENTIAL:
MICROPIPET FILLED WITH HIGH SALT
For this method, recording pipet has a very
fine tip and is filled with a high salt solution
(e.g. 3M KCl), so that pipet has very low
resistance.
In this way, the voltage measured by amplifier
accurately reflects the voltage across the cell
membrane.
(method not useful in very small cells due
to pipet salt poisoning of cells)
Rpipet
Rm (actual)
Vpipet
Vm (actual)
Vm (measured) = Vm (actual) + Vpipet
when Rpipet <<< Rm (actual)
Vm (measured) = Vm (actual)
MEASURING THE RESTING MEMBRANE POTENTIAL:
PATCH PIPET IN “WHOLE-CELL” CONFIGURATION
Patch pipet filled with cytoplasm-like solution is touched to cell membrane;
with negative pressure, the pipet makes a very tight
“cell-attached” or “on-cell” seal onto membrane (leak resistance > 10 GW)
Applying gentle suction can break the membrane inside the pipet, making pipet fluid
contiguous with the cytoplasm. This is the “whole-cell” configuration.
When break is made into cell, the pipet can record the membrane potential
TWO TYPES OF PROTEIN COMPLEXES CONTRIBUTE TO ESTABLISHING
THE RESTING MEMBRANE POTENTIAL
ION PUMP -- drives a specific ion or group of ions from one side
of the plasma membrane to the other side
Pumps drive ions ONE-WAY and use energy from ATP hydrolysis
to make the process energetically favorable
ION CHANNEL -- protein complex containing a small pore which
allows a specific ion or group of ions to pass
Flow of ions through channels is PASSIVE and is driven by the
prevailing chemical and electrical gradients
A channel is an ion-specific resistor with a certain conductance ( g )
For most channels, the conductance is the same for ions flowing IN or OUT
Other channels allow ions to pass with greater conductance in one direction;
these are called RECTIFYING CHANNELS
e.g., a channel with greater conductance of inward current is called
an inwardly rectifying channel
Na/K ATPase PUMP
2 K+
Na+/K+ ATPase USES ENERGY FROM
ATP HYDROLYSIS TO PUMP
SODIUM IONS OUT OF CELL &
POTASSIUM IONS INTO CELL
AT A 3 Na+ : 2 K+ RATIO
CONSEQUENCES OF PUMP ACTIVITY
[ K+ ]in
[ Na+ ]in
>>
[ K+ ]out
<<
[ Na+ ]out
Net positive charge pumped out
of cell causes a matching amount
of permeable chloride anions to move
out passively through channels
[ Cl- ]in
<<
[ Cl- ]out
outside
inside
3 Na+
ATP
ADP + Pi
IONS CHANNELS
K+
POTASSIUM
CHANNEL
(non-gated, “leak”)
outside
inside
Some types of ion channels are “gated”, meaning the
ion-selective pore can be either open or shut (not in-between)
Such channels can be gated by ligands, phosphorylation, or voltage
Other types of ion channels are open all the time
These channels referred to as “leak” channels
Na+
POTASSIUM CHANNELS FAVOR A NEGATIVE MEMBRANE POTENTIAL
Potassium channels are the most abundant leak channels in neurons.
Because the Na/K pump makes [K+]in >> [K+]out , potassium ions move
outwards through channels due to the chemical driving potential, EK .
(EK can be thought of as a potassium “battery”)
Net outward ion flow continues until opposed by a membrane
potential, Vm , of equal force built up in the membrane capacitor.
Cl-
K+
Na+
Cl-
K+
Na+
out
+ - + -
out
++++
in
+ - + -
in
- - - -
Cl-
K+
Na+
A-
When Vm = 0, large K+ efflux
Cl-
K+
Na+
A-
AT EQUILIBRIUM
When Vm = EK, zero net K+ flux
CIRCUIT REPRESENTATION OF POTASSIUM CONDUCTANCE,
POTASSIUM BATTERY, AND MEMBRANE CAPACITANCE
Cl-
Na+
Cl-
K+
out
+ - + -
out
++++
in
+ - + -
in
- - - -
K+ Na+ A+
Cl-
When Vm = 0, large K efflux
gK
+
-
Cl-
K+
Na+
A-
+
When Vm = EK, zero net K flux
gK
IK
CM
EK
Na+
K+
VM = 0
EK
+
IK = 0
+++
CM _ _ _
-
What is the strength of the potassium battery EK ???
