Electrical properties of cells: Action Potentials

Question 1

Definition of action potentials

Answer 1

Membrane potentials of a specific cell location rapidly rises and falls
This depolarization causes adjacent locations to depolarize as well

Question 2

Definition of resistive transmembrane currents

Answer 2

Ion flows through channel

Question 3

Definition of capacitative transmembrane currents

Answer 3

Ion approaches 1 surface of membrane and another is expelled from the other side

Question 4

Definition of saltatory conduction

Answer 4

Activation of Nodes of Ranvier from a distance

Action potentials can jump over the myelin sheath so the velocity of the action potential is faster

Question 5

What are the stages in an action potential

Answer 5

Rest
Depolarization
Repolarisation
Hyperpolarisation

Question 6

Properties of the action potential

Answer 6

Triggered by depolarization
Threshold of depolarization required
All of nothing principle
Propagates without decrement
At peak:Vm approaches Ena
After AP, membrane is inexcusable during refractory period

Question 7

Use of ion channels

Answer 7

AP due to current flow through Na, K voltage gated channels
Channels can be open/closed, highly selective
Probability of opening and closing determined by voltage across channel

Question 8

Changing ion permeabilities

Answer 8

If a cell becomes permeable to an ion, will move down its electrochemical gradient, drive Vm => equilibrium potential
During AP, membrane becomes more permeable to Na+ first, then K+
Membrane driven to Ena and then to Ek

Question 9

What happens when Vm is driven towards Ena

Answer 9

Depolarization of membrane
Na+ channels open fast
Na+ influx
Increases amount of depolarization

Positive feedback loop, prolonged depolarization causes Na+ channel inactivation

Question 10

What happens when Vm is driven towards Rk

Answer 10

Depolarization of membrane
K+ channels open slow
K+ efflux
Repolarization of membrane

Question 11

Why is there a threshold

Answer 11

At rest Pk, is much larger than Pna

Flow of K+ out can counteract flow of Na+ in

Question 12

How much charge separation needed for 100mV

Answer 12

1picomole/cm^2 for 100mV

V few ions need to be separated with negligible consequences

Question 13

How is the action potential propagated

Answer 13

Along axoplasm (Rlongitudinal)
Across membrane (Rmembrane)

Question 14

Describe the path of injected currents

Answer 14

Transmembrane currents either
Resistive, ion flows through channel
Capacitative, ion approaches 1 surface of membrane and another is expelled from the other side

Question 15

Describe saltatory conduction and how myelination increases conduction velocity

Longitudinal resistance?
Membrane resistance?
Membrane capacitance?

Answer 15

As axon diameter increases, longitudinal R decreases

Increased membrane resistance (ion channels)
-current forced through axoplasm to next node

Decreased membrane capacitance

• charge not stored at membrane
• small change in ion conc => AP

Question 16

Describe unmyelinated axons and their ability to conduct slowly’

Longitudinal resistance?
Membrane resistance?
Membrane capacitance?

Answer 16

Thinner axons, higher longitudinal R

Decreased membrane resistance
-current dissipates more quickly, voltage falls more rapidly

Increased membrane capacitance

• voltage changes more slowly
• large change in ion conc => AP

Question 17

How is the resting membrane potential restored during hyperpolarisation?

Answer 17

The K channels close to prevent further efflux of K out of the cell.
The membrane is naturally leaky to both Na and K even when the channels are closed.

Question 18

What is the difference between the absolute refractory period and the relative refractory period?

Answer 18

Absolute, during the depolarisation. No further AP can fire when Na are already open

Relative, during repolarisation. AP can fire but the stimulus has to be greater to overcome the efflux of K

Question 19

Compare the flow of Na and K during the action potential and why

Answer 19

Na influx is faster as there is a inward force of -120mV

K efflux is slower as there is an outward force of +20mV