Module 3: Action potential

Question 1

What is depolarisation?

Answer 1

1. Inside of membrane becomes more positive, Na+ coming into neuron
2. more positive

Question 2

What is repolarisation?

Answer 2

1. return to resting membrane potential from being depolarised
2. back to normal

Question 3

What is hyperpolarisation?

Answer 3

1. inside becomes more negative, K+ leaving neuron
2. more negative

Question 4

what triggers an Action potential in neurons?

Answer 4

a local or graded potential that is large enough to trigger the positive feedback cycle

Question 5

what is the general threshold to trigger an action potential?

Answer 5

~50mV

Question 6

describe the features of an action potential

Answer 6

1. fast (1-2 ms)
2. travel long distances without diminishing
3. initiated in axon hillock

Question 7

describe the ion flow of an action potential

Answer 7

1. 70mV: both channels closed
2. ripple of current causes a positive feeback cycle until the threshold is reached
3. on reaching -50mV threshold all Na+ channels open and flood the neuron, rapidly depolarising it to +30mV
4. once at +30mV the Na+ channels become inactive and the K+ channels open
5. K+ channels cause repolarisation, dropping the voltage
6. once voltage reaches -70mV the Na+ channels become closed
7. K+channels still open, neuron becomes hyperpolar and K+ channel closes
8. passive facilitated diffusion occurs due to the leak channels and the resting membrane potential is achieved again

Question 8

when does NA+ channel close?

Answer 8

1. open
2. closed
3. inactivated

Question 9

describe how the sodium channel is positioned in the closed position

Answer 9

the gate is closed, the ball is at rest, no ions can pass through but it is ready to open again at the next action potential

Question 10

describe how the sodium channel is positioned in the inactivated position

Answer 10

the gate is open, the ball is blocking the channel so sodium ions cannot pass through. It isn’t able to open again until it moves to the closed position at the end of repolarisation

Question 11

describe how the sodium channel is positioned in the open position

Answer 11

gate is open and ball is at rest, sodium ions can pass down heir gradient as they wish

Question 12

when do potassium channels become open in an action potential?

Answer 12

at the peak of the action potenetial

Question 13

when do potassium channels become closed in an action potential?

Answer 13

when the action potential becomes hyper polarised (lower than -70mV)

Question 14

what is the positive feedback cycle in an action potential?

Answer 14

when a local or graded potential is big enough to cause a ripple effect within the Na+ channels, opening enough to trigger more opening, which floods the neuron with positive ions, making it depolarise more and more until an action potential is achieved

Question 15

what is action potential propagation?

Answer 15

the process of the action potential travelling from the axon hillock to the end of the axon

Question 16

what is an absolute refractory period?

Answer 16

1. when all the Na+ channels are open or inactive no more action potentials can be generated
2. limits firing frequency
3. action potential travel in one direction

Question 17

what is a relative refactory period?

Answer 17

1. action potential generated if a large stimulus is applied
2. some Na+ closed

Question 18

what affects the speed of an action potential?

Answer 18

1. diamter of the axon: thicker is faster
2. temperature: not relevant humans as we are hod blooded, but faster the warmer
3. degree of myelination: more myelination faster action potential AKA saltatory conduction

Question 19

what is saltatory conduction?

Answer 19

when the myelination of an axon enables the action potential to jump along the axon to the next node of Ranvier, speeding up the pace