Saltatory conduction

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For the definition of saltation, see Saltation (biology).
Structure of a typical neuron
Saltatory conduction occurs only on myelinated axons.

Saltatory conduction (from the Latin saltare, to hop or leap) is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials. Absent nodes (uninsulated sections) separated by the myelin sheaths, an electrical potential spike must be rebuilt many more times along the axonal membrane. Thus the conduction we describe here is propagation of ion exchanges across a nerve cell membrane, and is not similar to conduction of electrons in an electrical circuit.

Mechanism[edit]

In order for a signal to travel along the axon of a nerve cell, a charge must build up across the axonal membrane. This spike in voltage is what it termed an Action Potential (AP). Creation of AP's must be repeated many times at structures along the axon known as ion gates.[1] However with myelin sheaths insulating the axon an AP is observed to "jump" to the next ion gate which occurs at a neighboring Ranvier node. Thus in myelinated axons, action potentials can "hop" along the axon, by which process the signal travels faster by skipping around the insulated sections. (The action potential only moves in one direction, because the sodium channels at the previous node of Ranvier are inactivated, and cannot regenerate another action potential, even when depolarized. The charge will passively depolarize the adjacent node of Ranvier to threshold, triggering an action potential in this region and subsequently depolarizing the next node, and so on.) This phenomenon was discovered by Ichiji Tasaki[2][3] and Andrew Huxley[4] and their colleagues.

Energy efficiency[edit]

Apart from increasing the speed of the nerve impulse, the myelin sheath helps in reducing energy expenditure at 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.[citation needed]

Distribution[edit]

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 Fenneropenaeus chinensis and Marsupenaeus japonicus,[5][6][7] as well as a median giant fiber of an earthworm.[8] Saltatory conduction has also been found in the small- and medium-sized myelinated fibers of Penaeus shrimp.[9]

See also[edit]

References[edit]

  1. ^ Tamarkin, Dawn. "Saltatory Conduction of APs". Retrieved 6 May 2014. 
  2. ^ 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
  3. ^ 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
  4. ^ Huxley AF, Stämpfli R. Evidence for saltatory conduction in peripheral myelinated nerve fibres. J Physiol. 108:315-39, 1949. PMID 16991863
  5. ^ 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
  6. ^ Hsu K, Tan TP, Chen FS. Saltatory conduction in the myelinated giant fiber of shrimp (Penaeus orientalis). KexueTongbao 20:380-382, 1975
  7. ^ 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
  8. ^ 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
  9. ^ 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

External links[edit]