Download Nerve Impulse Transmission

April 7th 2014
Date
Neurons - Excitable Cells
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Excitable cells = electrically
active cells
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There is polarity, or a difference
in electrical charge between the
inside and outside of the neuron
=> called membrane potential
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Inside the cell: more negatively
charged
Axomembrane Ion Channels & Pumps
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Ions involved - Sodium (outside of
neuron) & Potassium (inside of
neuron)
✤
Voltage-gated channels (do not use
ATP) are sensitive to voltage changes
and facilitate transportation of Na+
and K+ across the membrane ✤
Sodium-potassium pumps (use ATP)
actively pump Na+ & K+ across the
membrane to maintain the membrane
potential (negative interior of the cell)
Axomembrane Ion Channels
Axomembrane Ion Pumps
Maintenance of Membrane Potential
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https://www.youtube.com/watch?v=NCE8baQaiK8
Nerve Impulse Transmission
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As electrical impulses travel through neurons, there is
a series of membrane potential shifts
✤
These shifts are due to the movement of Na+ & K+
ions (charged molecules) across the membrane
Nerve Impulse Transmission: Action Potential
✤
If a stimulus (e.g. pressure, sound, etc.) is strong
enough, a nerve impulse is initiated => action
potential
✤
Action potential: when the membrane potential
rapidly rises and falls - this acts as the signal/message
to be repeated & sent along the neuron's axon
Action Potential Stages
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1. Resting State
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2. Threshold
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3. Depolarization
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4. Repolarization
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5. Recovery (refractory) Period
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6. Back to Resting State
1. Resting Stage
✤
The sodium & potassium voltage-gated channels in the
membrane are closed
✤
Hence, membrane is not permeable to these ions - cannot
move in or out of the neuron ✤
Inside the cell: more negative
✤
Resting membrane potential: -70mV
2. Threshold
✤
A stimulus triggers the opening of some sodium
voltage-gated channels and Na+ flow into the neuron ✤
Inside the cell: becomes less negative
✤
If the membrane potential reaches -55mV =>
threshold has been reached and an action potential is
initiated
3. Depolarization
✤
Once a stimulus surpasses the threshold, an action potential
is triggered ✤
The membrane becomes permeable to sodium => all sodium
voltage-gated channels open and Na+ rush into the neuron
✤
Inside the cell: becomes more positive (+30mV at peak)
4. Repolarization
✤
When depolarization peaks at +30mV, potassium voltagegated channels open and K+ rush out of the neuron
✤
Sodium gates begin closing
✤
Inside the cell: becomes negative again => restoring the
resting potential at -70mV (but ions are in reverse positions)
5. Recovery Period
✤
At -70mV, all sodium & potassium voltage-gated channels close
✤
Inside the cell: becomes more negative than the resting
membrane potential ✤
To restore Na+ and K+ ions to their original conditions, sodiumpotassium pumps (using ATP) actively pump Na+ & K+ back
across the membrane until the resting state is re-established
6. Return to Resting State
✤
Re-establishment of resting state allows the
conduction of another impulse
Action Potential Propagation
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https://www.youtube.com/watch?v=Sa1wM750Rvs
Action Potential Propagation
All or None Response
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If the threshold (-55mV) has been reached, an action
potential will be generated
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Each action potential is equal to all other action
potentials (stronger stimulus ≠ bigger impulse)
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Stronger stimuli produce a greater # of impulses i.e.
more neurons involved or one neuron conducting a
series of impulses
Myelination & Action Potential Propagation
Myelination & Action Potential Propagation
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https://www.youtube.com/watch?v=DJe3_3XsBOg
Reminder
✤
PNS & Neuron Quiz on Wednesday
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