VM = EK
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
EX = z
Nernst Equation
[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
58 mV log 5
EK+ = 1
130
= - 82 mV
z = +1
VOLTAGE-CURRENT RELATION OF THE POTASSIUM BATTERY
IK
Cl- K+Na+
Cl- K+Na+
out
+ - + -
out
++++
in
+ - + -
in
- - - -
A-
When Vm = 0,
large K+ efflux
IK = 2 pA
Cl-
K+
Na+
A-
slope = gK = 25 pS
Vm
EK = - 82 mV
When Vm = EK,
zero net K+ flux
IK = 0 pA
Conductivity of single K channel
gK = 25 pS
out
out
IK = 2 pA
Total K conductivity (gK )
gK = 25 pS
IK = 0
Cl-
K+ Na+
IK = 2 pA
gK = 25 pS
in
EK = - 82 mV
in
=
gK
X
NK
where NK is # K channels
+
+
gK
EK = - 82 mV
IK
=
gK
Vm
=
x ( Vm - EK )
EK + IK RK
gK and Cm DETERMINE HOW FAST Vm CHANGES TO EK
Cl-
Na+
K+
out
+ - + -
in
+ - + -
Cl-
Vm (mV)
K+ Na+
A-
channels
open
0
- 82
T
t
T ~ Cm / gK
gK , the greater the potassium current ( IK ) and
the faster the transition to the potassium Nernst potential ( E )
K
The greater the value of
Cm , the longer the potassium current ( IK ) and
the slower the transition to the potassium Nernst potential ( E )
K
The greater the value of
RESTING POTENTIAL SET BY RELATIVE PERMEABILITIES
OF K+, Na+, & Cl- IONS
Nernst Potential
Relative Permeability (P)
EK = - 82.1 mV
ENa = + 84.8 mV
ECl = - 63.6 mV
1.0
0.05
0.2
Resting membrane potential reflects the relative permeabilities
of each ion and the Nernst potential of each ion
Vm =
gK EK + gNa ENa + gCl ECl
gK + gNa + gCl
~
=
PK EK + PNa ENa + PCl ECl
PK + PNa + PCl
When the resting membrane potential is achieved, there is
ongoing influx of sodium and a matching efflux of potassium.
Na/K ATPase is continually needed to keep the ion gradients
from running down over time
THE GOLDMAN EQUATION
PK EK + PNa ENa + PCl ECl
Vm =
PK + PNa + PCl
from before
EX =
Nernst equatiion
Goldman equation
58 mV log [X]out
z
Vm = 58 mV log10
[X]in
(
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
When one ion is by far the most permeable, Goldman eq. reduces to Nernst eq.
)
RELATIVE PERMEABILITY & THE RESTING POTENTIAL
PK EK + PNa ENa + PCl ECl
Vm =
PK + PNa + PCl
[K+]o = 5 mM
[Na+]o = 145 mM
[K+]i = 130 mM [Na+]i = 5 mM
EK = - 82.1 mV
ENa = 84.8 mV
PK = 1
PNa = 0.05
[Cl-]o = 100 mM
[Cl-]i = 8 mM
ECl = - 63.6 mV
PCl = 0.2
Vm = - 72.4 mV
GRAPHIC AND CIRCUIT REPRESENTATIONS OF ION FLOWS
ACROSS THE MEMBRANE AT THE RESTING POTENTIAL
out
+++
+++
+++
+++
in
- - -
- - -
- - -
- - -
K+
K+
EK = - 82.1 mV
+++
- - -
- - -
gK = 2 nS
RK = 0.5 GW
+
EK = - 82.1 mV
+++ out
---
Vm =
in -72.4 mV
Vm = - 72.4 mV
Na+
ENa = + 84.8 mV
out
out
INa = - 15.7 pA
IK = 19.4 pA
+++
Cl-
IK + INa + ICl = 0
-
Na+
AT STEADY STATE:
out
in
K+
Cl-
gNa = 0.1 nS
RNa = 10 GW
+
ENa = + 84.8 mV
ICl = -3.5 pA
K+
gCl
RCl
-
K+
= 0.4 nS
=
2.5 GW
+
K+
ECl = - 63.6 mV
in
EK + IKRK = Vm = ENa + INaRNa = ECl + IClRCl
-82.1 mV + (19.4 pA)(0.5 GW) = -72.4 mV = +84.8 mV + (-15.7 pA)(10 GW) = -63.6 mV + (-3.5 pA)(2.5 GW)
INCREASING SODIUM PERMEABILITY UNDERLIES SODIUM INFLUX
AND MEMBRANE DEPOLARIZATION DURING ACTION POTENTIAL
During action potential, the number of open sodium channels increases dramatically
Nernst Potential
EK = - 82 mV
ENa = + 85 mV
ECl = - 64 mV
Prest
Paction-potential
1.0
0.05
0.2
1.0
5.0
0.2
Rest
GOLDMAN EQUATION-PREDICTED Vm
- 70 mV
During Action Potential
+ 36 mV
When sodium channels open, sodium ions flow in rapidly because of the
negative membrane potential and the strong inward sodium battery
Inward sodium current depolarizes membrane and moves it towards the
positive potential predicted by Goldman’s equation
(this positive potential is never fully achieved due to additional channel dynamics)
Next Lecture: MEASURING MEMBRANE CONDUCTANCE AND CAPACITANCE &
VOLTAGE-CLAMP RECORDING
REQUIRED READING: Kandel text, Chapters 8, 9 (beginning), pgs 140-153