Joseph T. Ho wrote:
>> Saltatory conduction basically is what happens when axons are myelinated. The
> myelin acts as an insulator between the nodes of Ranvier. These nodes are
> areas of the axon that are not covered by the myelin sheath. A depolarization
> of the membrane "jumps" from node to node. Because of the increased
> insultation, there is a high resistance across the axon membrane. The
> depolarization travels between the nodes basically instantaneously, like
> electricity in a wire. It is only at the nodes that the action potential is
> regenerated, because on these areas have the voltage-gated Na channels, etc.
>> If this doesn't make sense then perhaps someone else can explain it. This was
> my first attempt at explaining it to someone else. :)
>> ----------------------------------------------------------------------------
> Joseph T. Ho, MS I jtho at u.washington.edu> School of Medicine University of Washington
> ----------------------------------------------------------------------------
One correction and a few additions. Internodal conduction velocity is
not infinite. The membrane capacitance must be (dis)charged, and this
takes time. Myelination therefore serves two purposes. The first is to
reduce the current leak, forcing all of the current to flow through the
nodes where the channels are located. This is an increase in the space
constant of the axon. The secone, and more important function, is to
reduce the membrane capacitance. This reduces the time constant for
(dis)charging and directly results in the increase in AP velocity. Now
you have an inhomogeneous cable. In the internodal regions, velocity is
high because the membrane time constant is low (a function of
capacitance). In the nodal regions, velocity is low because the
membrane time constant is high. For further reference, see Aidley
(1989) The Physiology of Excitable Cells, 3rd ed. and Ch. 7 of Kandel et
al. (1991) Principles of Neural Science, 3rd ed.
Dr. L.W. Haynes
Asst. Prof. Physiology and Biophysics
University of Calgary