L5-6 Resting Potential

Question 1

What determines the chemical driving force?

Answer 1

The difference in concentration of molecules across the plasma membrane.

W_chem = - RT ln (Co/Ci)

ln(1) = 0, so if concentrations are equal there will be 0 chem driving force

Question 2

What determines the Electrical Driving Force?

Answer 2

For ions in solutions,
determined by the charge on the ion and the existing
charge across the plasma membrane.

W_el = q (Vi - Vo)

Question 3

What is the Electrochemical Driving Force?

Answer 3

For ions in solution,
determined by adding the electrical driving force to
the chemical driving force.

(determines if ions move in or out)

W_EC = W_chem + W_el

Question 4

What is Electrochemical Equilibrium?

Answer 4

For a given ion in
solution, the state at which the electrochemical
driving force is equal to zero (i.e., the sum of
electrical plus chemical driving forces is equal to 0)

When W_EC = 0

Question 5

What is Equilibrium Potential (E_x)

Answer 5

For a given ion in
solution, the membrane voltage at which
electrochemical equilibrium is achieved.

Question 6

What is the Nernst Potential?

Answer 6

Voltage at Electrochemical Equilibrium

E_X = 61/z log ([X]o/ [X]i)

z = ion valence

Question 7

What is the electrochemical driving force on K⁺ at rest?

How could it reach electrochemical equilibrium?

Answer 7

The electrochemical driving
forces on K⁺ drive K⁺ out of the cell.

The chemical driving force out is greater than the electrical driving force in.

Electrochemical equilibrium could be reached by increasing the electrical driving force in (hyperpolarize cell so that positive charge is driven in)

For K⁺ E_K = -94 mV (hyperpolarized)

Question 8

What is the electrochemical driving force on Cl^- at rest?

How could electrochemical equilibrium be reached?

Answer 8

Cl^- is driven out of a cell at rest because the electrical driving force out is greater than the chemical driving force in.

Electrochemical equilibrium could be reached by depolarizing (make less negative) the cell do decrease the electrical driving force out

E_Cl = -86 mV

Question 9

What is the electrochemical driving force on Na⁺ at rest?

How could electrochemical equilibrium be reached?

Answer 9

The EC force drives Na⁺ in, both chemical and electrical driving forces drive Na⁺ in.

EC Equilibrium could be reached by depolarizing (make less negative) cell do decrease the electrical driving force in.

E_Na = +70 mV

Question 10

What is the ion channel current equation?

Answer 10

I_ion = g_ion (V_m - V_ion)

I = current

g = conductance

Question 11

What defines Inward Ionic Current and Outward Ionic Current?

Answer 11

• *Inward** ionic current is depolarizing from entering positive ions or exiting negative ions.
• *Represented on EC Chart with “up arrow”** from V_m to V_ion
• *Outward** ionic current is hyperpolarizing from entering negative ions or exiting positive ions.
• *Represented on EC Chart with “down arrow”** from V_m to V_ion

example: up arrow on Cl^- means Cl^- is exiting and depolarizing the cell to move towards EC equilibrium

Question 12

How would a cell’s resting membrane potential be affected by increasing Na+ permeability/conductance?

How would this affect the extracellular K⁺ concentration and EC driving force?

(this is just my interpretation of slide 19)

Answer 12

Increasing a cell’s permeability to Na⁺ would cause more Na⁺ to enter the cell resulting in depolarization. Less resistance to Na⁺ inward chemical and electrical driving forces and thus a greater EC driving force in.

Depolarizing the cell would move V_m away from E_K, increasing the EC driving force out for K⁺ thus increasing extracellular [K⁺]

(according to workshop 2 this might not be correct)

Question 13

What is the role of Na-K ATPase?

Answer 13

Pumps out 3 Na and pumps in 2 K, maintains E_Na and E_K in spite of passive leak of Na and K.

Pumping out a net 1 positive charge / ATP produces a continuous outward ionic current that tends to hyperpolarize the membrane.

Question 14

Summary of Ionic Currents at Resting
Potential:

Equation for sum of all ionic currents?

Answer 14

At resting potential, the total ionic current is zero:
I_K + I_Na + I_Ca + I_Cl + I_ATPase = 0

Question 15

Summary of factors contributing to
Resting Potential:

1. Intracellular proteins
2. Ion permeability
3. Na-K ATPase
4. The resting membrane is not at equilibrium. It is in a steady state defined by:

Answer 15

1. Intracellular proteins are negatively charged at intracellular pH
2. The resting permeability to K+ and Cl- is higher than the resting permeability to Na+. Cl- diffuses readily across the plasma membrane such that ECl= EK (Donnan equilibrium). EK is maintained at a constant value by the Na - K ATPase. The relative permeabilities at rest are:
   P_K : P_Cl : P_Na
   1.0 : 0.1 : 0.03
3. The Na - K ATPase pumps out 1 positive charge per ATP hydrolyzed, thus producing an outward ionic current (IATPase).
4. Steady state defined by:
   I_ion = g_Na (V_m - E_Na) + g_K (V_m - E_K) + g_Cl (V_m - E_Cl) + g_Ca (V_m - E_Ca) + I_ATPase = 0