chapter 4: the action potential 高毓儒 institute of physiology, school of medicine national...
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Chapter 4: The Action PotentialChapter 4: The Action Potential
高毓儒
Institute of Physiology, School of Medicine National Yang-Ming University2826-7086 [email protected]
NeuroscienceNeuroscience
OutlineOutline
Introduction Properties of the action potential The action potential – in theory The action potential – in reality Action potential conduction Action potential, axons, and dendrites Review
IntroductionIntroduction Action Potential
Action potential vs. electricity Electrical charge of ions vs. generator Non-degraded vs. degraded conduction All-or-none vs. adjustable characteristic Encoding by frequency and pattern vs.
magnitude of electrical power
AP-PropertiesAP-Properties Measurement
AP-PropertiesAP-Properties The Up and Down
GenerationAP-PropertiesAP-Properties
Generation
Concept of threshold Concept of all-or-none
AP-PropertiesAP-Properties
Generation
Absolute refractory period Relative refractory period
AP-PropertiesAP-Properties
AP-in TheoryAP-in Theory Current and Conductance
A simplified model at resting state (0 - 80 mV)
AP-in TheoryAP-in Theory Current and Conductance
A simplified model - upon stimulation (-80 – 62 mV)
AP-in TheoryAP-in Theory Current and Conductance
A simplified model upon stimulation (62 - -80 mV)
AP-in RealityAP-in Reality Voltage-Gated Na Channel
Structure – 4 domains
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AP-in RealityAP-in Reality Voltage-Gated Na Channel
Structure – 6 helices for each domain
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AP-in RealityAP-in Reality Voltage-Gated Na Channel
Structure – domains for specificities
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AP-in RealityAP-in Reality Voltage-Gated Na Channel
Depolarization and pore opening
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AP-in RealityAP-in Reality Voltage-Gated Na Channel
Pore selectivity
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AP-in RealityAP-in Reality Voltage-Gated Na Channel
Patch-clamp technique
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AP-in RealityAP-in Reality Voltage-Gated Na Channel
Functional properties
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AP-in RealityAP-in Reality Voltage-Gated Na Channel
Functional properties
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AP-in RealityAP-in Reality Voltage-Gated Na Channel
Open with little delay.
Stay open for only 1 ms and then close (inactivate).
Cannot be opened again by depolarization until the membrane potential returns to a negative value near threshold.
The overshoot is limited by inactivation.
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Characteristics
AP-in RealityAP-in Reality Voltage-Gated Na Channel
Opining a single channel does not result in action potential.
The membrane of axon contains thousands of Na channel per m .
Concerted action within 1 ms explains the rapidly rising phase of action potential.
Inactivation of Na channel accounts for the absolute refractory period.
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Reminders
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AP-in RealityAP-in Reality Voltage-Gated Na Channel
Effect of TTX and Saxitoxin – channel blocker
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Toxins
AP-in RealityAP-in Reality Voltage-Gated Na Channel
Batrachotoxin (Frog) – lower the threshold and stay open
Toxins from Lilies and Buttercups
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Toxins
AP-in RealityAP-in Reality Voltage-Gated K Channel
Inactivation of Na channels (the 1st factor) A transient increase in K conductance Also open in response to depolarization with 1 ms
delay - delay rectifiers (the 2nd factor) Na -K pump working in the background at all
time (the 3rd factor)
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Repolarization
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AP-in RealityAP-in Reality Overall Changes in Ionic Currents
AP-in RealityAP-in Reality Overall Changes in Ionic Currents
AP-in RealityAP-in Reality Overall Changes in Ionic Currents
AP ConductionAP Conduction Propagation
Orthodromic conduction (10 m/s) Mechanism of all-or-none
Characteristics
AP ConductionAP Conduction
Only one direction and no turning back Influenced by axonal size and number of
voltage-gated channels Axonal excitability Local anesthetics
Propagation
Characteristics
AP ConductionAP Conduction Myelin and Saltatory Conduction
Insulation by myelin
AP ConductionAP Conduction Myelin and Saltatory Conduction
Break of insulation for ionic currents to generate AP
AP, Axons and DendritesAP, Axons and Dendrites Difference
The membrane of dendrites and cell bodies do not have enough voltage-gated sodium channels.
They do not generate AP. The spike-initiation zone (axonal hillock)
fires the first AP.
AP, Axons and DendritesAP, Axons and Dendrites Difference
Action Potential Review
Define membrane potential, Na equilibrium potential. Which of these, if any, changes during the course of an action potential?
What ions carry the early inward and late outward currents during the action potential?
Why is the action potential referred to as “all-or-none”?
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Action Potential Review
Some voltage-gated K are known as delay rectifiers. What would happen if these channels took much longer than normal to open?
What parts of the cell would you see the labeling of TTX? What would be the consequence?
How does action potential conduction velocity vary with axonal diameter? Why?
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