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  • What you will see in this presentation

  • is the development of myelin

  • in the peripheral nervous system

  • and the propagation of the action potential

  • along a myelinated axon

  • You should already have

  • a good understanding

  • of the Schwann Cell

  • and action potential.

  • The Schwann Cell forms a protective coating

  • around the axon

  • Scwann cells start to develop in the embryo

  • and continue to increase the wrapping around

  • the axon through childhood.

  • This increases the thickness

  • which peaks in adolescence.

  • This is why teenagers have such quick responses.

  • The Schwann cell contains

  • typical cell organelles and membrane structure

  • Notice as the Schwann cell surrounds the axon

  • the nucleus is squeezed to the outside wrapping of the cell.

  • This outer wrapping of the Schwann cell is called the "neurolemma."

  • The inner lining is made up of

  • layers upon layers of cell membrane.

  • This inner wrapping is called the myelin sheath.

  • The cell membrane

  • called the fluid mosaic model

  • is made up of a bilayer of lipids

  • integrated with proteins

  • The thicker the myelin

  • in other words, the more layers

  • of cell membrane making up the myelin

  • the more advantageous it is to the axon.

  • One advantage is the regeneration

  • of severed axons

  • Another advantage is an increase

  • in the speed of propagation of the action potential

  • along the axon.

  • The rest of this presentation will concentrate

  • on the increased speed of action potential

  • down the length of the axon

  • Here is the neuron

  • And you can see the repeated Schwann cell

  • membrane forming the myelin

  • Note that there's a small space

  • between the Schwann cells

  • where the axon is not covered

  • by the cell

  • These spaces are called Nodes of Ranvier.

  • From what you already know

  • action potentials occur at the axon hillock and

  • continue to be repeated

  • away from the cell body

  • much like dominoes falling one after another

  • An action potential starts

  • on a polarized membrane

  • which is negative 70

  • A stimulus causes the sodium gates to open slightly

  • and sodium starts to trickle into the cell

  • If the cell reaches -60 or threshold

  • the sodium gates open wide

  • and sodium floods in

  • bringing the inside of the axon to +30

  • At this point the

  • sodium gates close and potassium gates open

  • Potassium starts to pour out of the cell

  • This allows the neuron to become polarized again

  • Then the sodium potassium pumps

  • starts to actively transport sodium out

  • and potassium back into the neuron

  • First look at the propagation

  • of the action potential in the unmyelinated axon

  • Propagation is the repeating

  • of action potentials down the axon.

  • The action potential is repeated

  • because as the sodium comes in

  • it diffuses to adjacent areas within the axon

  • As the sodium increases in this area

  • threshold is reached

  • Sodium gates open wide

  • sodium rushes in

  • causing depolarization

  • and an action potential

  • as the sodium enters this area

  • it diffuses through the ectoplasm

  • and another action potential is created

  • This continues down the length of the axon.

  • Now look at the myelinated axon.

  • The same process applies

  • An action potential develops

  • And as the sodium comes in

  • it diffuses through the axon

  • It continues to diffuse through the portion

  • of the axon wrapped in myelin

  • The increased sodium concentration

  • reaches the Node of Ranvier

  • increases the ectoplasm

  • to -60 and

  • depolarization occurs.

  • The sodium gates open wide

  • sodium floods in

  • and we have an action potential

  • Again. The sodium diffuse

  • through the ectoplasm, reaching the next node

  • an action potential develops

  • The process is continued down the myelinated axon

  • passing from node to node

  • Compare the unmyelinated axon with the myelinated

  • You can see that action potential reach

  • the end of the myelinated axon more rapidly

  • The speed of the propagation

  • is faster going node to node than

  • action potentials that develop adjacent

  • to the previous action potential

What you will see in this presentation

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B2 H-INT US axon action potential sodium action potential myelin

The Schwann Cell and Action Potential

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