Saltatory conduction

:"For Saltation definition, and other use disambiguation, see Saltation"Saltatory conduction (from the Latin "saltare", to hop or leap) is a means by which action potentials are transmitted along myelinated nerve fibers.

Mechanism

Because the cytoplasm of the axon is electrically conductive, and because the myelin inhibits charge leakage through the membrane, depolarization at one node of Ranvier is sufficient to elevate the voltage at a neighboring node to the threshold for action potential initiation. Thus in myelinated axons, action potentials do not propagate as waves, but recur at successive nodes and in effect "hop" along the axon, by which process they travel faster than they would otherwise. This process is outlined as the charge will passively spread to the next node of Ranvier to depolarize it to threshold which will then trigger an action potential in this region which will then passively spread to the next node and so on. This phenomenon was discovered by Ichiji Tasaki [Tasaki, I. The electro-saltatory transmission of the nerve impulse and the effect of narcosis upon the nerve fiber. Am J Physiol 127: 211-227, 1939] [Tasaki, I. and Takeuchi, T. Der am Ranvierschen Knoten entstehende Aktionsstrom und seine Bedeutung für die Erregungsleitung. Pflügers Arch ges Physiol. 244: 696-711, 1941] and Andrew Huxley [Huxley AF, Stämpfli R. Evidence for saltatory conduction in peripheral myelinated nerve fibres.J Physiol. 108:315-39, 1949. PMID 16991863] and their colleagues.

peed

The speed of the signal from one node to the other is the speed of the induced electromagnetic wave, that is, the speed of light in interaction with transparent materials like the cytoplasm. It should be noted, however, that any individual ion only moves with the drift velocity, less than a millimeter per second. It can be compared with a line of marbles pushing on each other - when poking the marble in one end then each marble only moves slightly, but this small effect on the marble in the other end is almost instantaneous. Myelinated axons serve the purpose of letting this small effect reach as far as possible before having to be enhanced again in the next node by activating ion channels anew. It is this activation of ion channels in each node of myelinated axons that makes the total signal speed sub-electromagnetic - if there was a perfectly insulating myelin sheath, making nodes unnecessary, then it would transduce the signal with almost the speed of light. However, such a myelin sheath would be too large and inconvenient to be efficient.

Other advantages

Apart from increasing the speed of the nerve impulse, the myelin sheath helps in reducing energy expenditure as the area of depolarization and hence the amount of sodium/potassium ions that need to be pumped to bring the concentration back to normal, is decreased.

Locations

Saltatory conduction had been found exclusively in the myelinated nerve fibers of vertebrates, but was later discovered in a pair of medial myelinated giant fibers of "Penaeus orientalis" (chinensie) and "Penaeus japonicus" [Hsu K, Tan TP, Chen FS. On the excitation and saltatory conduction in the giant fiber of shrimp (Penaeus orientalis). Proceedings of the 14th National Congress of the Chinese Association for Physiological Sciences. 1964, Aug. 7-15, Dalian, p. 17] [Hsu K, Tan TP, Chen FS. Saltatory conduction in the myelinated giant fiber of shrimp (Penaeus orientalis). KexueTongbao 20:380-382, 1975] [Kusano K, La Vail MM. Impulse conduction in the shrimp medullated giant fiber with special reference to the structure of functionally excitable areas.. J Comp Neurol. 142:481-494, 1971] , as well as a median giant fiber of an earthworm [Gunther J. Impulse conduction in the myelinated giant fibers of the earthworm. Structure and function of the dorsal nodes in the median giant fiber. J Comp Neurol. 168:505-531, 1976] . Saltatory conduction has also been found in the small- and medium-sized myelinated fibers of "Penaeus" shrimp [Xu (Hsu) K, Terakawa S. Saltatory conduction and a novel type of excitable fenestra in shrimp myelinated nerve fibers. Jap J Physiol. 43 (suppl. 1), S285-S293] .

ee also

*Saltation (biology)
*Saltation (geology)

References