Transmembrane Potential

Question 1

Net Electrical Potential Energy Difference (Vm)

Answer 1

Algebraic sum of contribution from each permeable ion
Results from separation of positive charge from negative chart within the membrane during the transmembrane diffusion of each permeable ion down its respective electrochemical gradient

Question 2

Na/K Pump

Answer 2

Energy from ATP hydrolysis drives 3 Na+ ions from the cell cytoplasm in exchange for the uptake of 2K+ ions into the cell cytoplasm.

Question 3

Goldman, Hodgkin and Katz

Answer 3

Utilizing the classic Nerst-Plank electrochemical diffusion eqn for ionic current in an aqueous solution to derive equations for transmembrane ionic currents and the steady-state transmembrane diffusion potential.

Question 4

GHK transmembrane current equation

Answer 4

Current is a function of the membrane potential, and concentration gradient and permeability.
Assuming that the membrane permeability stays constant

Question 5

At Vk (equilibrium potential)

Answer 5

The outwardly-directed driving force generated by the transmembrane K+ concentration different rapidly becomes equal (and opposite to) the inwardly-directed driving force generated by the K+ electrical potential energy different

The net transmembrane flux of K+ is 0, the outward K+ flux rapidly becomes equal and opposite to the inward K+ flux

Question 6

Equilibrium Potential of Ions

Answer 6

Ek = -88mV
Ecl= -47 mV
Ena = 61 mV
Eca = 123 mV

Resting Vm= -70 mV

Question 7

Mammalian Nerve Cells

Answer 7

Ecl is more negative than Vm because active transport mechanism exist in the cell membrane to move Cl- against its eletrochemical graident out of the cell

Question 8

Mammalian Skeletal Muscle Cells

Answer 8

Ecl is equal to Vm because no active transport mechanisms exist in the cell membrane to move Cl- against its electrochemical gradient either into or out of the cell

Question 9

GHK constant field equation for the steady state diffusion potential

Answer 9

Vm= -RT/F x ln[(Pk[K]i + Pna[Na[i + Pcl[Cl]o)/(Pk[K]o = PNa[Na}o + Pcl[CL]i)}

eqn useful in predicting the effect of the permeabilities and ionic concentration gradients on the magnitude of the transmembrane Vm for a nerve or skeletal muscle fiber membrane

Question 10

Vm for skeletal muscle cell

Answer 10

Cl- does not contribute to the Em across a resting mammalian skeletal muscle cell plasma membrane because:

1. Cl is in equilibrium across the membrane (no active transport pumps for Cl-)
2. Cl- current is zero

Note - Cl- flows into a transiently depolorized cell to to repolorize the cell to its resting value to terminate the action potential

Question 11

Vm for skeletal muscle cell equation

Answer 11

Vm = -RT/F x ln([K+)1 + a[Na}i)/([K+]o + a[Na]o)]

a = Pna/Pk

Question 12

Phsyiological Importance of alpha (Pna/Pka ratio)

Answer 12

Absolutle value of VM is inversely proportional to alpha, as alpha is increased, the membrane becomes less polarized
Mechanism for increasing alpha involves increasing Pna resulting from an increase in the number of open transmembrane Na+ channels during the generation of an AP