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Flashcards in Action Potentials Deck (17)
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what is the resting potential?

-65 mV


what is depolarization?

rising phase due to Na+ influx (membrane potential gets less negative --> more positive from -65 mV to +40 mV)


what is hyperpolarization

falling phase due to K+ efflux (membrane potential becomes more negative from +40 mV to below -65 mV)


overshoot phase

depolarization above the threshold value
-greater depolarization produces more spikes at higher frequency


undershoot phase

afterhyperpolarization phase with refractory period (cannot be stimulated) and relative refractory period (needs greater stimulation)
-due to open voltage-gated K+ channels that gradually close and return to resting membrane potential


relationship between sodium and depolarization

lowered external Na+ results in smaller and slower APs
-important control is to return the external solution to normal (so if they are in very hypotonic solution without Na+ influx, return it to a higher Na+ solution and APs will return)


4 voltage-sensitive mechanisms during action potentials

1. activation of Na+ conductance (Na+ influx down concentration gradient) --> depolarization
2. delayed activation of K+ conductance (K+ efflux down concentration gradient) --> hyperpolarization
3. inactivation of Na+ conductance (later merely close)
4. closing of voltage-gated K+ channels


how were changes in Na+ and K+ conductances discovered?

voltage-clamp recordings via voltage and ligand-gated channels to measure change with time and membrane potential
-injects current into the cell that is equal and opposite to the current flowing through the voltage-gated channels
-negative feedback loop prevents voltage across the membrane from changing


how to use voltage-clamp recordings

the amount of current injected by clamp to keep voltage constant is a measure of the current flowing across the membrane
-routinely used during development of new drugs


what 2 currents the voltage-clamp technique reveals

1. early inward current (Na+)
2. late outward current (K+)
both change with time


tetrodotoxin (TTX)

blocks early Na+ channels without affecting K+ channels
-from puffer fish


tetraethylammonium bromide (TEA)

blocks late K+ channels without affecting Na+ channels
-also an ACh receptor blocker


AP propagation

requires both active and passive current flow
-active: gating of voltage-gated channels and associated Na+ influx
-passive: depolarization wave that precedes AP (Na influx travels further to depolarize the other areas)
--discharging membrane capacitance leads to Na+ channel activation



wrapping of glial cells in cell membranes around axon (equivalent to increasing membrane thickness 100x)
-increases insulation to reduce leak of passive flow and decrease capacitance


capacitance equation

C = area/distance (distance = total thickness)
decreases with myelination


conduction in myelinated fibers

1. fast, passive potentials between nodes of Ranvier
2. generation of AP in nodes (boosting stations)
3. saltatory conduction (APs jump from node to node)
unmyelinated is 0.5 to 1.0 m/s, but myelinated is 150 m/s


nodes of Ranvier

gap in myelin sheath separated by 1 or 2 mm
-contain full complement of Na+ and K+ channels
-generate APs

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