Conduction Block

    
      Re: Conduction Block
    


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Posted by CCF Neurology MD on September 12, 1997 at 23:59:50:

In Reply to: Conduction Block posted by Sid on September 09, 1997 at 10:11:28:

: Please explain conduction block for me.
  thanks!
  Sid
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Dear Sid:
I do not know your background. I can generalize, however, that the precise concept of a conduction block is quite difficult to understand for anyone who does not have a fair concept of peripheral nerve electrophysiology. This includes most physicians who are not neurologists.
The term "conduction block" refers to an electrophysiological phenomenon observed most commonly is a disease state of nerve fibers called demyelination (loss of myelin, the fatty insulation material around nerve fibers). Although conduction block can occur both in the central nervous system (brain, spinal cord, and optic nerves in MS, for instance) and in the peripheral nervous system (peripheral nerves or spinal roots in Guillain-Barre syndrome, or in some focal neuropathies, for instance), it is most easily measured in latter using a test called electromyography (EMG). As you might know, nerves resemble cables somewhat. Each nerve has a large number of nerve fibers which differ from, but could be considered analogous to wires in a cable. Each nerve fiber consists of a central long structure called the axon which is surrounded by a sheath of myelin, the latter being formed by cells called Schwann cells. Signals are conducted along nerve fibers by virtue of the dynamic electrical and ion-flux state of the membrane of the axon. In normal large nerve fibers, the velocity of these electrical signals is about 40-50 meters/secon. This high velocity is observed because the signal tends to "hop" along the length of the axon at regular points of discontinuity in the myelin sheath (nodes). A loss of the myelin has two consequences. Firstly, the velocity of signal conduction can slow down. Secondly, and more importantly, some of the nerve fibers in a nerve can FAIL to transmit the signal beyond the area of demyelination, although the axon is preserved. This latter effect is the conduction block. It is observed at EMG when the response on stimulating a nerve distally (closer to muscle) is higher than the response on stimulating the same nerve more proximally. The importance of the conduction block lies in the fact that it is the results in deficit such as weakness, and that it is usually reversible.
If this question is related to a medical problem you have, or someone you know has, you are welcome to seek a second opinion, if you so wish, from neuromuscular specialists at the Cleveland Clinic.
The number t call for appointments is [phone], or [phone] toll free.
Good luck!