David S. Campion, M.D.

Nerve Conduction

On this page is an animated diagrammatic representation of Nerve conduction in the normal, damaged and repaired state.

NORMAL NERVE: Nerves are really bundles of thousands of individual nerve fibers and it is simplest to think of each nerve fiber as a" wire" with a central core called the axon (in green) – like the copper wire in the center of an electrical connection – and an insulation in blue, called the myelin sheath, on the outside. Don’t forget however that this is a living dynamic tissue that needs blood vessels to supply nourishment and oxygen. The basic function of the nerve is to conduct electrical signals from the spinal cord to the muscles (motor nerves), and sensory information from the skin, joints, tendons and a host of other sensory organs to the spinal cord (sensory nerves). More recently it has been realized that there are substances – termed trophic substances – that are made in the motor and sensory cells in and next to the spinal cord that are transported down the nerve and keep the nerve in good condition. The fastest of these trophic substances travel from the cord to the hand in two or three days, but the slower ones, that are probably very important for healing, can take up to three years! This is one reason why repair after damage should never be considered complete until three years after injury.

In the animated diagram we see the central core - the axon -in green, the myelin "insulation" in blue and the electrical signal traveling along the nerve. There is a gap in the insulation every millimeter or two – the nodal gap – and the electrical signal travels along the nerve by jumping from node to node. Essentially the node acts like a booster or relay station and at each node there is a very small delay. Any interruption in the nodes stops the signal at the site of damage. The signals travel very fast - about 60M/sec (120 miles per hour), and that of course is why we can so quickly pull away from an hot object!

DAMAGED NERVE: Here damaged is represented by a section of the insulating myelin that is in black and the signal is abruptly stopped at this site. If this a motor nerve then paralysis of the muscle will occur, and if this is a sensory nerve then numbness, tingling, in-coordination or other changes in sensation will be experienced. Damage of this form, called demyelination, is what happens when a nerve is subjected to repeated compression, stretching or long standing lack of oxygen or nourishment. The most common example of this is Carpal Tunnel Syndrome. The carpal tunnel at the wrist contains the median nerve and many tendons that flex the fingers. In this instance the median nerve, that takes sensation from the thumb, index, middle and half of the ring finger, is damaged by the repeated trauma that can result from repetitive actions such as typing.

Although not represented here, another less common but very serious form of damage, sometimes caused by toxic substances, is when the axon dies. From the site of damage the axon disappears down the remaining length of the nerve and the signal is completely lost. The nerve only recovers by a new axon forming and growing from the site of damage to the end of the nerve.

REPAIRED NERVE: This represents diagrammatically the repair that occurs after the myelin "insulation" has been damaged - for example in Carpal Tunnel Syndrome. The nodes reform but in the repaired region of the nerve the nodes are much closer together and for this reason the signal, having to traverse more nodes than normal, is slowed. In the animated diagram if you compare the normal nerve to the repaired nerve then it is clear that the signal arrives at the end of the repaired nerve later than in the normal nerve.

The EDX physician tests for this by stimulating the nerve at a point above the damage, and recording either on a sensory or motor (muscle) nerve below the damage. Very extensive studies have determined the normal values for each nerve.

Nerve Conduction

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