DRAFT: This module has unpublished changes.


 What is the importance of the myelin sheath?





An axon that has myelin sheath has the capability to send out electrical impulses in more efficiently and at a faster speed.


In a myelinated axon, the axon is completely insulated except in certain sections known as Rode of Ranvier. In the node of Ranvier, we have the ion channels where exchange of ions takes place. From one node to the next there are action potentials  that travel at a velocity of about 25m/s. The myelin sheath decreases capacitance but increases resitivity. The voltage increases in sections where the capacitance is low and therefore sending signals out fast.


In unmyelinated axons, the signals travel by means of diffusion of Na+ ions. As the sodium ions travel forward this causes an action potential which therefore opnes the ion channels. Since signal impulses are not as fast as the myelinated axons, its speed depends on the cross section of the axon.



What are the four currents ?


  • Current outside of the  axon
  • Current inside of the axon
  • Current through the capacitive component
  • Current through the resistive component

Capacitance and resistance are two factors that must be taken into consideration because the membrane does leak, therefore changing the current across an axon.





What is the electric circuit of a myelinated axon?


The image below represents the electric circuit of a myelinated axon. As we can see the higher the resistance, the faster the electrical impulse will travel. Insulation provides an efficient means of sending out electrical impulses. 




As seen below, in a myelinated sheath, there are two action potentials because in each node of Ranvier the capacitacitance is low in each break. In an unmyelinated axong, electrical resitivity is low because nothing is insullating the axon.









DRAFT: This module has unpublished changes.