LECTURE 3: ION CHANNELS

1
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
2
MEASURING THE RESTING MEMBRANE POTENTIAL MICROPIP
ET 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)
Rm (actual)
Rpipet
Vpipet
Vm (actual)
Vm (measured) Vm (actual) Vpipet
when Rpipet ltltlt Rm (actual)
Vm (measured) Vm (actual)
3
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 gt 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
4
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
5
Na/K ATPase PUMP
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 gtgt K out Na
in ltlt 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 ltlt Cl- out
6
IONS CHANNELS
K
outside
Na
POTASSIUM CHANNEL (non-gated, leak)
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
7
POTASSIUM CHANNELS FAVOR A NEGATIVE MEMBRANE
POTENTIAL
Potassium channels are the most abundant leak
channels in neurons. Because the Na/K pump makes
Kin gtgt Kout , 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.
AT EQUILIBRIUM When Vm EK, zero net K flux
When Vm 0, large K efflux
8
CIRCUIT REPRESENTATION OF POTASSIUM
CONDUCTANCE, POTASSIUM BATTERY, AND MEMBRANE
CAPACITANCE
When Vm 0, large K efflux
When Vm EK, zero net K flux
gK
gK
IK
IK 0
_ _ _
CM
CM
VM 0
VM EK


EK
EK
-
-
What is the strength of the potassium battery EK
???
9
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
Nernst Equation
Where EX is the chemical potential and z is the
charge of ion X
z 1
Kin 130 mM
Kout 5 mM
For potassium
5
58 mV
log
EK
- 82 mV
1
130
10
VOLTAGE-CURRENT RELATION OF THE POTASSIUM BATTERY
When Vm 0, large K efflux IK 2 pA
When Vm EK, zero net K flux IK 0 pA
Conductivity of single K channel gK 25
pS Total K conductivity (gK ) gK gK X
NK where NK is K channels
IK gK x ( Vm - EK ) Vm EK IK RK
11
gK and Cm DETERMINE HOW FAST Vm CHANGES TO EK
Vm (mV)
channels open
0
- 82
T
t
T Cm / gK
The greater the value of gK , the greater the
potassium current ( IK ) and the faster the
transition to the potassium Nernst potential (
EK) The greater the value of Cm , the longer the
potassium current ( IK ) and the slower the
transition to the potassium Nernst potential ( EK)
12
RESTING POTENTIAL SET BY RELATIVE
PERMEABILITIES OF K, Na, Cl- IONS
Nernst Potential Relative Permeability
(P)
EK - 82.1 mV 1.0 ENa
84.8 mV 0.05 ECl -
63.6 mV 0.2
Resting membrane potential reflects the relative
permeabilities of each ion and the Nernst
potential of each ion
gK EK gNa ENa gCl ECl
PK EK PNa ENa PCl ECl

Vm

gK gNa gCl
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
13
THE GOLDMAN EQUATION
from before
Nernst equatiion
Goldman equation
The greater an ions 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.
14
RELATIVE PERMEABILITY THE RESTING POTENTIAL
Ko
5 mM
145 mM
Nao
Cl-o
100 mM
Ki
130 mM
5 mM
8 mM
Nai
Cl-i
EK
- 82.1 mV
ENa
84.8 mV
ECl
- 63.6 mV
PK
PNa
PCl
0.2
0.05
1
Vm - 72.4 mV
15
GRAPHIC AND CIRCUIT REPRESENTATIONS OF ION
FLOWS ACROSS THE MEMBRANE AT THE RESTING POTENTIAL
Cl-






out
Vm - 72.4 mV
- - -
- - -
- - -
- - -
- - -
- - -
in
Cl-
ENa 84.8 mV
EK - 82.1 mV
out
gCl 0.4 nS
out



RCl 2.5 GW
Vm
ICl -3.5 pA
-
-
-
in
-72.4 mV
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)
16
INCREASING SODIUM PERMEABILITY UNDERLIES SODIUM
INFLUX AND MEMBRANE DEPOLARIZATION DURING ACTION
POTENTIAL
During action potential, the number of open
sodium channels increases dramatically
Rest During Action Potential
- 70 mV
36 mV
GOLDMAN EQUATION-PREDICTED Vm
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 Goldmans equation (this
positive potential is never fully achieved due to
additional channel dynamics)
17
Next Lecture MEASURING MEMBRANE CONDUCTANCE AND
CAPACITANCE VOLTAGE-CLAMP RECORDING
REQUIRED READING Kandel text, Chapters 8, 9
(beginning), pgs 140-153