The Myelin Project

Transcription

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