Ionic Basis of Membrane Potential

Transcription

Ionic Basis of Membrane Potential
Dr Sergey Kasparov
School of Medical Sciences, Room E9
Teaching
Teaching home
home page:
page:
http://www.bristol.ac.uk/phys-pharm/personal/virallab/teaching/downloads.php
http://www.bristol.ac.uk/phys-pharm/personal/virallab/teaching/downloads.php
1.Phospholipid bi-layer is the key component of all cellular
membranes.
Water
Water (aqueous)
(aqueous) phase:
phase:
charged,
charged, polar
polar molecules
molecules and
and
ions
ions can
can dissolve
dissolve and
and move.
move.
Water
Water (aqueous)
(aqueous) phase:
phase:
charged,
charged, polar
polar molecules
molecules and
and
ions
ions can
can dissolve
dissolve and
and move.
move.
Lipid
Lipid (hydrophobic)
(hydrophobic) phase:
phase:
charged,
charged, polar
polar molecules
molecules
cannot
cannot dissolve
dissolve and
and move.
move.
2.Bioelectricity is generated by the ions moving across cellular
membranes.
Concentration
Concentration of
of selected
selected solutes
solutes in
in intracellular
intracellular fluid
fluid and
and
extracellular
extracellular fluid
fluid in
in millimols
millimols
mM
mM160
160
140
140
120
120
100
100
Inside
Inside the
the cell
cell
80
80
In
In extracellular
extracellular
fluid
fluid
60
60
40
40
+
CCa
a 22+
+
MMg
g 22+
NNa
a ++
KK ++
00
IInno
CCl orr
lH
H
g
gaa
C
C
nniic OO3
c P 3 -Phh
oo
AAm sspph
mi
h
inno aatte
oa e
acc
iidds
s
GGl
luuc
coo
ssee
AAT
TPP
PPrr
oott
eeiin
n
20
20
Remember these numbers (they may vary to some extent in different cells)
IN
OUT
K++
140
4
Na++
15
145
0.001
1.8
4
115
2+
Ca2+
Cl--
CONCENTRATIONS
CONCENTRATIONS ARE
ARE IN
IN MILLIMOLS
MILLIMOLS
If the membrane was freely permeable to these ions…
their concentration would be identical on both sides.
3.
3. Ions
Ions cannot
cannot pass
pass through
through the
the membranes
membranes –– they
they are
are too
too hyrdophylic
hyrdophylic for
for
that.
that. Therefore
Therefore they
they use
use channels
channels –– special
special proteins
proteins embedded
embedded into
into the
the
plasma
plasma membrane.
membrane.
Forces which determine the direction of transport
across the membrane
Passive vs active transport
Passive
- along the concentration
gradient and using the energy of
this gradient
Active
– uses energy supplied by the
cell to special proteins called
PUMPS
- against the concentration
gradient
These guys
guys are
are
It is the pumps which are responsible for the generation of These
called
called pumps!
pumps!
ionic gradients across the membranes of neurones
Chemical vs Electrical Driving Force
Chemical driving force
Chemical vs Electrical Driving Force
+
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Electrical driving force
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Chemical and Electrical Driving Forces may combine to
create the Electrochemical Driving Force
+_ _+ +
_
+_
+_
+ _
_ +
+
_
+ _
+
+
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_+
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4mM
+
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+
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+
_
+ What will happen if the potential of this
mV _
+ _ -70
_ _
membrane decreases to -10 mV?
+
+
+ +
140 mM
+
K++
Conclusion: movement of charged particles
such as ions, across the membrane depends on
electro-chemical driving force (the sum of the
force generated by chemical gradient and the
force generated by electric field).
_
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_
_
_+
_
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_
+
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Net flux
+
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V
+
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Net flux
+
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++
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++
++
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V
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+
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Net flux
+
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V
+
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+
The Reversal Potential:
Potential of the membrane at which the
electrical driving force is exactly equal the
chemical driving force and therefore
THE NET FLUX of this particular ion is NIL.
Reversal potential can be calculated using Nernst equation:
61.5mV Log (C /C
Eion =
out
Z
in)
61.5 is a calculated constant derived from Universal gas constant, the
temperature (37oC) and Faraday electrical constant
Z – is a valence of an ion
Using this equation:
ENa
Na = 61.5/1 * log (145/15) ≈ 60.5 mV
EKK = 61.5/1 * log (4/140)
≈ -95 mV
This means that in a hypothetical neurone sodium flux through the
open channels will tend to bring membrane potential toward +60.5
mV, while potassium flux will bring it toward -89 mV.
But
But this
this could
could only
only happen
happen if
if these
these ions
ions were
were allowed
allowed to
to freely
freely flow
flow
through
through the
the membrane…
membrane… and
and they
they are
are not!
not!
Because the membrane is essentially impermeable to ions, they have
to use ion channels to pass between intra- and extracellular space.
AA leak
leak channel
channel
AA gated
gated channel
channel
1. When neurone is at “rest” its membrane potential is negative, this is
called “resting membrane potential”.
2. At rest the main ion flux is K++, but also there are small Na+ and Cl-fluxes. Since the potential at “rest” does not change, it means that the
sum of these currents is zero.
3. Permeability of the membrane to an ion (NOT actually the membrane
but the channels passing that particular ion) may vary, because the
channels may open and close.
The equation which takes into account impacts of all these 3 ions and
predicts the resting membrane potential is called “Goldman-HodgkinKatz” or “Goldman” or “Constant Field” equation.
Vm = 61.5 mV x log ( P
)
]
PK [Kout] +PNa[Naout] + PCl[Clin]
K
[Kin] +PNa[Nain] + PCl[Clout

Ionic Basis of Membrane Potential

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