Saltatory conduction

For the definition of saltation in evolutionary biology, see Saltation (biology).

Action potential propagation in myelinated neurons is faster than in unmyelinated neurons because of saltatory conduction.

Dendrite

Soma

Axon

Axon hillock

Nucleus

Node of
Ranvier

Axon terminal

Schwann cell

Myelin sheath

Saltatory conduction occurs only on myelinated axons.

Propagation of action potential along myelinated nerve fiber

In neuroscience, saltatory conduction (from Latin saltus 'leap, jump') is the propagation of action potentials along myelinated axons 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). 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. In the CNS, nerve cells have been shown to individually alter the size of the nodes to tune conduction speeds.

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.

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 japonicus shrimp, as well as in a median giant fiber of an earthworm. Saltatory conduction has also been found in the small- and medium-sized myelinated fibers of Penaeus shrimp.

History of research

In 1939, Ichiji Tasaki discovered saltatory conduction through experiments on isolated single-nerve fibers of the Japanese Toad. Tasaki was experimenting with anaesthetics and noticed a lack of conduction when three or more nodes were anesthetized, leading to his hypothesis. 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.

Tasaki's hypothesis was confirmed by Andrew Huxley and Robert Stämpfli in peripheral myelinated nerve fibers in 1949 through experiments with isolated frog nerves. Bernhard Frankenhaeuser proved that this was true in undissected frog nerves as well, ending scholarly debate.

See also

• Bioelectrochemistry
• Cable theory
• Electrophysiology
• Ephaptic coupling – Form of nervous system communication
• GHK current equation – Expression of the ionic flux across a cell membranePages displaying short descriptions of redirect targets
• Goldman equation – Generalization of the Nernst equation for the membrane potential
• Hindmarsh–Rose model – Of the spiking-bursting behavior of a neuron
• Hodgkin–Huxley model – Describes how neurons transmit electric signals
• Neurotransmission – Impulse transmission between neurons
• Patch clamp – Laboratory technique in electrophysiology
• Quantitative models of the action potential
• Myelination – Fatty substance that surrounds nerve cell axons to insulate them and increase transmission speedPages displaying short descriptions of redirect targets

Further reading

• Saladin K. "Saltatory conduction". Biology Online.
• Anatomy & physiology : the unity of form and function (6th ed.). McGraw-Hill. 2011. ISBN 978-0-07-768033-6.

External links

• Saltatory conduction - Scholarpedia
• cell biology - Why is saltatory conduction in myelinated axons faster than continuous conduction in unmyelinated axons?