Myelin sheath consists of layers that wrap around axons, leaving small sections of exposed axon at the nodes of Ranvier. The action potential is regenerated only at these nodes, rather than continuously along the fiber like in unmyelinated fibers. The resistance across the layers of myelin is high, limiting transmembrane currents to the nodes of Ranvier. This allows for the action potential to be regenerated at each successive node, leading to saltatory conduction. The local currents enter the node and conduct to the next node, bringing each node to threshold in a short period of time. This jumping of the action potential from node to node is what is known as saltatory conduction. The functional consequences of myelination can be observed when comparing squid and mammalian axons. Despite having smaller diameters, mammalian axons conduct at comparable or even faster speeds due to myelination. Unmyelinated squid giant axons have a diameter of 500 µm and a conduction velocity of around 20 m/sec. In mammals, axon diameters range from 0.2 to 20 µm, and only fibers larger than 1-2 µm are myelinated. Unmyelinated mammalian nerve fibers, with diameters less than 1-2 µm, have conduction velocities less than 2 m/sec. In contrast, myelinated mammalian fibers with a diameter of 10 µm have conduction velocities around 50 m/sec, more than twice that of squid giant axons. This increase in conduction velocity achieved through myelination allows for greater neuronal connectivity without significantly increasing the volume of the central nervous system. This is a crucial factor in the evolution of mammalian nervous systems with their vast numbers of neurons capable of generating both reflexes and complex mental processes.
CHAPTER 5 Generation and Conduction of Action Potentials •Fig. 5.10 A, Schematicillustrations,incrosssectionandlongitudinalsectionthroughanodeofRanvier,ofaSchwanncellwrappedaroundanaxontoformmyelin.NotethattheaxonisexposedtotheextracellularspaceonlyatthenodeofRanvier.B, Viewoftwonodesandtheinterveninginternodeofmyelin.C, Saltatoryconductioninamyelinatedaxonwithaplotoftheactionpotentiallocationalongtheaxon(x-axis)versustime(y-axis).Notetheshorttimetakenfortheactionpotentialtotraversethelargedistancebetweennodes(shallow sloped lines ontheplot)becauseofthehighresistanceandlowcapacitanceoftheinternodalregion.Incontrast,theactionpotentialslowsasitcrosseseachnode(steep sloped line segments). (B, RedrawnfromSquiresLRetal:Fundamental Neuroscience. 2nded.SanDiego,CA:AcademicPress;2002.C, RedrawnfromBlankenshipJ:Neurophysiology. Philadelphia:
Mosby;2002.) at the nodes of Ranvier and are not found between them. Thus the action potential is regenerated only at the nodes of Ranvier (0.3-2 mm apart) rather than being regenerated continuously along the fiber, as is the case in an unmyelinated fiber. Resistance to the flow of ions across the many layers that make up the myelin sheath is so high that transmembrane currents are largely restricted to the short stretches of naked plasma membrane that are present at the nodes of Ranvier (see
Fig. 5.10C ). Therefore, the action potential is regenerated at each successive node. The local currents entering the node are almost entirely conducted from one node to the next node, bringing each node to threshold in about 20 µsec. Thus the action potential appears to “jump” from one node of Ranvier to the next, and the process is called saltatory (from the Latin word saltare, “to leap”) conduction (
Fig. 5.11
Functional Consequences of Myelination
The functional consequences of myelination can be highlighted by a comparison of squid and mammalian axons. Although human nerve fibers are much smaller in diameter than squid giant axons, human axons conduct at comparable or even faster speeds because of myelination. The unmyelinated squid giant axon has a 500-µm diameter and a conduction velocity of about 20 m/sec. In mammals, axon diameters range from about 0.2 to 20 µm, and all fibers with diameters larger than 1-2 µm are myelinated. An unmyelinated mammalian nerve fiber, which has a diameter of less than 1 to 2 µm, has a conduction velocity of less than 2 m/sec (see
Fig. 5.9 ), as expected because of its smaller diameter in comparison to the squid giant axon. In contrast, a 10-µm myelinated mammalian fiber has a conduction velocity in the range of 50 m/sec, more than twice that of the 500-µm squid giant axon, despite being 1/50 of its diameter. Thus the high conduction velocity with far narrower axons achieved by myelination allows a tremendous increase in neuronal connectivity without enormously expanding the volume of the CNS. This is certainly one factor that enabled the evolution of mammalian nervous systems with their huge numbers of neurons that are able to generate everything from fast reflexes to efficient and complex mental processing.