Saltatory conduction

Saltatory conduction (from the Latin saltare, to hop or leap) is a means by which action potentials are transmitted along myelinated nerve fibers.

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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^[1][2] and Andrew Huxley^[3] and their colleagues.

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.

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 ^[4][5][6], as well as a median giant fiber of an earthworm^[7]. Saltatory conduction has also been found in the small- and medium-sized myelinated fibers of Penaeus shrimp^[8].

References

1. ^ 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
2. ^ 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
3. ^ Huxley AF, Stämpfli R. Evidence for saltatory conduction in peripheral myelinated nerve fibres. J Physiol. 108:315-39, 1949. PMID 16991863
4. ^ 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
5. ^ Hsu K, Tan TP, Chen FS. Saltatory conduction in the myelinated giant fiber of shrimp (Penaeus orientalis). KexueTongbao 20:380-382, 1975
6. ^ 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
7. ^ 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
8. ^ 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