Action potentials

Question 1

What causes changes in membrane potentials?

Answer 1

Ionic movements acrosss the cell membrane

Question 2

What does the direction of the change in potential (hyperpolarization and depolarisation ) depend on?

Answer 2

• The direction of the movement of the ion

- the charge carried by the ion

Question 3

What is the driving force for Na and what happens when it is negative?

Answer 3

Vm (membrane potential) - Ena ( equilibrium potential for Na+)
-when its negative, inward movement of Na+ occurs

Question 4

How is the current carried by Na+ calculated?

Answer 4

Ina= gna ( Vm - Ena)
Ina- current
gna- Na+ conductance

Question 5

Why does Na+ generate inward current?

Answer 5

• the concentration gradient is inward

- electrical gradient is inward

Question 6

Why does K+ generate an outwards current?

Answer 6

-the concentration gradient is outward and has an energy which exceeds the electrical gradient which is inward

Question 7

What is the driving force of K+ efflux and what happens when its positive?

Answer 7

Vm- Ek

-when positive, outward movement of K+ occurs

Question 8

How is the K+ current calculated?

Answer 8

Ik= gk (Vm - Ek)

gk- K+ conductance

Question 9

What happens to the membrane potential when Na+ channels open?

Answer 9

The membrane potential is driven towards ENa (equilibrium potential of Na)

Question 10

What happens to the membrane potential when K+ channels open?

Answer 10

The membrane potential is driven towards EK (equilibrium potential of K)

Question 11

What are ion channels?

Answer 11

protein complexes that span the lipid bilayer to form a central pathway that allows rapid flow of selected ions

Question 12

What are the different ways that ion channels are gated?

Answer 12

• membrane voltage (voltage-gated channels)
• Chemical substances ( ligand-gated)
• physical stimuli- mechanical / thermal

Question 13

What are the ions channels responsible for the action potentials in neurones?

Answer 13

• Voltage -activated Na+ channels (depolarising)

- volatge-activated K+ channels (hyperpolarizing )

Question 14

What is the resting potential of a membrane?

Answer 14

-70mV

Question 15

Describe an action potential in neurone

Answer 15

• depolarising stimulus
• Na+ flows down electrical gradient so membrane potential moves to ENa+
• hyperpolarization occurs when K+ channels open and reverse system

Question 16

What cause the undershoot in an action potential of a neurone?

Answer 16

Delayed closure of K+ channels

Question 17

What causes the overshoot in an action potential of a neurone?

Answer 17

Delayed closure of Na+ channels

Question 18

What are action potentials?

Answer 18

Brief electrical signals which the polarity of the nerve cell membrane is momentarily (2 sec) reversed

Question 19

How do action potentials travel long distances?

Answer 19

They propagate along nerve cell axons with constant magnitude and velocity for a given axon

Question 20

What does “all or none” in terms of action potentials mean?

Answer 20

Action potential are generated when threshold is reached, if the threshold is not reached (sub threshold), no action potential is created

Question 21

What does it mean by the activation of Na+ channels is self-reinforcing?

Answer 21

The opening of a few channels causes further channels to open which causes further depolarisation etc..- POSITIVE FEEDBACK

Question 22

What does it mean by the activation of K+ channels is self-limiting?

Answer 22

The outward movement of K+ causes repolarisation which turns off the stimulus for opening - NEGATIVE FEEDBACK

Question 23

What are both Na+ and K+ channels activated by ?

Answer 23

Membrane depolarisation

Question 24

Describe the conducting state of Na+ channels?

Answer 24

-depolarization initially opens Na channels into an open state

Question 25

What are the 2 non-conducting states of Na channels?

Answer 25

- during maintained depolarisation, the channels are open but in an inactivated state - after repolarisation, channels close

Question 26

What is the refractory period and what causes it?

Answer 26

- There is no secondary action potential even when a nerve is stimulated - Inactivation contributes to the repolarizing phase of the action potential and is responsible for the refractory period

Question 27

what are the 2 types of refractory periods?

Answer 27

- absolute | - relative

Question 28

Describe the absolute refractory period.

Answer 28

no stimulus, however strong, can elicit a second action potential (all Na+ channels inactivated)

Question 29

Describe relative refractory period.

Answer 29

a stronger than normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels)

Question 30

What is caused by the nerve cell membrane being 'leaky'?

Answer 30

Passive signals do not spread far from their site of origin due to current loss across membrane

Question 31

What does the distance over which the current spreads depend on?

Answer 31

- membrane resistance (rm) | - axial resistance of the axoplasm (ri)

Question 32

What increases action potential velocity?

Answer 32

- decrease ri ( increasing axon diameter) | - increase rm (adding myelin providing Schwann cells)

Question 33

What is saltatory conduction in myelinated axons?

Answer 33

the action potential jumps from one node of Ranvier to the next

Question 34

what lies at nodes of ranvier?

Answer 34

Clusters of Voltage- activated Na+ channels