BB Lecture 18: Membrane Excitability

Question 1

What is passive membrane behavior?

Answer 1

when a membrane only contains voltage-independent “leak” channels and current flows resulting in voltage changes as predicted by Ohm’s law (with delay imposed by capacitance) with direct return to original Vm when the current ends

Question 2

What is active membrane behavior?

Answer 2

membranes that include voltage-dependent channels

arise from dynamics of currents flowing through these channels as their conductances respond to changes in Vm

Question 3

What are excitable membranes?

Answer 3

membranes that respond to a threshold depolarization with an action potential
demonstrate most dramatic consequence of active behavior

Question 4

What is an action potential?

Answer 4

an all or none sterotyped sequence of changes in Vm

Question 5

What can excitable cells be useful for?

Answer 5

1. some can be specialized to act as pacemakers (spontaneously and rhythmically generate action potentials)
2. propagation of action potentials without attenuation

Question 6

What are the key features of an action potential in a neuron?

Answer 6

1. the threshold of initiation
2. the reversal of membrane polarity at the peak
3. the afterhyperpolarization before the return of Vm to its resting value

Question 7

What is the absolute refractory period?

Answer 7

a brief period after an action potential where it is impossible to initiate another (no matter how strong the stimulus)

Question 8

When does the refractory period end in neurons?

Answer 8

when the full complement of Na+ channels have been de-inactivated

Question 9

What limits the speed of a propagation of an action potential in neurons?

Answer 9

kinetics of Na+ channel activation

Question 10

What are the key characteristics of heart excitable muscle cell action potentials?

Answer 10

1. a long phase of depolarization compared to neurons
2. prominent use of Ca2+ as the depolarizing charge carrier
3. spend a substantial portion of their lives in a depolarized state (because mechanical pumping action of the heart relies on contractions that last hundreds of milliseconds)

Question 11

What are the phases of a ventricular action potential?

Answer 11

0 (upstroke)
1 (early/fast repolarization)
2 (plateau)
3 (repolarization) 
4 (at rest/diastole)

Question 12

What happens in Phase 0 of a ventricular action potential?

Answer 12

membrane is depolarized to the threshold for action potential initiation (activation of voltage gated Na+ channels)

Question 13

What happens in Phase 1 of a ventricular action potential?

Answer 13

partial repolarization as most (but not all) Na+ channels inactivate; Kirs close (due to depolarization)

Question 14

What factors impact the termination of the plateau phase of ventricular action potentials?

Answer 14

1. the slowly increasing K+ curretn that is contributed by various voltage-gated K+ channels
2. diminishing Ca2+ current as the voltage-gated Ca2+ channels slowly inactivate

Question 15

What happens during Phase 4 of a ventricular action potential?

Answer 15

well polarized to about -90mV and inward rectifying Kir channels provide dominant conductance

Question 16

What happens during Phase 2 of a ventricular action potential?

Answer 16

delayed opening of voltage gated Ca2+ channels that inactivate with very slow kinetics; smaller inward current through remaining non-inactivated voltage-gated Na+ channels; voltage-gated K+ channels slowly activate