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Announcements

Announcements. Tutorial next Thursday, Oct 9 Submit questions to me Mid-term schedule Go vote!. Today Action Potential Conduction Next Lecture Synaptic Transmission. Action Potential Conduction. Axon hillock Region of neuron where AP usually starts. Action Potential Conduction.

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Announcements

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  1. Announcements • Tutorial next Thursday, Oct 9 • Submit questions to me • Mid-term schedule • Go vote!

  2. Today • Action Potential Conduction • Next Lecture • Synaptic Transmission

  3. Action Potential Conduction Axon hillock Region of neuron where AP usually starts

  4. Action Potential Conduction Why are Action Potentials needed? • First look at current flow without APs. • Second look at current with APs

  5. Passive Current Flow Inject current axon Record voltage

  6. Inject current axon Record voltage 0.63V0 voltage V= Vo – 0.63Vo 0 distance Length constant

  7. Passive Current • Current decays very rapidly along the length of an axon • The length constant is the distance over which the potential drops to 63% of the highest value • Typical length constants range only from 1-5 mm

  8. Length Constant • Depends on: • Resistance across the membrane (‘leakiness’) • Longitudinal resistance to current flow (varies with axon diameter)

  9. Passive Current Flow Membrane Resistance Inject current axon Longitudinal Resistance

  10. Action Potential Conduction Stimulate Action Potential axon Record voltage

  11. Action Potential Conduction • APs constant amplitude at all points along the axon

  12. Na+ Inject current

  13. Na+ Inject current

  14. Sequence of Events leading to AP propagation • Stimulus opens Na+ channels & cause AP • Depolarizing current flows down the axon • Local depolarization opens Na+ channels downstream & initiate a new AP • Na+ channels close (inactivate) & K+ channels open • Local depolarization opens Na+ channels downstream and initiate a new AP

  15. Na+ Inject current

  16. Conduction Velocity Inject current Measure distance between recording sites axon Record voltage Measure time between APs

  17. Axon diameter • Myelination Small unmyelinated  0.5 m/s Large myelinated  120 m/s

  18. Myelinated nerve Node of Ranvier Myelin Formed by: Schwann cells (periphery) Oligodendrocytes (central)

  19. Myelin

  20. Na+ Myelin Saltatory conduction

  21. Myelin Myelin increases speed of conduction by: • Increasing membrane resistance • Reduces ‘leakiness’  length constant • Voltage-gated channels only at Node of Ranvier • APs generated only at the Node

  22. Mulitple Sclerosis • Demyelination of axons • Impaired AP conduction • Symptom depends on nerves affected • Optic nerve  blindness • Motor nerves  weakness or paralysis

  23. Summary & Key Concepts • Currents flow passively down axon • decay described by length constant • Action potentials propagation due to sequential opening of Na+ channels in response to local depolarization • Conduction velocity determined by axon diameter and myelination - length constant • Myelin  trans-membrane resistance and Na+ channels only at Nodes • Saltatory conduction

  24. Sample Question • AP conduction velocity will increase following an increase in the electrical resistance of the: • Axon membrane • Node of Ranvier • Schwann cell

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