Lecture 6.F10

Lecture 6.F10 - Neuroscience 106: Lecture 6 Action...

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Neuroscience 106: Lecture 6 – Action Potential to Synaptic Potential ASSIGNMENTS: In Bear et al read Chapter 7. TODAY'S LECTURE: ACTION POTENTIALS (cont) AND POSTSYNAPTIC POTENTIALS I. Action Potential Conduction Velocity A. The figure below shows a “snapshot” of the different voltage values along an axon during an action potential. It also shows the fluxes of sodium and potassium at different points of the action potential and how these contribute to the voltage changes occurring at those points. II. This figure also shows the flow of ions inside the axon, ahead of the action potential. The main factor that affects conduction velocity is the rate of propagation of these depolarizing positive ions in front of the leading edge of the action potential. Na + K + Action Potential’s direction of travel Opening voltage-gated sodium channels allows rapid sodium influx. Na + -channel inactivation Opening Voltage-gated K channels allows K efflux. 1
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A. The opening of voltage-gated Na + channels allows an influx of Na + which produces depolarization, causing more voltage-gated Na + channels to open. ... This is a positive feedback system and is the action in action potential. B. The further and faster the positive ions travel down the axon, the faster the action potential is propagated. Two factors influence the rate of ion travel down the axon: 1. The internal resistance (R i ) of the axon to flow of ions. 2. The membrane resistance (R m ) of the axon to ions crossing the membrane. C. Changing either R i or R m will change the conduction velocity. So, how is conduction velocity increased? 1. Conduction velocity is increased by increasing the axon diameter, because larger diameter axons have lower internal resistance . This is the strategy taken by invertebrates (the maximum conduction velocity in invertebrates is 20 meters/second) which sometimes have giant axons. 2. Conduction velocity can also be increased by increasing membrane resistance (R m ). This is done with myelination (by oligodendroglia or Schwann cells), and this is the strategy taken by vertebrates (maximum conduction velocity in vertebrates is ~ 100 meters/second even though the axons diameters are relatively small). III. Myelin and membrane resistance (R m ) A. Below is a figure of myelin around the axon: 2
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B. Why does increasing R m (e.g., by myelination) increase conduction velocity?
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Lecture 6.F10 - Neuroscience 106: Lecture 6 Action...

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