In neuroscience, saltatory conduction (from Latinsaltus 'leap, jump') is the propagation of action potentials along myelinatedaxons from one node of Ranvier to the next, increasing the conduction velocity of action potentials. The uninsulated nodes of Ranvier are the only places along the axon where ions are exchanged across the axon membrane, regenerating the action potential between regions of the axon that are insulated by myelin, unlike electrical conduction in a simple circuit.
Mechanism
Myelinated axons only allow action potentials to occur at the unmyelinated nodes of Ranvier that occur between the myelinated internodes. It is by this restriction that saltatory conduction propagates an action potential along the axon of a neuron at rates significantly higher than would be possible in unmyelinated axons (150 m/s compared from 0.5 to 10 m/s).[1] As sodium rushes into the node it creates an electrical force which pushes on the ions already inside the axon. This rapid conduction of electrical signal reaches the next node and creates another action potential, thus refreshing the signal. In this manner, saltatory conduction allows electrical nerve signals to be propagated long distances at high rates without any degradation of the signal. Although the action potential appears to jump along the axon, this phenomenon is actually just the rapid conduction of the signal inside the myelinated portion of the axon.
If the entire surface of an axon were insulated, action potentials could not be regenerated along the axon resulting in signal degradation.[citation needed] In the CNS, nerve cells have been shown to individually alter the size of the nodes to tune conduction speeds.[2]
Energy efficiency
In addition to increasing the speed of the nerve impulse, the myelin sheath helps in reducing energy expenditure over the axon membrane as a whole, because the amount of sodium and potassium ions that need to be pumped to bring the concentrations back to the resting state following each action potential is decreased.[3]
Distribution
Saltatory conduction occurs widely in the myelinated nerve fibers of vertebrates, but was later discovered in a pair of medial myelinated giant fibers of Fenneropenaeus chinensis and Marsupenaeus japonicusshrimp,[4][5][6] as well as in 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]
History of research
In 1939, Ichiji Tasaki discovered saltatory conduction through experiments on isolated single-nerve fibers of the Japanese Toad[9]. Tasaki was experimenting with anaesthetics and noticed a lack of conduction when three or more nodes were anesthetized, leading to his hypothesis[10]. During World War II, Tasaki was not able to publish in American journals and had to send manuscripts to Germany via the Siberian railroad. He only heard of their publication after the war ended[10].
Tasaki's hypothesis was confirmed by Andrew Huxley and Robert Stämpfli in peripheral myelinated nerve fibers in 1949 through experiments with isolated frog nerves[11]. Bernhard Frankenhaeuser proved that this was true in undissected frog nerves as well, ending scholarly debate[12].
Myelination – Fatty substance that surrounds nerve cell axons to insulate them and increase transmission speedPages displaying short descriptions of redirect targets
^Hsu K, Tan TP, Chen FS (August 1964). "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 Science: 7–15.
^Hsu K, Tan TP, Chen FS (1975). "Saltatory conduction in the myelinated giant fiber of shrimp (Penaeus orientalis)". KexueTongbao. 20: 380–382.
^Kusano K, LaVail MM (August 1971). "Impulse conduction in the shrimp medullated giant fiber with special reference to the structure of functionally excitable areas". The Journal of Comparative Neurology. 142 (4): 481–94. doi:10.1002/cne.901420406. PMID5111883. S2CID33273673.
^Günther J (August 1976). "Impulse conduction in the myelinated giant fibers of the earthworm. Structure and function of the dorsal nodes in the median giant fiber". The Journal of Comparative Neurology. 168 (4): 505–31. doi:10.1002/cne.901680405. PMID939820. S2CID11826323.
^Xu K, Terakawa S (1993). "Saltatory conduction and a novel type of excitable fenestra in shrimp myelinated nerve fibers". The Japanese Journal of Physiology. 43 Suppl 1: S285-93. PMID8271510.
^Tasaki, Ichiji (31 Jan 1939). "Electric Stimulation and the Excitatory Process in the Nerve Fiber". American Journal of Physiology. 125 (2): 380–395. doi:10.1152/ajplegacy.1939.125.2.380.
