Module 3: action potentials

Question 1

what sends actions potentials in a neuron?

Answer 1

the Axon

Question 2

what other words are used to describe an action potential?

Answer 2

spike
impulse

Question 3

what is depolarisation in a neuron?

Answer 3

when the ICF becomes more positive than the resting membrane potential (-70mV+)

Question 4

why does hyperpolarisation happen normally?

Answer 4

Na+ entering the cell

Question 5

what is repolarisation in a neuron?

Answer 5

when the cell has gone from resting to depolarise and it goes back to resting again

Question 6

why does repolarisation happen normally?

Answer 6

more K+ leaving the cell

Question 7

what is hyperpolarisation?

Answer 7

when the ICF becomes more negative than the resting membrane potential (-70mV-)

Question 8

what triggers an Action potential in neurons?

Answer 8

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

Question 9

describe the features of an action potential

Answer 9

1. large change in voltage
2. very brief (~1-2ms)
3. revirsible

Question 10

are local/graded potentials the same size?

Answer 10

No

Question 11

what is the general threshold to trigger an action potential?

Answer 11

~50mV

Question 12

where are action potentials initiaiated?

Answer 12

in the axon hillock

Question 13

are action potentials the same size?

Answer 13

yes ~+30mV

Question 14

what are the 3 states of the voltage gates sodium channel?

Answer 14

1. open
2. closed
3. inactivated

Question 15

describe how the sodium channel is positioned in the open position

Answer 15

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

Question 16

describe how the sodium channel is positioned in the closed position

Answer 16

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 17

describe how the sodium channel is positioned in the inactivated position

Answer 17

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 18

when do sodium channels open during an action potential?

Answer 18

at the threshold of ~-50mV

Question 19

when do sodium channels become inactive in an action potential?

Answer 19

at the peak of the action potential ~+30mV

Question 20

when do sodium channels become closed in an action potential?

Answer 20

either before the thereshold or during repolarisation, when it reaches -70mV resting potential

Question 21

what are the 2 states of the potassium channel?

Answer 21

1. open
2. closed

Question 22

when do potassium channels become open in an action potential?

Answer 22

at the peak of the action potenetial

Question 23

when do potassium channels become closed in an action potential?

Answer 23

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

Question 23

what is the positive feedback cycle in an action potential?

Answer 23

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 24

describe the ion flow of an action potential

Answer 24

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 25

how do local anaesthetics work?

Answer 25

they keep the Na+ channels in the inactive position so they cannot go back to closed, then open, which means an action potential cannot be triggered, causing the targeted sensory neurons to stop working, which in turn makes your face numb

Question 26

what is action potential propagation?

Answer 26

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

Question 27

what is an absolute refactory period?

Answer 27

when all the Na+ channels are open or inactive so no more action potentials can be generated

Question 28

at what point is an abosolute refactory period?

Answer 28

between the threshold and near back to resting potential

Question 29

what is a relative refactory period?

Answer 29

when an action potential could be generated but it would need a large stimulus as
1. some Na+ channels are ready but some are still inactive, so cannot open or
2. all Na+ channels are ready but it’s hyperpolarised

Question 30

what limits an action potential being triggered again?

Answer 30

the amount of Na+ channels open, as indicated by the refactory periods

Question 31

what direction does an action potential travel?

Answer 31

from the axon hillock, away from the soma, to the terminal

Question 32

what affects the speed of an action potential?

Answer 32

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 33

what is saltatory conduction?

Answer 33

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

Question 34

what is a node of Ranvier?

Answer 34

a space on an axon between the myelinated sheaths that exposes the Na+ and K+ gated voltage channels

Question 35

describe multiple sclerosis

Answer 35

a degenerative disease where the body attacks the myelination of the neurons and breaks them down.
symptoms: pain, tingling, numbness, walking difficulty, fatigue, muscle spasms. etc

Question 36

how does action potential nuance itself when it always triggers at +30mV?

Answer 36

frequency of action potentials, more potentials=stronger stimulus