Excitable cells 1: Action potentials

Question 1

What is required for current (units A, symbol I) to flow?

Answer 1

A potential difference between 2 points (V) with a conductor connecting them

Question 2

How are conductance and resistance related?

Answer 2

Conductance is inversely proportional to resistance

G = 1/R

Question 3

Ohms law?

Answer 3

I = V/R

Question 4

Explain (membrane) capacitance

Answer 4

• The store of charge that builds up on the membrane for a given voltage
• Quantity of charge required to change membrane potential by a given value
  i. e. ‘How much charge must move across membrane’ -> ion channels
• Acts to slow changes of membrane potential

Question 5

Main cell factors which influence capacitance

Answer 5

Surface area / Insulator thickness

affected by myleination (increases insulator thickness) -> reduced capacitance

(and cell size)

Question 6

What components affect time course of changes to membrane potential? General time frame in neurones?

Answer 6

Capacitor (lipid bi-layer)
Resistor (Ion channels)

High capacitance = larger time constant
20-50 milliseconds typical in neurons

Question 7

Equation that relates charge, voltage and capacitance?

Answer 7

Q = C.V

Question 8

Equation which relates current, voltage and capacitance?

Answer 8

I = C . dV/dt

Question 9

How are membrane capacitance and membrane resistance associated? Draw Diagram

Answer 9

Cm and Rm are in parallel with eachother

                Inside
    \_\_\_\_\_\_\_ - \_\_\_\_\_\_\_
   I                                   I
   \                                   I
   /                                   I                        
 Rm                        (capacitor)   Cm
   \                                   I
   /                                   I
   I \_\_\_\_\_\_\_ - \_\_\_\_\_\_\_ I
                Outside

Question 10

What is time constant? Equation?

Answer 10

Time constant = time for time (t) for (delta)Vm to change by 63% of (delta)Vmax

Time constant = Rm . Cm

Question 11

What is the principle of cable theory?

Answer 11

In long cellular processes (eg axons, dendrites and muscle fibres), passive responses decrease with distance from stimulus

Question 12

What is length constant? What does it indicate and when is it greatest? equation?

Answer 12

Length constant = distance from current injection point (x) when voltage signal has decreased to ~ 37 %

( (=delta)Vm = (delta)Vmax . e(-x/λ) )

Gives indication of electronic conduction properties
In physiological conditions it is greatest if
1) Rm is high - low channel density or high insulation (myelin)
2) Ra is low - diameter of membrane process is high