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• 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
Inject current
axon
Record
voltage
Inject current
axon
Record
voltage
0.63V0
voltage
V= Vo – 0.63Vo
0
Length constant
distance
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 Flow
Inject current
Membrane Resistance
axon
Longitudinal Resistance
Action Potential Conduction
Stimulate Action Potential
axon
Record
voltage
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
Inject current
Measure distance between
recording sites
axon
Record
voltage
Measure time between APs
distance
conduction velocity=
time
1. Axon diameter
2. Myelination
Small unmyelinated  0.5 m/s
Large myelinated  120 m/s
Myelinated nerve
Node of Ranvier
Myelin Formed by:
Schwann cells (periphery)
Oligodendrocytes (central)
Myelin
Na+
Myelin
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