lecture 4 Action Potentials

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Biology/Animal Physiol & Neurosc
BIPN 100
Laurie Smith

10/7/13 Lecture 4  This is a brief large change in trans-membrane potential, this is an action potential.  What causes change? And ionic current across membrane, in this case 2 ionic currents.  The rising phase depends on the opening of VG sodium channels  When the membrane channels open, the membrane permeability to sodium opens  There is a driving force on sodium at rest, and when channel opens there is a conductance so now there is an ionic current.  (-) driving force = driving force is inward for sodium  Any negative driving force is inward on a positively charged ion  A change in total trans-membrane current (added new current), changes membrane potential, and sodium entering the cell -> more positive charge in cell that changes trans-membrane potential and makes it more inside positive  The depolarization that caused the voltage gated sodium channels to open, also causes voltage gated K channels to open  Voltage gated Na opens very fast, the kinetics of this conformational change is very fast  The opening and closing of voltage gated K channels happens much more slowly, it’s a property of the proteins, but they are both opened by the same depolarization  When the action potential peaks, the membrane becomes more inside negative again, which is caused by another feature of VG NA channel, there is an activation gate and an inactivation particle  When the activation gate opens, the inactivation particle moves into the pore blocking it -> inactivated channel (this is based purely on time)  Initial depolarization causes channel to open because activation gate swings open, then as a function of time, the inactivation particle swings into position and channel is inactivated  By this time, voltage gated K channels start to open  The driving force on K is outward(+) in most conditions, so K leaves the cell, carrying positive charge out of the cell, therefore the membrane becomes more inside negative again,  The conductance for sodium is dropping and the conductance of K increases, so membrane returns to becoming more inside negative  In many neurons, the membrane potential falls below resting potential and gets more inside negative because K is still exiting while the conductance is still nonzero so membrane potential is continues to become more negative  When an ionic current flows, it brings membrane potential toward the equilibrium potential for that ion/current  How inside positive can membrane get during rising phase??  As long as Eion is inside driving force, sodium will continue to enter and membrane will continue to get more inside positive(until channels inactive) until it equals the eq. potential to sodium, when they equal then there is no driving force  Sodium can make it as inside positive as the sodium eq. potential but no more inside positive than that.  As long as there is an outward driving force on potassium, these channels are open and the membrane potential will shift toward the equilibrium potential for the K ion  It can get as inside negative as Epotassium, because when the membrane potential = equilibrium potential for K, there is no driving force and no further potassium current  Action potential = transient change in trans membrane potential caused by ionic current by two diff. selective protein channels (Na & K) which are opened by depolarization. Then the currents flow as long as channels are open  THERE IS A LOOP, inactivation breaks the loop for Na  Positive for Na  Negative for K  Inactivation is removed when the membrane gets inside negative again (pretty close to rest)  Voltage gated K channels don’t have inactivation particles, they have an activation gate which closes when the membrane gets inside negative again  It is the current through those channels that indirectly leads to the conditions in which the channels clos
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