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