Resting (membrane) Potential

DENT/OBHS 131Neuroscience 2009

dendritic synaptic inputstransfer to the somagenerate APsaxonal propagation

ionic basis of RMPAP initiation & propagation

Electrical signaling in neurons

Learning Objectives

1. Explain how the concentration gradient of potassium ions across the membrane gives rise to the resting membrane potential

2. Compute the equilibrium potential of an ion using the Nernst equation

3. Predict the effect of changing the concentration of an ion (or it’s relative permeability) on the membrane potential

How familiar are you with resting and active properties of membranes?1. Not at all

2. Somewhat

3. Very

4. Intimately

The major ion involved in setting the resting membrane potential is...

1. Sodium

2. Calcium

3. Chloride

4. Potassium

5. Bicarbonate

6. Hydrogen

Learning Objective #1

Explain how the concentration gradient of potassium ions across the membrane gives rise to the resting membrane potential

The RMP

The membrane acts to……

separate and maintain (pumps) gradients of solutions with different concentrations of charged ions

selectively allow certain ionic species (K+) to cross the membrane…

initial conditions:different distribution of a K-saltmembrane is only permeable to Kthere is no potential difference across the membrane

at equilibrium:K ions diffuse down concentration gradientanions are left behind: net negativity develops inside the cellfurther movement of ions is opposed by the potential difference

Electrical difference….. IN vs. OUT

+- 0 mV

DS Weiss

Electrical difference….. IN vs. OUT

+-

-70 mV

0 mV

DS Weiss

Can we calculate the potential?

The Nernst equation determines the voltage at which the electrical and chemical forces for an ion (X) are balanced; there is NO net movement of ions.

Ex = ln [x]outside

[x]inside

RTzF

Learning Objective #2

Compute the equilibrium potential of an ion using the Nernst equation

if K is 10-fold higher on the inside

in excitable cells the RMP is primarily determined by K ions

Ex = ln [K]OUT

[K]IN

RTzF

= log 10100

60z =-60 mV

The Nernst potential for K+

If we lowered the [K+]OUT

10-fold to 1 mM, the RMP would…..1. Not change

2. Hyperpolarize

3. Depolarize

Ex = ln [K]OUT

[K]IN

RTzF

= log 1100

60z =-120 mV

The Nernst potential for K+

What about hyperkalemia?

Learning Objective #3

Predict the effect of changing the concentration of an ion (or it’s relative permeability) on the membrane potential

Other ions affect RMP

different ions have different distributions

cell membrane is not uniformly permeable (“leaky”) to all ions

relative permeability of an ion determines its contribution to the RMP

a small permeability to Na and Cl offsets some of the potential set up by K

in reality the cell membrane is < negative than EK

Concentrations of other ions…..

ion [X]in [X]out Eq. (mV)

K 155 4 -98

Na 12 145 +67

Cl 4.2 123 -90

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 or equilibrium potential)

membranepotential (mV)

EK

ENa

RMP

+67

-90-98

ECl

General rule(s)

ion flux explanation

driving force on an ion X will vary with MP = (Em - Ex)

Ohm’s law V = IR = I/g, or transformed I = gV Ix = gx (Em - Ex)

there will be no current if: no channels for ion X are open (no conductance, g)

no driving force (MP is at Ex)