Myelin conduction

[scilearner]

Hello everyone,

 

I don't understand how depolarisation jumps when axons are myelinated. Ok myelin is an insulator so in that section no current passes through. But how does that make the current jump, when current reached myelin wouldn't it be inhibited, why does it still travel. Also what do they mean by developing current sinks at nodes of ranvier. Thanks

[SMF]

I presume that you already know that an action potential progresses down an axon incrementally as depolarization of a section of membrane initiates depolarization next door, and this triggers further depolarization. Membranes are usually negatively charged on the inside, relative to the outside, so depolarization makes the interior less negative, or even positively charged. Changes in membrane potential is produced by movement of ions across the membrane. Depolarization to a trigger point opens voltage sensitive sodium ion channels and this initiates an action potential whereby the entry of positive sodium ions, across the membrane, depolarizes it more (goes positive), and then potassium ion channels open to release potassium from the axon to recharge the membrane to a negative interior state.

 

In myelinated axons the sections between the nodes are sealed off so that ions cannot cross the membrane. Instead the depolarization, carried by an ionic current, progresses longitudinally down the axon until the next small segment of node membrane, where ions can cross, and an action potential is initiated. This is a bit simplistic, but the deal is that the spread of depolarization down the axon to the next node is much faster than the progression of an action potential, so myelinated axons conduct information at a much higher speed.

 

Said slightly differently-- in an unmyelinated axon the spread of depolarization of an action potential progresses from one house to the one next door, but in a mylelinated axon the spread of depolarization, between nodes, is a whole block to the next cross street. SM

Edited January 13, 2011 by SMF