Lecture 2: Membrane potentials and action potentials

Question 1

How do you measure membrane potential?

Answer 1

By placing a reference electrode outside the cell (zero-volt level) and another electrode is placed inside the cell - it measures a voltage difference that is negative compared with the outside.

Question 2

What are ion channels?

Answer 2

permeable pores in the membrane that open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces

Question 3

How is a membrane potential generated?

Answer 3

due to diffusion through a selectively permeable membrane

Question 4

When is electrochemical equilibrium achieved?

Answer 4

when electrical force prevents further diffusion across the membrane

Question 5

What is the equilibrium potential?

Answer 5

the potential at which electrochemical equilibrium has been reached - potential that prevents diffusion of ion down its concentration gradient.

Question 6

How can the equilibrium potential be calculated?

Answer 6

Using the Nernst equation

Question 7

Which ions contribute to the real membrane potential?

Answer 7

K+, Na+ and Cl-

Question 8

What is the size of each ion’s contribution to the real membrane potential proportional to?

Answer 8

How permeable the membrane is to the ion

Question 9

What does the GHK equation describe?

Answer 9

The resting membrane potential

Question 10

What is mean by depolarisation?

Answer 10

membrane potential increases from negative towards 0 and becomes positive

Question 11

What is meant by repolarisation?

Answer 11

membrane potential decreases towards resting potential

Question 12

What is meant by overshoot?

Answer 12

membrane potential becomes more positive

Question 13

What is meant by hyperpolarisation?

Answer 13

membrane potential decreases beyond resting potential

Question 14

What are the most important ions for the resting potential of neurons?

Answer 14

Na+ and K+

Question 15

What does P mean in the GHK equation?

Answer 15

Permeability or channel open probability

Question 16

What does it mean if P=0?

Answer 16

channel is 100% closed

Question 17

What does it mean if P=1?

Answer 17

channel is 100% open

Question 18

What does it mean if P = 0.5?

Answer 18

channel is open 50% of the time

Question 19

What are graded potentials?

Answer 19

Change in membrane potential in response to stimulation

Question 20

What are the 2 key features of graded potentials?

Answer 20

• the size depends on the strength of the stimulus

- they dissipate with distance from the stimulus as charge ‘leaks’ from the axon as the impulse propagates

Question 21

Where do graded potentials occur?

Answer 21

at synapses and in sensory receptors

Question 22

What do graded potentials contribute to?

Answer 22

initiating or preventing action potentials

Question 23

In what cells do action potentials occur?

Answer 23

in excitable cells (mainly neurons and muscle cells but also in some endocrine tissues)

Question 24

What roles do action potentials have?

Answer 24

• cell-to-cell communication and can be used to activate intracellular processes

Question 25

What does permeability depend on?

Answer 25

conformational state of ion channels

Question 26

How is an ion channel opened?

Answer 26

by membrane depolarisation

Question 27

How is an ion channel inactivated?

Answer 27

by sustained depolarisation

Question 28

How is an ion channel closed?

Answer 28

by membrane hyperpolarisation/repolarisation

Question 29

What are the 5 phases of the action potential?

Answer 29

1. resting membrane potential 2. depolarising stimulus 3. upstroke 4. repolarisation 5. after-hyperpolarisation

Question 30

What happens in phase 1 of an action potential?

Answer 30

RESTING MEMBRANE POTENTIAL: - P(K) > P(Na) therefore potassium moves out of the cell and very little sodium comes in, - nearer equilibrium potential for K+

Question 31

What happens in phase 2 of an action potential?

Answer 31

DEPOLARISING STIMULUS: - stimulus depolarises membrane potential, - moves it in positive direction towards threshold

Question 32

What happens in phase 3 of an action potential?

Answer 32

UPSTROKE: - starts at threshold potential - increased P(Na) because VGSCs open quickly - increased P(K) as VGKCs open slowly - less K+ leaving than Na+ entering - membrane potential moves toward Na+ equilibrium potential

Question 33

What happens in phase 4 of an action potential?

Answer 33

REPOLARISATION: - decreased P(Na) as VGSCs close --> Na+ entry stops - increased P(K) as more VGKCs open and remain open --> K+ leaves cell down electrochemical gradient - membrane potential moves towards K+ equilibrium potential

Question 34

What is the absolute refractory period?

Answer 34

Is the period of time during which a second action potential ABSOLUTELY cannot be initiated, no matter how large the applied stimulus is - the activation and inactivation gates are closed

Question 35

What happens in phase 5 of an action potential?

Answer 35

AFTER-HYPERPOLARISATION: - as membrane potential moves closer to K+ equilibrium some VGKCs close - returns to resting potential

Question 36

What is the relative refractory period?

Answer 36

stronger than normal stimulus required to trigger an action potential - inactivation gate is open

Question 37

When is an action potential triggered?

Answer 37

Once the threshold potential is reached

Question 38

What are action potentials described as?

Answer 38

'All-or-nothing' events

Question 39

When does the positive feedback behaviour of the depolarisation cycle stop?

Answer 39

When the voltage-gated Na+ channels inactivate (i.e. closed and voltage-insensitive)

Question 40

How is the electrochemical equilibrium restored following the action potential?

Answer 40

by K+ and Na+ ions moving through non voltage-gated ion channels and some ions are exchanged through pumps (like Na+K+ATPase)

Question 41

Where are the voltage-gated channels mostly located?

Answer 41

At the Nodes of Ranvier formed by gaps between the myelin sheath surrounding the axon

Question 42

What factors influence conduction velocity?

Answer 42

- axon diameter (Larger diameter = faster AP) | - myelination (more myelin = faster AP)

Question 43

What does passive propagation result from?

Answer 43

Passive (graded) propagation results from a local change in ionic conductance (e.g. synaptic or sensory that produces a local current) that spreads along a stretch of membrane becoming exponentially smaller

Question 44

What is the process called when APs propagate along an axon using nodes of Ranvier?

Answer 44

saltatory conduction