Conduction velocity in myelinated axons

The function of the myelin sheath is to increase the conduction velocity significantly with relatively slight increase in fiber diameter. The evolution of myelination has allowed vertebrates to have a large number of fast conducting axons without taking up too much cable space.

As the myelin sheath contains largely of plasma membrane it has large phospholipid content, providing it a high electrical resistance. The local circuit currents are forced to take paths of smaller resistance via the electrolyte solution around the sheath. The effect is that local circuits are recognized, not between adjacent areas of membrane as they are in nonmyelinated axons, however between adjacent nodes of Ranvier, that are relatively far apart. The local circuit current ahead of action potential arriving at next downstream node causes it to depolarize beyond the threshold and trigger an action potential. In these way action potentials looks to jump from node to node, a mechanism termed as the saltatory conduction. The density of Navs is about 100-fold bigger at nodes than in nonmyelinated axon membrane and the node threshold is accordingly much lower. This greatly reduces the risk of nodes not firing in response to local circuit currents weakened by the long distances they should spread.

The Conduction velocity (θ) is faster in myelinated than nonmyelinated axons for two reasons which are as shown below:

- The presence of a myelin sheath is functionally equal to increasing the thickness of the axonal membrane about 100-fold. This greatly decreases the amount of charge stored across the membrane that means that much less time is taken to depolarize it. More technically put, myelination decreases the membrane capacitance (c_m), as the capacitance of a parallel plate conductor is inversely proportional to the thickness of the insulator, and as θ ∝ 1/c_m, the lower is the capacitance the greater the speed of action potential propagation.

- The time taken up for Navs to respond to depolarization is less, as only channels at nodes have to be activated. In nonmyelinated axon each little area of membrane has to be depolarized and respond in succession. For a myelinated axon, though, only the node membrane requires to be excited. The Conduction velocities of myelinated axons vary from about 7 to 100 ms^-1. Since with nonmyelinated axons, the velocity depends on diameter, a, while the relationship is even simpler as:

                                                                   θ = ka.