excitable membranes
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Transcript excitable membranes
excitable
membranes
resting membrane potential
Basic Neuroscience NBL 120 (2007)
overview
electrical signaling
dendritic synaptic inputs
transfer to the soma
generate APs
axonal propagation
ionic basis of RMP
passive membrane properties
AP initiation & propagation
resting membrane potential
electrochemical gradients
equilibrium potentials - Nernst equation
driving forces – Ohm’s Law
GHK equation
what does the membrane do?
separate and maintain (pumps)
gradients of solutions with different
concentrations of charged ions
selectively allow certain ionic species to
cross the membrane
hence…..
measurement
+
-
0 mV
DS Weiss
measurement
+
-
0 mV
-70 mV
DS Weiss
resting membrane potential
what is it?
electrical potential difference between the inside
and outside of the cell
why does it exist?
differences in the concentrations of charged ions
inside and outside the cell
selective permeability of the membrane to certain
ions
active pumping of ions across the membrane
diffusion
Einstein
Brownian motion (diffusion)
random-walk
ions appear to move down their
concentration gradients
very fast over short distances
for small molecules / ions ≈ 1 m in 1 ms
how to create a RMP
electrochemical Na-K exchanger (& others)
pumps ions against their concentration gradients
2K ions in and 3Na ions out
net negativity to the RMP
requires energy (ATP)
not generally required for maintaining the RMP in the
absence of activity, but necessary for setting up initial
conditions by creating concentration gradients
initial conditions:
different distribution
of a K-salt
membrane is only
permeable to K
there is no potential
difference across the
membrane
at equilibrium:
K ions diffuse down
concentration gradient
anions are left
behind: net negativity
develops inside the cell
further movement of
ions is opposed by the
potential difference
r.m.p. review
Try here?
can we calculate the potential?
[x]outside
RT
Ex =
ln
[x]inside
zF
this equation determines the voltage at
which the electrical and chemical forces
are balanced; there is NO net movement
of ions.
the Nernst potential for K
if K is 10-fold higher on the inside
[K]o
RT [x]outside 60
Ex =
ln
= z log
= -60 mV
[K]i
zF [x]inside
in excitable cells the RMP is primarily
determined by K ions.………………….but
there are other ions…..
ion
[X]in
[X]out
Eq. (mV)
K
155
4
-98
Na
12
145
+67
Cl
4.2
123
-90
general rule
relationship between:
membrane potential
ion equilibrium potentials
ENa
+67
membrane
potential (mV)
RMP
ECl
EK
-90
-98
if the membrane becomes more permeable to
one ion over other ions then the membrane
potential will move towards the equilibrium
potential for that ion (basis of AP). DRIVING
FORCE
artificial manipulation of MP - reverse direction of
current flow (hence reversal potential)
other ions affect RMP
different ions have different distributions
e.g. Na high outside / K high inside
cell membrane is not uniformly permeable (“leaky”)
to all ions
relative permeability of an ion determines its
contribution to the RMP
Goldman-Hodgkin-Katz (GHK) equation
a small permeability to Na and Cl offsets some of
the potential set up by K
in reality the cell membrane is a < negative than EK
calculating the true RMP
driving force on an ion X will vary with MP
= (Em - Ex)
Ohm’s law
V = IR = Ig, or transformed I = gV
Ix = gx (Em - Ex)
there will be no current if:
no channels for ion X are open (no conductance)
no driving force (MP is at Ex)
chord conductance equation
IK=gK (Em-EK)
INa=gNa (Em-ENa)
ICl=gCl (Em-ECl)
at steady state:
IK + INa + ICl = 0
therefore: gK (Em-EK) + gNa (Em-ENa) + gCl (Em-ECl) = 0
gK
gNa
gCl
Em =
E +
E +
E
gK+gNa+gCl K gK+gNa+gCl Na
gK+gNa+gCl Cl
GHK equation
relative permeabilities
ionic concentrations
PK[K]o + PNa[Na]o + PCl[Cl]i
RT
Em =
ln
F
PK[K]i + PNa[Na]i + PCl[Cl]o