Introduction to Neurons In Action

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Introduction to Neurons In Action

• Graphs and questions about graphs stemming from
  this series of exercises from Neurons in Action.
• Effect of stimuli with different current on the
  action potential
• Effect of temperature on the action potential
• Effect of Nai and Nao on the action potential
• Effect of Ko on the action potential

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Graph 1 The two plots on the above graph are
the same in all conditions except that they
received stimuli with different amplitudes (0.06
and 0.07 nA). Can you tell which plot goes with
which stimulus value? How?
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Graph 1

• The first one is a sub-threshold stimulus (0.06
  nA).
• Action potentials are an all-or-nothing
  phenomenon if they do not reach threshold, the
  Na VGC dont open, the action potential is not
  produced, and vM goes back to resting.
• The second one is a threshold stimulus (0.07 nA).
  When the membrane is charged enough such that it
  reaches threshold, the Na VGC open and we get an
  action potential.

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Graph 2 The two Action Potentials above are
made by a different stimulus amplitude (0.07 and
0.08 nA). Can you tell which action potential is
associated with which stimulus amplitude? How?
Is one AP higher than the other?
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Graph 2

• The faster AP is associated with the higher
  stimulus value. When you increase the stimulus
  (from .07 to .08 nA), it takes less time for the
  membrane to get to the point where it becomes
  permeable. This is significant because it means
  that it takes less time for the membrane to reach
  threshold and for the Na VGC to open.
• Stronger the signal, more rapidly the action
  potential is produced.
• There is NO difference in the height of the two
  APs if you measure from the point of the stimulus
  to the top of the AP. Only looks higher because
  the initial stimulus shifted it up.

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Graph 3 The two plots on this graph have the
same stimulus amplitude (0.07 nA), but one is at
6.3C and the other is at 9.3C. Can you tell
which plot goes with which temperature? How?
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Graph 3

• The plot that produces no AP is associated with
  the higher temperature because increasing the
  temperature increases the fluidity and
  leakiness of the membrane, decreasing
  resistance, and making it harder to build up a
  charge, and harder to get to threshold.
• The plot that produces an AP is associated with
  the lower temperature because decreasing the
  temperature decreases the fluidity and
  leakiness of the membrane, resistance goes up,
  making it easier to build up and maintain a
  charge and get to threshold.

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Graph 4 The two Action Potentials above have
the same stimulus amplitude (0.08 nA), but one is
at 6.3C and the other is at 12.3C. Can you tell
which plot goes with which temperature? How?
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Graph 4

• The faster, more narrow AP is associated with the
  higher temperature. The higher temperature allows
  the Na VGC to change shape more rapidly. The
  flow of ions, in coincidence with this, will be
  more rapid.
• The slower, more broad AP is associated with the
  lower temperature. The Na VGC opens up more
  slowly at lower temperatures, and the flow of
  ions will be less rapid.

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Graphs 3 4

• Based on the two graphs that look at the effects
  of temperature on an action potential, what can
  be said for the production of an action potential
  at a higher temperature?

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Graphs 3 4

• Producing an action potential at a higher
  temperature is more difficult, due to the fact
  that increasing the temperature also increases
  the leakiness of the membrane, and reduces the
  resistance. It is harder to build up and maintain
  a charge under these conditions, making getting
  to threshold difficult.
• Increasing the temperature works to increase the
  ease with which the Na VGC change shapes and
  allow the flow of sodium current in, meaning that
  the AP actually occurs much faster at higher
  temperatures.

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Graph 5 These two action potentials have the
same stimulus amplitude (0.07 nA), but one has a
higher Nao. Can you tell which action potential
has a higher Nao. How?
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Graph 5

• The AP with the higher amplitude and faster onset
  is associated with the higher Nao. The reason
  for this is that increasing the amount of
  extracellular sodium will create a greater
  potential gradient so that the sodium will
  diffuse into the cell faster after the Na VGC
  are opened, making the AP faster and the peak for
  the AP higher.

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Graph 6 These two action potentials have the
same stimulus amplitude (0.08 nA), but one has a
lower Nai. Can you tell which action potential
has the lower Nai? How?
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Graph 6

• The AP with the faster onset and the higher
  amplitude is associated with the lower values of
  Nai. The reason for this is that decreasing the
  Nai increases the potential gradient, and once
  the Na VGC are activated, Na will diffuse more
  quickly into the cell. This trend is very similar
  to the trend of increasing the Nao.

