Action potential

Question 1

what does depolarization mean?

Answer 1

membrane potential becomes less negative (inside cell more positive)

Question 2

what does hyperpolarization?

Answer 2

membrane potential becomes more negative

Question 3

what is influx?

Answer 3

direction of ion movement into the cell

Question 4

what is efflux?

Answer 4

direction of ion movement out of the cell

Question 5

what does movement of ions across membrane require?

Answer 5

• channels= 2 types of channels for ions: leak (open all the time) & gated (open/close to specific stimuli)
• some transporters e.g. secondary transporters etc

Question 6

what is driving force for sodium channel? (voltage-gated channel)

Answer 6

membrane potential for nerve cell - sodium membrane potential
(-70 - 60 =m-130)

Question 7

what is Na+ conductance?

Answer 7

it’s related to whether sodium channel is open or closed (one of the factors required for movement across membrane - route)

Question 8

what is value of driving force for potassium ion?

Answer 8

= -70 - -90 = -130 mV (means negative inside cell so Na+ move inside to try as attracted to -ve area)

Question 9

what is value of driving force for potassium ion?

Answer 9

The driving force for K+ efflux is simply (Vm - EK = -70 - -90 = +20). When positive (in this case +20 mV, note the double negative) outward movement of K+ occurs

Question 10

what type of polarization is
a) Na+ channel opening?
b) K+ channel opening?

Answer 10

a) depolarisation (making inside more +ve/ less negative)
b) hyperpolarization (making inside more -ve)

Question 11

what are the different types of gated channels?

Answer 11

• voltage gated
• ligand gated
• physical stimuli, physical or mechanical changes in plasma membrane

Question 12

what is the importance of the threshold value?

Answer 12

action potential occurs if membrane potential hits threshold value

Question 13

what is the upstroke phase of the action potential of neurons?

Answer 13

depolarisation
= neuron receives stimulus and once threshold reached, opening of voltage-gated Na+ channels →influx of Na+ into cell making interior less negative/more positive (depolarisation)

• this involves a positive feedback loop where influx of sodium ions causes further depolarisation leading to generation of action potential

Question 14

what is the downstroke phase of action potential of neurons?

Answer 14

repolarization
- at peak of action potential, closing of Na+ channels and opening of K+ channels →K+ ions move out of cell restoring negative charge inside cell and bringing membrane potential to normal state

Question 15

what is the undershoot stage of action potential of neurons?

Answer 15

hyperpolarization
- can occur in some cases, it is when membrane potential may briefly dip below resting membrane potential due to K+ channels continuing to allow K+ ions to leave the cell

Question 16

what are action potentials?

Answer 16

brief electrical signals in which the polarity of the nerve cell membrane is momentarily (about 2 millisec) reversed
- they’re generated when threshold is reached (all or nothing)

Question 17

does sodium or potassium have longer conductance?

Answer 17

K has longer conductance than Na

Question 18

describe the positive feedback of sodium channels?

Answer 18

they’re self re-inforcing →opening of a few channels causes further channels to open and causes further depolarization etc (increase in Na+ conductance → inward Na+ current →depolarisation, this loops)

Question 19

describe the negative feedback of K+ channels?

Answer 19

they’re self limiting →outward movement of K+ causes repolarization which turns off stimulus for opening (depolarisation →increase in K+ conductance →outward K+ current → repolarization)

Question 20

what are the 3 states of sodium channel?

Answer 20

1. start with being closed
2. they then open in response to depolarisation
3. maintained depolarisation means they are inactivated (for downstroke/repolarisation stage)
4. they then get closed and cycle can start again

Question 21

what is the absolute refractory period?

Answer 21

no stimulus, no matter how strong, can elicit a 2nd action potential (all Na+ channels are in inactivated state)

Question 22

what is the relative refractory period?

Answer 22

stronger than normal stimulus is required, stronger stimulus may elicit a second action potential (mixed population of inactivated and closed channels, plus the membrane is hyperpolarized)

Question 23

where do we find clusters of voltage gated channels?

Answer 23

at gaps in between myelin sheath on axons - called nodes of ranvier

Question 24

what is difference for myelinated axons compared to non-myelinated axons?

Answer 24

conduction is much faster in myelinated
- in un-myelinated axons involves passive spread of current
- in myelinated axons is by saltatory conduction

Question 25

when is
a) the refractory period?
b) the relative refractory period?

Answer 25

a) downstroke
b) undershoot