Last Modified: 2011-07-05
membrane are given a negative sign and currents that flow outward across the
membrane are given a positive sign.
produces inward Na current (INa) which depolarizes the membrane even more,
which leads to a higher Na conductance (gNa). This is a regenerative
process, with positive feedback. There is also a slow increase in gK.
the conductance is turned on it turns itself off and the Na conductance decreases again to its resting level. This is the major reason why the action potential is self-limiting
current increases and hyperpolarizes the membrane. gK is still on after g
Na has turned off, so at this point Vm is closer to EK than before the
action potential started. This is called the after-hyperpolarization.
After the hyperpolarization, gK turns off (because of the hyperpolarization,
not because of inactivation) and gK returns to the resting state.
there is no action potential. (This suggests that Na+ is important)
potentials. For potassium, we preload the cell with 42K+ and measure the
efflux. Using this method we calculate that for a single action potential, about 3 x 10^-12 moles per square cm of Na+ enters the cell and the same amount of
K+ leaves the cell.
changes the charge on the membrane, NOT because the ionic concentration in
the cytoplasm changes.
*Na+ and K+ flow mean Na+ and K+ current, and these currents flow as a result of increases in gNa and gK brought about by the depolarization.
on) both voltage and time. When gNa and gK change they cause currents
that produce changes in membrane voltage, which causes another change in gNa and gK, and so on.
CLAMP to keep the voltage across the membrane constant so that for each set
voltage they could measure the currents as a function of time, without letting the current flow due to the conductance change affect the membrane potential, as it does during the actual action potential. With the
voltage constant, the current records enable you to calculate the way the
conductances change with time.
induces a change in the conductance of the membrane -- we measure the
membrane current as a function of time after the voltage step. This allows
the measurement of conductance as a function of time. Then repeat the experiment by stepping to a different Vm. This will lead to a description of the properties of the channels, and at the end we'll see if this describes the action potential.
amplifier, the amplifier sends a current across resistor C until A and B are
at the same voltage. This is the same as the membrane current because the electronics supplies an equal and opposite current to keep the voltage constant. Voltage command
at A (from Vref) can set and hold Vm.
axis. If we vary the concentration of sodium outside the cell, we change
this reversal potential (Vrev), so we conclude that the early current is
carried by sodium.
(TTX). This lets us see the 'pure' late (K+) current.
INa = gNa (Vm - ENa)
IK = gK (Vm - EK)
Note that the conductance depends on voltage -- this is a
voltage-dependent resistance - NOT OHMIC.
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