Resting (membrane) Potential DENT/OBHS 131 Neuroscience 2009

Electrical signaling in neurons dendritic synaptic inputs transfer to the soma generate APs axonal propagation ionic basis of RMP AP initiation & propagation

Learning Objectives Explain how the concentration gradient of potassium ions across the membrane gives rise to the resting membrane potential Compute the equilibrium potential of an ion using the Nernst equation 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? Not at all Somewhat Very Intimately

The major ion involved in setting the resting membrane potential is... Sodium Calcium Chloride Potassium Bicarbonate Hydrogen 10

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: 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

Electrical difference….. IN vs. OUT + - 0 mV DS Weiss

Electrical difference….. IN vs. OUT + - 0 mV -70 mV DS Weiss

Can we calculate the potential? Ex = ln [x]outside [x]inside RT zF 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.

Learning Objective #2 Compute the equilibrium potential of an ion using the Nernst equation

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 Ex = ln [K]OUT [K]IN RT zF = log 10 100 60 z = -60 mV

If we lowered the [K+]OUT 10-fold to 1 mM, the RMP would….. Not change Hyperpolarize Depolarize 10

The Nernst potential for K+ What about hyperkalemia? Ex = ln [K]OUT [K]IN RT zF = log 1 100 60 z = -120 mV

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

General rule(s) relationship between: membrane potential (mV) EK ENa RMP +67 -90 -98 ECl 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)

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)