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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
Why are Action Potentials needed?1. First look at current flow without APs.
2. Second look at current with APs
Passive Current Flow
Recordvoltage
Inject current
axon
axon
voltage
distance
0.63V0
0Length constant
V= Vo – 0.63Vo
Inject current
Recordvoltage
Passive Current
1. Current decays very rapidly along the length of an axon
2. The length constant is the distance over which the potential drops to 63% of the highest value
3. Typical length constants range only from
1-5 mm
Length Constant
• Depends on:
1. Resistance across the membrane (‘leakiness’)
2. Longitudinal resistance to current flow (varies with axon diameter)
Passive Current FlowInject current
axon
Membrane Resistance
Longitudinal Resistance
Action Potential Conduction
Recordvoltage
Stimulate Action Potential
axon
Action Potential Conduction
1. APs constant amplitude at all points along the axon
Na+Inject current
Na+Inject current
Sequence of Events leading to AP propagation
1. Stimulus opens Na+ channels & cause AP
2. Depolarizing current flows down the axon
3. Local depolarization opens Na+ channels
downstream & initiate a new AP
4. Na+ channels close (inactivate) & K+ channels open
5. Local depolarization opens Na+ channels
downstream and initiate a new AP
Na+Inject current
Conduction Velocity
Recordvoltage
Inject current
axon
Measure distance betweenrecording sites
Measure time between APs
distanceconduction velocity=
time
1. Axon diameter2. Myelination
Small unmyelinated 0.5 m/sLarge myelinated 120 m/s
Myelinated nerve
Myelin Formed by: Schwann cells (periphery)Oligodendrocytes (central)
Node of Ranvier
Myelin
MyelinNa+
Saltatory conduction
Myelin
Myelin increases speed of conduction by:
1. Increasing membrane resistance• Reduces ‘leakiness’ length constant
2. Voltage-gated channels only at Node of Ranvier
• APs generated only at the Node
Mulitple Sclerosis
• Demyelination of axons– Impaired AP conduction– Symptom depends on nerves affected
• Optic nerve blindness• Motor nerves weakness or paralysis
Summary & Key Concepts
1. Currents flow passively down axon• decay described by length constant
2. Action potentials propagation due to sequential opening of Na+ channels in response to local depolarization
3. Conduction velocity determined by axon diameter and myelination - length constant
4. Myelin trans-membrane resistance and Na+ channels only at Nodes
• Saltatory conduction
Sample Question
• AP conduction velocity will increase following an increase in the electrical resistance of the:
A. Axon membrane
B. Node of Ranvier
C. Schwann cell