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22 Cards in this Set
- Front
- Back
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Do bulk ion concentrations change significantly during an action potential?
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NO
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What is the major factor that gives rise to an action potential, conductance or concentration??
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CONDUCTANCE. It doesn't really matter what the concentrations are if the membrane is not permeable to the ion.
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What causes changes in membrane conductance?
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1. selective permeability = the membrane can decide to be permeable to one ion vs another - and it can change its mind.
2. gated channels can open and close in response to Em. |
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Which ion channels have positive feedback? What does that mean?
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Na+ channels OPEN with depolarization. Na+ flows into the cell and the area becomes MORE depolarized so Na+ wants to stay open.
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Which ion channels exhibit negative feedback? What does this mean?
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K+ channels have negative feedback. They OPEN with depolarization BUT as the membrane becomes MORE DEPOLARIZED THE CHANNELS SHUT.
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What are the opening and closing of ion channels dependent on?
Why? |
Voltage.
Think of the charge across the membrane as an electric field. An electric field can do WORK. The energy stored in Vm do the work of changing ion conformation. |
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Why is S4 different than the other parts of a voltage gated ion channel?
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S4 is amphipathic - has hydrophobic/philic residues q 3rd position. these charges (Arg) sense the electric field. S4 feels the change and moves S6 to open the gate.
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What does this mean:
g = nPoY |
the conductance (permeability) = the number of channels (n) times the Probability that the channel is open (Po) times the conductance of a single channel (Y).
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What is accommodation with regard to Na+ channels?
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A cell can depolarize slowly beyond it's normal threshold for firing without evoking an action potential.
This is because Na+ channels inactivate and fewer are available for the positive feedback cycle. |
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What does Vm-Ex describe?
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For example. If Vm is -70 and Ek is -90...the driving force is low.
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What is Rm?
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loss of current through membrane leak conductance.
Current going OUT a) to charge the capacitance or b) is just lost instead of DOWN to propagate the AP. |
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What is the length constant, lambda?
What does it depend on? |
lambda = the loss of current with distance
lambda = Rm/Ri that means the loss of current through leak conductance/longitudinal resistance |
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What is Ri?
What does Ri depend on? |
Ri is the longitudinal resistance.
Ri depends on the the diameter of the axon. If the diameter is bigger there will be more charge carriers and more current will be lost as the current flows down the axon. |
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What does large Rm mean?
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MORE CURRENT MAKES IT DOWN THE AXON.
resistance to leaking out peripherally is LOW = more current to make it down axon. |
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What does having a small Ri mean?
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Small Ri means LESS RESISTANCE DOWN THE AXON = Longer lambda.
This is because there are bigger local currents because of more local charge carriers to accommodate the big diameter. |
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What does it mean to have a long Lambda?
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the longer the length constant -the more current makes it down the axon. (little current decrement).
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The success of AP propagation is a direct result of magnitude of what?
What is a safety factor? |
LOCAL CURRENTS
The safety factor means that the magnitude of local currents is usually a lot larger than necessary to make an AP. |
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Increasing the axon diameter has what effect on Ri and Length constant?
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Increasing axon diameter increases the length constant and decreases the Ri.
lambda = Rm/Ri |
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Why doesn't the decrease of Rm cause the length constant to become shorter with increasing axon diameter?
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When you increase axon diameter - you decrease both Ri and Rm...but you decrease the Ri MORE...so the overall length constant increases.
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What does it mean if length constant is increased??
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You increase the distance over which the current decays. A longer length constant means the current is traveling farther.
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What happens to capacitance (the amount of charge that the membrane holds) with a lot of myelin?
What happens to membrane resistance? |
the capacitance decreases as the thickness of the myelin increases.
Membrane resistance increases = less goes through leak channels (Rm). |
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What is the primary cause of disease in multiple sclerosis?
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Velocity of current is depressed because local currents are lost through membrane (where myelin should be) - there is also increased increased capacitance = current that goes to charge the membrane instead of down it.
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