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The action potential is used by
neurons that need to send information over long distances.
- Example:
- The neuron that carries touch information from the great toe (hallux) to the medulla can reach lengths of 2 meters in humans, or 6 meters in a giraffe.
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The action potential results from the
opening and closing of voltage-gated channels
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The axonal membrane has voltage-gated sodium channels with a special property:
when the voltage becomes less negative, the channels open.
Because the driving force (concentration + electrical forces) on sodium is pushing it into the cell, sodium ions rush in and make the cell membrane less negative inside. This opens up more voltage-gated sodium channels, and the neuron becomes less negative, and so on.
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Action Potential:
1. Resting State
All voltage-gated Na+ and K+ channels are closed. The axon plasma membrane potential: small buildup of negative charges along inside surface of membrane and an equal buildup of positive charges along outside surface of membrane.
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Stimulus causes
depolarization to threshold.
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Threshold is
the point at which depolarization will trigger an action potential.
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The self-regenerating process has a "tipping-point":
at some point, enough voltage-gated channels open that the voltage changes that result will open up all voltage-gated sodium channels.
Tipping point = threshold
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Action Potential:
2. Depolarizing Phase
When membrane potential of axon reaches threshold, the Na+ channel activation gates open. As Na+ ions move through these channels into the neuron, a buildup of positive charges forms along inside surface of membrane and the membrane becomes depolarized.
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Action Potential:
3. Repolarizing phase begins:
Na+ channel inactivation gates close and K+ channels open. The membrane starts to become repolarized as some K+ ions leave the neuron and a few negative charges begin to buildup along the inside surface of the membrane.
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Action Potential:
4. Repolarizing Phase continues:
K+ outflow continues. As more K+ ions leave the neuron, more negative charges build up along inside surface of the membrane. K+ outflow eventually restores resting membrane potential. Na+ channel inactivation gates open. Return to resting state when K+ gates close.
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Steps in the Action Potential:
-Resting potential
-Threshold
- -Depolariztion
- *Na+ channels open
- PEAK
- *Na+ channels close (inactivate)
- *K+ channels open
-Repolarization
-After-hyperpolarization (also called hyperpolarization)
Return to resting potential
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When firing an action potential is impossible, we call this the
Absolute refractory period.
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When it is merely difficult to fire an action potential, because not all of the voltage-gated channels have reset, we call this the
Relative refractory period: it is not impossible, but relatively difficult to fire a new spike.
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The axonal membrane is said to be refractory, meaning
resistant to change. As more and more voltage-gated channels "reset" to their resting state, it becomes possible to fire a new action potential.
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Action Potential begins at the
Trigger Zone
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Trigger Zone
-Near the axon hillock
-Voltage-gated Na+ and K+ channels begin here and continue along the axon
-No voltage-gated channels in cell body or dendrites
-Passive spread of current in both directions from trigger zone
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Propagation of the Action Potential:
*Action potential begins at the trigger zone
*Some differences between myelinated and unmyelinated axons
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In virtually all neurons, there are no voltage-gated channels in the
dendrites and cell body. No action potentials can occur here.
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Electric Current Flow:
*Think of a nerve axon as a sort of cable
- *Ions flow in one direction or another to cause a flow of electric current.
- -remember I=V/R
- *g is conductance
- *current equals voltage times conductance
- *bigger "cable"= more conductance= more current
- *bigger voltage=more electrical potential= more current
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Moving further down the axon, however, the ripples encounter a fresh patch of axonal membrane where the voltage-gated channels have not yet been activated. These "fresh" voltage-gated channels are activated, and
a new action potential is triggered at this point.
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Propagation of the Action Potential: Continuous Conduction in Unmyelinated Axons.....
The action potential is forced to move in only one direction. Back toward the trigger zone, the voltage-gated channels are still in their
absolute refractory phase.
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The only "fresh" active membrane is found toward the _____ ________, so the action potential ______ that direction.
axon terminal; moves
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Saltatory conduction in myelinated axons.
Myelin sheath is interrupted at
nodes of Ranvier
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Propagation of the Action Potential:
Saltatory Conduction in Myelinated Axons...
A mechanism operates when the axon is ensheathed in an insulating layer of ______. The insulation keeps ______ of ions from depleting the "wave" of voltage _____, and the neuronal axon is tuned so that the height of the depolarizing "wave" is just enough to reach ________ at the next ____ of _______.
myelin; leakage; change; threshold; node; Ranvier
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Propagation of the Action Potential:
Saltatory Conduction in Myelinated Axons....
Voltage-gated sodium and potassium channels are only found at the nodes of _______. There's no point in having them under the blanket of myelin. They are all clustered at high density in the ___ ______.
Ranvier; node region
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The action potential is only triggered at _____. It therefore appears to ____ from node to ____, a phenomenon called ________ _________.
nodes; jump; node; saltatory conduction
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Distribution of Ions Across the Neuronal Membrane is Altered by the Action Potential:
Give the approximate concentration for inside & outside for the given ion.
Sodium (Na+)
Inside -10
Outside - 140
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Distribution of Ions Across the Neuronal Membrane is Altered by the Action Potential:
Give the approximate concentration for inside & outside for the given ion.
Potassium (K+)
Inside - 140
Outside - 4
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Distribution of Ions Across the Neuronal Membrane is Altered by the Action Potential:
Give the approximate concentration for inside & outside for the given ion.
Chloride (Cl-)
Inside - 20
Outside - 103
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Distribution of Ions Across the Neuronal Membrane is Altered by the Action Potential:
Give the approximate concentration for inside & outside for the given ion.
Calcium (Ca++)
Inside - zero
Outside - 5
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