MCP 27: Ion Channels and Action Potentials Flashcards
threshold
voltage at which action potential starts, usually around -50mV
overshoot
how much greater than zero the action potential goes
undershoot
how much under the resting potential the action potential goes, because K+ channels remain open for a longer time
tetrodotoxin (TTX)
blocks the fast voltage-gated sodium channels
voltage clamp
keeps voltage constant and you can measure the current even when conductance is changing, any change in current is a result from a change in conductance (G); gives data on CONDUCTANCE CHANGES OVER TIME!
current clamp
keeps current constant
features of potassium channels
slow to open, do not inactivate, responsible for outward current of action potential
Tetraethylammonium (TEA)
blocks potassium channels
features of sodium channels
activate super fast, inactivates super fast, responsible for inward current, when you get to higher voltages, reverses direction
patch clamp experiment
can hold the voltage of a single ion channel constant and measure the current
relative refractory period
possible to get an action potential because Na+ channels have reactivated, but need stronger stimulus; because K+ channels do not deactivate and are causing hyperpolarization
absolute refractory
time when you cannot get an action potential because Na+ channels have deactivated
current moving backwards
doesn’t happen because Na+ are deactivated, unless you start an action potential in the middle of the axon
conduction velocity
speed of action potential measured in meters/second; depends on internal diameter of axon, internal resistance, membrane capacitance, myleination, activation kinetics of the Na+ channel
passive property of cables
when you apply a current to a cable, the voltage decreases with increasing distance
threshold
when the influx of sodium from the voltage gated channels is large enough to counterbalance the current produced by the K+ leakage channel, explosive action potential occurs
space constant
distance it takes for the action potential to fall to 37% of its original strength, increasing membrane resistance and axon diameter both increase space constant and allow action potential to travel farther. space constant=sqrt(Rm/Ri) where Ri=1/d
diameter of axon
decrease internal resistance, increase space constant, and internal velocity
role of calcium
higher Ca+2=decreased excitability, lower Ca+2=increased excitability; alters the way Na+ channels sense membrane potential because of shielding effects on the voltage sensor
hyperkalemic periodic paralysis or paramyotonia congenita
amino acid mutation in sequence encoding Na+ channel
cocaine
sodium channel blocker–other drugs ending in -aine
myelination
increases resistance and decreases capacitance
time constant
gives a measure of the speed of charging a capacitor, at t=tau, the charge on an initially uncharged capacitor is 63% its maximum value; on a fully charged capacitor, the charge q on the capacitor decreases exponentially with time depending on the rate constant. At time t=tau, the charge on the capacitor is reduced to 37% its original value. tau=Rm X Cm
capacitance
steals charge from the action potential
inward negative current
positive ions flowing into the cell
saxitoxin
blocks sodium channels
peak of action potential
Na+ channels deactivate, K+ leakage channels allow for some repolarization, delayed voltage-gated K+ channels activate and allow huge flux of K+ out of the cell
Nodes of Ranvier
block AP from occurring bc of myelin–AP can only happen at nodes (http://www.wisegeek.com/what-are-nodes-of-ranvier.htm#didyouknowout)
