action potential Flashcards
what does depolarization mean?
membrane potential becomes less negative (inside cell more positive)
what does hyperpolarization?
membrane potential becomes more negative
what is influx?
direction of ion movement into the cell
what is efflux?
direction of ion movement out of the cell
what does movement of ions across membrane require?
- 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
what is driving force for sodium channel? (voltage-gated channel)
membrane potential for nerve cell - sodium membrane potential
(-70 - 60 =m-130)
what is Na+ conductance?
it’s related to whether sodium channel is open or closed (one of the factors required for movement across membrane - route)
what is value of driving force for potassium ion?
= -70 - -90 = -130 mV (means negative inside cell so Na+ move inside to try as attracted to -ve area)
what is value of driving force for potassium ion?
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
what type of polarization is
a) Na+ channel opening?
b) K+ channel opening?
a) depolarisation (making inside more +ve/ less negative)
b) hyperpolarization (making inside more -ve)
what are the different types of gated channels?
- voltage gated
- ligand gated
- physical stimuli, physical or mechanical changes in plasma membrane
what is the importance of the threshold value?
action potential occurs if membrane potential hits threshold value
what is the upstroke phase of the action potential of neurons?
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
what is the downstroke phase of action potential of neurons?
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
what is the undershoot stage of action potential of neurons?
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
what are action potentials?
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)
does sodium or potassium have longer conductance?
K has longer conductance than Na
describe the positive feedback of sodium channels?
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)
describe the negative feedback of K+ channels?
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)
what are the 3 states of sodium channel?
- start with being closed
- they then open in response to depolarisation
- maintained depolarisation means they are inactivated (for downstroke/repolarisation stage)
- they then get closed and cycle can start again
what is the absolute refractory period?
no stimulus, no matter how strong, can elicit a 2nd action potential (all Na+ channels are in inactivated state)
what is the relative refractory period?
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)
where do we find clusters of voltage gated channels?
at gaps in between myelin sheath on axons - called nodes of ranvier
what is difference for myelinated axons compared to non-myelinated axons?
conduction is much faster in myelinated
- in un-myelinated axons involves passive spread of current
- in myelinated axons is by saltatory conduction
when is
a) the refractory period?
b) the relative refractory period?
a) downstroke
b) undershoot