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Graph 7 These two action potentials have the
same stimulus amplitude (0.08 nA), but one has a
higher Ko. Can you tell which AP has the higher
Ko? How?
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Graph 7

• The quicker action potential that never goes back
  to its original RMP is associated with the higher
  Ko.
• Increasing the amount of potassium outside will
  decrease the gradient potential potassium will
  be less likely to flow out of the cell, thereby
  not allowing the membrane to repolarize after an
  action potential. It also increases the
  excitability of the cell.

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The Membrane Tutorial

• Graphs and questions about graphs stemming from
  this series of exercises from Neurons in Action.
• Membranes Membrane Capacitance
• Membranes with Na/K Pumps
• Membranes with Leak Channels
• Membranes with Na K VG Channels

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Graph 8 Describe what is occurring in the above
graph using what you know about the properties of
a plain lipid bilayer. What is the reason for the
sudden leveling off of the plot?
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Graph 8

• In the graph, a small stimulating current is
  delivered to the membrane. The membrane, acting
  as a capacitor, is charged up. This charging
  accounts for the positive linear plot.
• When the stimulating current ceases, the membrane
  retains the charge that it has been building up
  because there is no path by which the current may
  leave. This is what accounts for the sudden
  leveling off of the plot.

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Graph 9 The two plots above represent two
different membranes charging up, and one membrane
is larger than the other. Can you determine which
of the plots is associated with the larger
membrane? How? Mention something about
capacitance in your answer.
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Graph 9

• The larger membrane would be associated with the
  smaller, more angular plot.
• The reason for this is that larger membranes are
  associated with higher capacitances. The larger
  the membrane, then, the longer it takes for the
  higher capacitance to charge up.

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Graph 10 Look at graph 10 and graph 8. What is
the difference between the starting point for
these two graphs? What accounts for this
difference? (You can read that it is the Na/K
pump but what does it do?)
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Graph 8 Graph 10

• Graph 10 begins at a much more negative value
  than does Graph 8. The reason for this, at least
  in part, is the presence of a Na/K pump.
• The Na/K pump is exporting one more positive
  charge than it is bringing in, and every time it
  does that, the interior of the cell is becoming
  more negative.

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Graph 11 What is significant about the membrane
depicted in this graph? What is the reason for
the sudden decrease in membrane voltage after
approximately 10 milliseconds?
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Graph 11

• Leak channels are present in the membrane. This
  makes it possible for the current that is
  building up in the membrane to escape it also
  makes it hard for the membrane to build up a
  charge, as demonstrated by the slow, exponential
  plot.
• The reason that there is a sudden decrease in the
  membrane voltage after 10 ms is the fact that the
  charge that has been building up is now quickly
  escaping through the leak channels.

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Graph 12 This membrane has HH channels. What
are the HH channels? What is significant about
this graph? How can you attribute the HH channels
to what you observe in this graph?
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Graph 12

• The HH channels are the Sodium and the Potassium
  VGC.
• This graph, in the presence of the HH channels,
  produces an AP.
• The injected current is going to take the
  membrane voltage up to a certain point
  (threshold), at which time the Na VGC open. The
  opening of these Na VGC have the effect of
  producing an AP with the typical depolarization,
  repolarozation, undershoot, and regaining of the
  vM.

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Membrane Voltages Capacitive Current Density

• On the following slide, match the graphs of
  Membrane Voltages VS Time (labeled A thru C) to
  their respective graphs of Capacitive Current
  Density VS Time (labeled 1 thru 3).

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A
1
2
B
3
C
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Membrane Voltages Capacitive Current Density

• A3
• B1
• C2

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Equilibrium Potentials

• Graphs and questions about graphs stemming from
  this series of exercises from Neurons in Action.
• Membranes with Ion-specific Conductance
• Effect of Conductance on Membrane Potential

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Graph 13 The above graph represents the resting
membrane potentials for two glial cells, one of
which is exposed to higher amount of Ko than
the other. Can you determine which plot has the
higher Ko? How? Consider the Nernst Equation.
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Graph 13

• The higher, less negative plot is associated with
  glial cell with the higher Ko.
• The Nernst equation states that
• EK RT/zF ln Ko/Ki
• The larger values of the Ko make the ratio
  larger taking the natural log of the larger
  ratio increases the EK (makes it less negative).
• Besides the Nernst equation, increasing the Ko
  in the cells external environment will decrease
  the likelihood of K from inside the cell flowing
  outward, thereby increasing the membrane
  potential.

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Graph 14 The above graph depicts the ENa, EK,
and the resting membrane potential of a cell. The
conductance of Na and K is not equal based on
the graph, can you determine which ion the cell
has more conductance for? How? (Conductance
Channel Density).
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Graph 14

• The conductance of the Na ion is higher. This is
  demonstrated by the fact that the membrane
  potential is closer to the ENa than the EK.

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