resting and action potentials

Question 1

what is the resting potential?

Answer 1

-70mV

Question 2

give 3 ways in which the movement of ions can be controlled

Answer 2

1. the phospholipid bilayer - prevents simple diffusion as ions are polar
2. channel proteins allow facilitated diffusion. However Na+ are voltage gated so require depolarisation to open
3. the sodium potassium pump - actively transports ions in/out

Question 3

what happens to establish the resting potential 
1. Na+/K+ pump
2. gradient
3. diffusion
FINAL STATEMENT

Answer 3

1. the sodium potassium pump actively transports sodium ions out of the axon and potassium ions into the axon using ATP
2. an electrochemical gradient is created as there more Na+ outside of the axon than there are K+ inside
3. So the K+ diffuses out down its electrochemical gradient however the Na+ is unable to difuse back in as the channels are voltage gated
   FINAL STATEMENT : therefore the membrane is more permeable to potassium ions than sodium ions

Question 4

recall the stages on an action potential graph

Answer 4

1. generator potential/threshold
2. depolarisation
3. repolarisation
4. hyperpolarisation
5. refractory period : absolute then relative

Question 5

describe the generator potential stage

Answer 5

• stimulus causes some voltage gated Na+ channels to open
• some Na+ diffuses into the axon along electrochem gradient
• this causes more Na+ channels to open
• threshold is reached

Question 6

describe the depolarisation stage

Answer 6

• Na+ continues diffusing in until membrane potential reaches +40 mV

Question 7

describe the repolarisation stage

Answer 7

• once the membrane potential reaches +40mV the voltage gated Na+ channels close
• the K+ channels open and K+ diffuses out
• this repolarises the axon

Question 8

describe the hyperpolarisation stage

Answer 8

• the K+ diffusing out causes an overshoot where the membrane potential is more negative than the resting potential (-95ish)
• so the K+ channels slow and close

Question 9

describe the refractory period

Answer 9

• when another action potential cannot be triggered
• ensure that they are 1-directional and discrete (don’t overlap)
• absolute refractory period is when the Na+ channels are completely inactive
• relative refractory period is when the Na+ channels are active but a strong stimulus is required