Ionic Basis Of Action Potential

Question 1

Concentration of K+ outside cell:

Answer 1

5 mM

Question 2

Concentration of Na+ outside cell:

Answer 2

140 mM

Question 3

Concentration of K+ inside cell:

Answer 3

140mM

Question 4

Concentration of Na+ inside cell:

Answer 4

10 mM

Question 5

What would happen to a cell that is only permeable to potassium?

Answer 5

Potassium equilibrium potential would be negative as positive K+ would leave cell

Question 6

What would happen if a cell is only permeable to sodium?

Answer 6

Sodium equilibrium potential would be positive as positive NA+ enter cell

Question 7

What are all animal cells at rest more permeable to?

Answer 7

More permeable to K+ than Na+

Question 8

How does resting membrane potential come about?

Answer 8

Very small number of K+ diffuse out down conc. gradient

Charge separation occurs as K+ ions not accompanied by anions

Interior electrical potential of cell becomes negative compared to extracellular solution

Question 9

How can you record resting membrane potentials?

Answer 9

Axon in saline solution

Add KCl filled microelectrodes (one in solution, one in axon)

Use a current stimulator and oscilloscope

Question 10

Ohm’s law:

Answer 10

V = IR

Question 11

Why is resting potential closer to K+ equilibrium potential?

Answer 11

As cell more permeable to K+

More K+ leave than Na+ entering

Question 12

What is depolarisation?

Answer 12

Movement of membrane potential to more positive value

Question 13

What is repolarisation?

Answer 13

Movement of membrane potential to more negative value

Question 14

What’s hyperpolarisation?

Answer 14

More negative than normal

Question 15

What happens when there is an injection of current in inexcitable cell?

Answer 15

Resting membrane potential depolarises slightly

Question 16

How are excitable membranes different to inexcitable membranes?

Answer 16

Excitable membranes have voltage-sensitive Na+ channels

Question 17

What happens when current is injected in excitable membrane?

Answer 17

Threshold potential is reached which causes conformational change to Na+ voltage-sensitive channels (they open)

Rapid depolarisation and action potential generated

Question 18

WHat happens after rapid depolarisation in excitable membrane?

Answer 18

Na+ channels quickly close again - causes repolarisation

Hyperpolarisation results

Return to resting potential

Question 19

What must happen for action potential to be generated?

Answer 19

Initial depolarisation must reach critical threshold

Depolarisation peaks around +30 mV (all-or-none response)

Repolarisation back to resting level

Question 20

How does membrane repolarisation?

Answer 20

Voltage sensitive K+ channels activated by membrane potential at threshold - open slower than Na+ channels

Question 21

Why is there hyperpolarisation?

Answer 21

As existing K+ voltage-sensitive channels still open

Question 22

Ratio of permeability at rest:

Answer 22

Na+ permeability : K+ permeability

             1            :           50

Question 23

What is the upstroke (depolarisation) caused by?

Answer 23

Rapid increase in Na+ permeability (x600)

Question 24

What is repolarisation associated with?

Answer 24

Delayed 10 fold increase in K+ permeability

Question 25

What is voltage-gated sodium channel like at resting potential?

Answer 25

Closed but capable of opening

-70 mV

Question 26

What is voltage-gated sodium channel like from threshold to peak potential?

Answer 26

Open (activated)

Rapid opening triggered by threshold potential

-50mV to +30 mV

Question 27

What is voltage-gated sodium channel like from peak to resting potential?

Answer 27

Closed and not capable of opening (inactivated)

+30 mV to -70 mV

Question 28

When do voltage-gated sodium channels become capable of opening again after repolarisation?

Answer 28

At resting potential -70 mV

Question 29

When are voltage-gated potassium channels closed?

Answer 29

At resting potential

Delayed opening triggered at threshold + remains closed to peak potential

-70 mV to +30 mV

Question 30

When are voltage-gated potassium channels open?

Answer 30

From peak potential through to after hyperpolarisation

+30 mV to -80 mV

Question 31

Why is this an example of positive feedback?

Answer 31

Na+ channels open -> increased Na+ permeability -> increased flow of Na+ into cell -> membrane depolarisation