Journal Article

Node of Ranvier length as a potential regulator of myelinated axon conduction speed

Arancibia-Cárcamo, I Lorena, Tohyama, Koujiro, Ishida, Kinji, Attwell, David, Cossell, Lee, Ford, Marc C

2017

Overview

Myelination speeds conduction of the nerve impulse, enhancing cognitive power. Changes of white matter structure contribute to learning, and are often assumed to reflect an altered number of myelin wraps. We now show that, in rat optic nerve and cerebral cortical axons, the node of Ranvier length varies over a 4.4-fold and 8.7-fold range respectively and that variation of the node length is much less along axons than between axons. Modelling predicts that these node length differences will alter conduction speed by ~20%, similar to the changes produced by altering the number of myelin wraps or the internode length. For a given change of conduction speed, the membrane area change needed at the node is >270-fold less than that needed in the myelin sheath. Thus, axon-specific adjustment of node of Ranvier length is potentially an energy-efficient and rapid mechanism for tuning the arrival time of information in the CNS. Information is transmitted around the nervous system as electrical signals passing along nerve cells. A fatty substance called myelin, which is wrapped around the nerve cells, increases the speed with which the signals travel along the nerve cells. This allows us to think and move faster than we would otherwise be able to do. The electrical signals start at small “nodes” between areas of myelin wrapping. Originally it was thought that we learn things mainly as a result of changes in the strength of connections between nerve cells, but recently it has been proposed that changes in myelin wrapping could also contribute to learning. Arancibia-Cárcamo, Ford, Cossell et al. investigated how much node structure varies in rat nerve cells, and whether differences in the length of nodes can fine-tune the activity of the nervous system. The experiments show that rat nerve cells do indeed have nodes with a range of different lengths. Calculations show that this could result in electrical signals moving at different speeds through different nerve cells. These findings raise the possibility that nerve cells actively alter the length of their nodes in order to alter their signal speed. The next step is to try to show experimentally that this happens during learning in animals.

• Publisher: eLife Science Publications, Ltd,eLife Sciences Publications Ltd,eLife Sciences Publications, Ltd
• Subject: Animals / axon / Axons / Axons - physiology / Biostatistics / Central nervous system / Cerebral Cortex - cytology / Cerebral Cortex - physiology / Cognitive ability / conduction speed / Cortex / Microscopy / Models, Biological / myelin / Myelination / Neural circuitry / Neural Conduction / Neuroscience / node of Ranvier / Optic nerve / Optic Nerve - cytology / Optic Nerve - physiology / Physiological aspects / Physiology / Ranvier's Nodes - physiology / Rats / Standard deviation / Substantia alba