Lecture 4: Active and Passive potentials

Question 1

Describe the differences between a passive and active potential

Answer 1

Passive potential: propagation doesn’t involve ion channels

• stimulus comes from current, creates passive potential that doesn’t reach threshold and trigger AP
• current leaks out through the membrane during propagation
• voltage change is round due to capacitance
• voltage decreases exponentially with distance

Active potential: propagation does involve ion channels, voltage-gated sodium channels, threshold before firing, large depolarization

Question 2

Why is membrane potential change rounded?

Answer 2

potential change comes from flow of ions (current). however, membrane is a capacitor and charges are stored, takes some time for ions to pull the charges away from the membrane and create the potential change

Question 3

What are 3 passive properties that govern transmission of passive potentials

Answer 3

1) membrane capacitance
2) membrane resistance
3) axial resistance
length constant = Rm/Ra - distance at which membrane potential is 1/e of original value

Question 4

What are the values of an AP?

Answer 4

RMP: -70
threshold: -55
peak: +30
K+ ion channels open at about -20

Question 5

Why are APs unidirectional?

Answer 5

due to refractory periods, makes those ion channels unresponsive, therefore no propagation occurs in that direction (absolute and relative)