Neuron Transmission

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Neuron Transmission

• Action Potential Propagation along the Axon

2
Introduction

• The Neuron
• Its Structure
• How it works
• Why it works
• New Concepts

3
The Neuron
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Types of Neurons

• Sensor Neurons
• Motor Neurons
• Inter Neurons

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Sensor Neurons

• Long Dendrites
• Short Axons
• Transmit from Central Nervous system

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Motor Neurons

• Long Axons
• Short Dendrites
• Transmit from CNS to Muscles

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Inter Neurons

• Found in Central Nervous System
• Connect Neuron to Neuron

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Structure of the Neuron
Dendrites are the Input Layer of the Neuron
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Structure of the Neuron
The Soma Nucleus regulates the metabolism of
the Neuron
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Structure of the Neuron
The Axon Carries the Output Signal to the
attached Dendrites
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Structure of the Neuron
Transmission takes place again between the
Boutons and Dendrites
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Structure of the Neuron
Neuron Signal Transmission
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Neuron Firing
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Neuron Firing
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Neuron Firing

• How does the signal travel?
• What makes it propagate?
• What stops it?

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Neuron Firing

• Chemical reaction creates Action Potential
• Action Potential Propagates along neuron
• AP induces chemical reaction at end of neuron

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Hogkin-HuxleySignal Transmission
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Hogkin-Huxley

• Chemical Model
• Creation of the Action Potential

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Hogkin-Huxley

• Electrical Model

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Hogkin-Huxley

• Derived Equations
• ic C dv/Dt
• iNa gNa (V-VNa)
• iK gK (V-VK)
• ir gr (V-Vr)

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The Action Potential

• 50-70 millivolt electrical potential
• Chemically induced by the cell
• Cell permeability changes to displace ions
• K and Na used to propagate potential
• Nodes of Ranvier important for speed of AP
• Axon has no loss of signal due to its properties

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The Action Potential

• What Happens to the Cell?

• Polarization
• Depolarization
• Re-polarization
• Hyper-polarization

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An Action Potential Example
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An Action Potential Example
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An Action Potential Example
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An Action Potential Example
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Propagation along the Axon

• Resting Membrane Potential State of Axon
• Low Na concentrations inside axon
• High K concentrations inside axon
• High Na concentrations outside axon
• Low K concentrations outside axon
• Cell permeability is high for K
• Cell permeability is low for Na Cl-

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Propagation along the Axon

• It is observed at resting state membrane
  potential is positive on outside of axon
• Thus inside axon is negatively charged
• Proof is Nernst Equation

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Propagation along the Axon
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Propagation along the Axon

• The Nernst Equation gives a good estimate of
  resting potential, but neglects the interaction
  of the Na and Cl- ions.

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Propagation along the Axon

• Real World Example Concentrations

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Propagation along the Axon

• The Goldman Equation accounts for all ionic
  interaction

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An Action Potential Example

• Squid Axon used for experiment
• Large size simplifies study
• Permeability measured
• Ion concentrations measured

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An Action Potential Example

• Squid Axon Evaluated

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An Action Potential Example

• AP Calculated

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Action Potential Propagation

• Once initiated how is AP transmited?

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Action Potential Propagation

• Simplify the A-H Model

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Action Potential Propagation

• Axon Circuit Model

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Action Potential Propagation

• Substitute Rt for infinitely long cable

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Action Potential Propagation

• Axon Circuit Model

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Action Potential Propagation

• Axon Circuit Model
• Voltage moves away from source in circuit
• Amplitude drops exponentially as signal travels

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References

• 1 Modeling and Simulation in Medicine and the
  Life Sciences
• Frank C. Hoppensteadt and Charles S.
  PeskinSpringer, NY, 2002
• 2 Bioelectromagnetism
• Jaakko Malmivuo and Robert Plonsey
• Oxford Press, NY 1995