W6L2

Question 1

Equilibrium potential - Na+ equation

Answer 1

z = +1
temperature = 27 degrees Celsius

ENa = 60 log ([Na]o / [Na]i)

mV units

Question 2

Equilibrium potential - Cl- equation

Answer 2

z = -1
temperature = 27 degrees Celsius

ECl = -60 log ([Cl]o / [Cl]i)

mV units

Question 3

Why do we need to know the equilibrium potential?

Answer 3

Equilibrium potential and membrane potential determine the direction and magnitude of ion movement (current)

Remember
- Ohm’s Law V=IR or I =GV
- G (conductance) = 1/R

Ohm’s law for ion X:
Ix = Gx (Em-EX)
where GX = conductance
Em = membrane potential
Ex = equilibrium potential for ion X

when (Em-EX) > 0, the current is outward (+)

when (Em-EX) < 0, the current is inward (-)

when Em = EX, the current is 0

Question 4

Current-voltage relationship

Answer 4

I-V curve

current on y axis, voltage on x axis

Slope, G, is conductance

Outward current is positive, inward current is negative

Reversal potential is when slope reaches x axis, and is when current changes from outward to inward, or vice versa

Question 5

Nernst equation

Answer 5

Membrane potential Em depends on permeability (P) to K+ ions

Good but not the best description of the resting membrane potential

Not very precise

Ko = K+ concentration outside
Ki = K+ concentration inside

Em = (RT/F) * ln ([K]o / [K]i) = Ek

Question 6

Goldman equation aka Goldman-Hodgkin-Katz equation Aka GHK equation

Answer 6

Membrane potential Em depends on 3 ions: Na+, K+ & Cl- permeabilities (P)
- more common condition than in Nernst equation

PNa = Na+ Permeability; PK = K+ P ; PCl = Cl-P Nao = extracellular Na+ concentration
Nai = intracellular Na+ concentration

Em = (RT/F) * ln [ (PNaNa_o + PkK_o + PClCl_i) / (PNaNa_i + PkK_i + PClCl_o) ]

Goldman equation reduces to Nernst Equation when there is only one permeable ion K+

Goldman equation reduces to Nernst equation when there is only one permeable ion Cl-
- Note Cl_i is in the numerator (z valence = -1)

Question 7

Why do we care about electrical capacitance?

Answer 7

Microelectrode impaled into a neuron, then you record responses

Impale at 0 mV

They predicted that current injection immediately causes depolarization (rectangle, straight line increase), but in reality, there is a slow depolarization (curve) which is due to a membrane property (membrane is thin and insulating, depends on capacitance)

Question 8

Capacitance

Answer 8

A capacitance (C) is defined by its ability to store charges (Q) across an insulator.

Equal and opposite charges are stored across C

Capacitance current (IC) can increase (or decrease) the number of charges across C.

Cell membrane is an insulator and can store charges just like a capacitance.

Question 9

Passive membrane model

Answer 9

Electrical model of the membrane is a resistance and a capacitance in parallel

I_m = I_C+ I_R

Question 10

Inputting current

Answer 10

Positive current depolarizes the membrane

Output voltage is slower than input current, slow to rise and slow to fall

Higher input current gives proportionally higher output voltage

Question 11

Action potential

Answer 11

Is a voltage-dependent response

All or none

Self-generating after threshold is exceeded

Overshoot; undershoot (after hyperpolarization aka AHP)

Question 12

Absolute and Relative Refractory Periods

Answer 12

Absolute refractive period
- occurs after the peak, during repolarization

Relative refractive period
- occurs after absolute refractory period
- needs stronger stimulus to reach threshold

Question 13

Passive membrane vs AP properties

Answer 13

Passive membrane
1. Graded (not one fixed amplitude)
2. Not self generating (no propagation)
3. Depolarizing or hyperpolarizing
4. No refractory period
5. Linear properties

AP properties
1. All or none (fixed amplitude)
2. Self generating (propagate)
3. Depolarizing followed by AHP
4. Absolute and relative refractory periods
5. Non-linear properties