Action Potential Flashcards
Why are axons constitutively poor conductors?
Salt-filled cytoplasm and membrane permeability creates high internal resistance and leaky membranes respectively.
Draw out activation and inactivation gates of Na channel at different points in AP
See handout
Change in intracellular K and Na post AP
Electrical force is so strong that small # of Na is needed to depolarize for AP (0.1 mM). In addition to the constitutive K/Na pump, only a small % of total ICF and ECF ions are fluxing in an AP
Na/K pump in AP
Pump is not active during AP and is not fast enough to impact depolarization/hyperpolarization events. Post-AP it is necessary to restore resting Na and K concentrations, otherwise cells would eventually lose Vm
Refractory Period
Absolute: Time following AP hyperpolarization phase when majority of Na channel inactivation gates are still closed. No amount of stimulus can produce AP.
Relative: many inactivation gates are still closed but enough are open that a stronger than normal signal could produce enough depolarization to reach and pass threshold
Accomodation
Neuron does not produce AP with slow depolarization even if it overcomes threshold depolarization magnitude. Enough inactivation gates are closed before critical # of NaV channels are activated to provide enough depolarization.
Can be caused by hyperkalemia
Threshold
When Na influx equals K eflux (equal relative permeability). Minimal amount of depolarization needed to start AP. If Na current “wins” Na permeability greatly increases and AP happens. If K current wins, cell hyperpolarizes and returns to rest.
Rising Phase Positive Feedback
Depolarization opens NaV channels which increases depolarization which opens more NaV channels….
Na channels as booster stations
Depolarization moves through cytosol in both directions from AP site, however the signal eventually dies from resistance. Clustered Na channels throughout length of neuron sense depolarization and open, renewing depolarization.
Myelin
Myelin increases conduction velocity by insulating neuron and reducing leakiness. Electrical resistance increased between ECF and ICF
Refractoriness
APs cannot reverse directions since inactivation gates on Na channels are closed for a short time after rising phase occurs. If depolarization tries to spread backwards it will die out because no Na channels are prepared to boost the signal.
Extracellular Ca and Threshold
Hyperkalemia slowly depolarizes the cell and accommodation prevents APs. Extracellular calcium stops this by binding to negative charges on the PM and making sodium channels feel like cell is hyperpolarized. This increases the threshold for APs, allowing them to happen.
Axon diameter effect on conduction velocity and threshold
Larger diameter reduces resistance which increases conduction velocity and reduces threshold.
Safety factor
Cell express 5 to 10 times more Na channels than necessary to create AP. This is important at neuron branch points where more Na channels are needed to propagate AP and for reducing refractory period for subsequent stimuli
Which neurons are myelinated?
Large axons tend to be myelinated since the relationship velocitydiameter maximizes conduction velocity.
Small axons tend not to be myelinated since the relationship velocityroot(diameter) is greater than velocity*diameter at small diameters.
