1. resting membrane potential
excitable membranes
resting membrane potential
Basic Neuroscience NBL 120 (2007)

2. overview electrical signaling dendritic synaptic inputs
transfer to the soma
generate APs
axonal propagation
ionic basis of RMP
passive membrane properties
AP initiation & propagation

3. resting membrane potential
electrochemical gradients
equilibrium potentials - Nernst equation
driving forces – Ohm’s Law
GHK equation

4. 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…..

5. measurement + -
0 mV
DS Weiss

6. measurement + -
0 mV
-70 mV
DS Weiss

7. 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

8. diffusion

9. 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

10. 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

11. initial conditions: at equilibrium: 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

12. r.m.p. review
Try here?

13. can we calculate the potential?RT
[x]outside
Ex = ln zF
[x]inside
this equation determines the voltage at which the electrical and chemical forces are balanced; there is NO net movement of ions.

14. the Nernst potential for K
if K is 10-fold higher on the inside
in excitable cells the RMP is primarily determined by K ions.………………….but
Ex = ln
[x]outside
[x]inside RT zF
= log
[K]o
[K]i 60 z =
-60 mV

15. there are other ions….. ion [X]in [X]out Eq. (mV) K 155 4 -98 Na 12
145
+67 Cl
4.2
123
-90

16. general rule
ENa
+67
membrane
potential (mV)
relationship between: membrane potential ion equilibrium potentials
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)
RMP
ECl
-90 EK
-98

17. 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

18. 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)

19. 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
gK+gNa+gCl
gNa
gK+gNa+gCl
gCl
gK+gNa+gCl
Em = EK ENa ECl

20. GHK equation relative permeabilities ionic concentrations
PK[K]o + PNa[Na]o + PCl[Cl]i
PK[K]i + PNa[Na]i + PCl[Cl]o RT F
Em = ln