Lecture 11: Electrical Properties of Cell Membranes

On the I-V curve, where is E_x?

the x-intercept

Does positive current flow inward or outward for positive ions?

outward current

Does negative current flow inward or outward for positive ions?

inward current

True or False: Ionic current is proportional to the electrochemical driving force.

true

What does the size of the ionic current depend on?

the difference between actual V_m and E_ion

Review the following equation:

I_ion = G_ion (V_m - E_ion)

What is G_ion?

the total conductance mediated by the ion in the cell’s membrane

True or False: Equivalent circuit represents parallel pathways for different ions across the membrane.

true

What is the model for electrical behavior of cell membranes?

an equivalent circuit

Electrical current carried by each ion flows via what type of branches of circuit?

via separate parallel branches of circuit

What is each branch of the circuit under control of?

• variable resistor: conductance provided by ion channels

• electromotive force: relation between V_m and E_ion

What is G_K / G_m?

the fractional conductance for K+ (what fraction of total conductance across the membrane is mediated by K+)

What is V_rest?

the sum of equilibrium potentials

True or False: An alpha value of less than 1 equates to a larger percentage of P_K—meaning the V_rest will become more depolarized.

true

What does the phospholipid bilayer have property of?

capacitance

What is capacitance?

ability to separate and store charge

What is capacitance proportional to?

proportional to cell surface area

True or False: A cell’s capacitance determines how quickly V_m can respond to a change in current.

true

What does a high C_m lead to?

a slower rate of change in V_m

Describe how the plasma membrane acts like a capacitor.

• capacitors are devices capable of storing storing charge and are two conducting materials separated by an insulator

  • intracellular and extracellular fluids = conductors

  • lipid membrane = insulator

What is the equation for capacitance?

C = Q / V

What is capacitance measured in?

Farads (1 coulomb/volt)

The total membrane current when V_m is changing has which two components?

• carried by ions through channels

• carried by ions as they move on and off membrane capacitor

What is capacitative current?

current that flows when V_m is changing; movement of charge

True or False: When voltage across capacitor changes, capacitor either gains or loses charge.

true

What happens when the switch closes?

• charge on capacitor discharges off capacitor and onto resistor

• delta V between in and out decays from initial V to zero

True or False: Voltage decay follows an exponential time course.

true

What is the time constant (tau)?

the time required for voltage to fall to 37% of initial value

What is the equation for the time constant?

tau = R_m x C_m

What is the value of the capacitative current before switch is closed?

at zero

What happens when the switch closes?

• charge begins to flow rapidly off capacitor

• charge on capacitor gradually falls—leading to the rate at which charge is flowing off capacitor (slope) also decreases

What is the equation for capacitative current?

I_capacitative = C_m (delta V / delta tau)

True or False: Capacitative current is proportional to the rate of change in voltage (change in voltage over change in time).

true

Regarding the relationship of the capacitor, does the change in current or the change in voltage hit its highest point first?

change in current

What is square pulse (fixed value) stimuli used to determine?

passive membrane properties (R_m and C_m) of a cell

With a current pulse injection, what happens to V_m?

V_m changes according to R x C characteristics of the membrane

With a voltage pulse injection, what happens?

• transient I_C flows as charge flows off capacitor

• maximal at beginning of square pulse

• falls exponentially

• at end of stimulus, mirror I_C flows in opposite direction

True or False: Use current responses to square pulse injections of voltage to calculate the total membrane resistance of a cell. R = delta V / delta I

true

True or False: Use current responses to square pulse injections of voltage to estimate total membrane capacitance of a cell. Measure area under the curve.

true